Weather by Buckley
Ref: Buckley, Hopkins, Whitaker (2004). Weather: A Visual Guide. Firefly Books.
___________________________________________________________________________
Summary
Weather is one of the last wild things. Although we can predict it, we cannot control it. It profoundly affects our culture, our economy, our well-being, and our daily lives. This book provides a comprehensive visual guide to weather in all its manifestations. It explains how the weather works, including hurricanes, tornadoes, floods, fires, and droughts. Weather also explains the effect of climate on plants, animals, and humans, and examines the latest research on changing climate, with particular reference to vital issues like global warming and ozone depletion.
This book traces long-term climate change over Earth's 4600-My history; explains the complex atmospheric forces that influence weather; examines the diversity of climate throughout the world and how plants, animals and people have adapted to it; analyzes the factors that interact to create violent weather extremes; and reviews the latest research into current climate change.
Climate: The variations in the weather over a 30 yr time period (at minimum).
Weather: The state of the atmosphere at any particular time and location.
Meteorology: The study of the weather.
___________________________________________________________________________
Weather Engine
Structure of the Sun: Thermonuclear fusion reactions in the dense core produce temperatures of some 15,000,000°C. Energy from the core diffuses outward in the form of photons (parcels of EM energy) through the radiative zone. It then cools and undergoes a boiling, convective motion through the convective zone. By the time it reaches the photosphere- the Sun's visible surface the gas is ~6000°C. Here, cooler areas appear as dark sunspots, and loops of gas known as prominences erupt from the surface. The chromosphere, a thin, cool layer, surrounds the photosphere. The Sun's vast, hot outer atmosphere, the corona, has temperatures of 1,000,000°C.
~46% of the sun’s radiation is in the form of visible light, while an equal amount is near IR radiation, which we feel as heat. The remainder is UV radiation, the form that causes sunburns in humans. The sun also emits a constant stream of high-energy particles as part of the solar wind.
Solar Wind: A stream of protons and electrons that flows into space from the Sun's corona at speeds of ~400 km/s. The solar wind causes Earth’s magnetosphere to become tear-shaped. Gusts in the wind can create geomagnetic disturbances, such as auroras, and even disrupt communications and power supply networks.
Coronal Mass Ejection: An eruption of gas often, but not always, associated with flares and prominences, which cause high-speed gusts in the solar wind.
Seasons: Earth is ~149M km from the Sun, and rotates around the Sun in an elliptical path, taking about 365 days to complete one orbit. Simultaneously, Earth rotates about its N-S axis in a CCW direction. Earth's axis (the imaginary line running between the North and South poles) is not perpendicular to the plane of its orbit around the Sun. So, depending on the time of year, some latitudes are tilted toward the Sun, while others are tilted away from the Sun. As Earth travels around the Sun on its annual orbit, solar rays reach the planet at different angles. For half the year, the Sun's rays fall most directly on the S. hemisphere; for the other half, on the N. hemisphere. When Earth's axis is tilted away from the Sun around the December solstice, the N. hemisphere receives less sunlight and experiences winter, while the S. hemisphere has summer. At the June solstice, the situation is reversed. In temperate climate zones, this cycle leads to the four seasons of spring, summer, autumn and winter.
Atmosphere: The gaseous envelope that surrounds Earth, protecting life from the harshness of space. The atmosphere's clouds, suspended particles and gas extend at least 100 km above the surface.
Hydrosphere: The liquid water component of Earth's system, consists of the oceans and other large bodies, which covers 71% of Earth’s surface and contains most of the planet’s water.
Cryosphere: The component of the planet made up of ice that includes glaciers and polar ice caps. A large proportion of the planet's freshwater is found in the cryosphere.
Lithosphere: The solid portion of Earth that includes the soil and rocks upon which we live. Nutrients from the atmosphere are fixed in the soil and used by plants in the biosphere.
Biosphere: Comprised of animals, plants and other organisms, as well as decaying organic matter.
Atmospheric Pressure: Caused by the weight of the column of air above, and is measured with a barometer. When simultaneous pressure readings are taken around the world, and lines of constant pressure (isobars) are drawn, a pattern emerges. Areas of high and low pressure appear, and these tend to circulate around Earth in well-defined bands. These pressure systems are closely associated with the weather experienced on the ground. High pressure usually produces fine weather, and low pressure is associated with unsettled conditions, sometimes with developing rainfall. As the atmosphere constantly works to restore equilibrium, air moves into low-pressure areas from surrounding areas of higher pressure. This movement of air from high to lower pressure areas (the flow is always in this direction) is known as wind.
Wind: The movement of air from high to lower pressure.
Global Winds: Uneven solar heating of Earth produces varying patterns of airflow, and hence varying weather at different latitudes. Intense heat reaches the tropics throughout the year and produces powerful convection currents. Warm air rises, creating a belt of low pressure around the equator. The air that rises eventually meets the troposphere, where it can rise no farther, gradually cools and sinks back to Earth's surface, at about 30° N. and S. latitudes. Some of the air from these latitudes, forced out by the sinking air, moves back toward the low pressure at the equator: this airflow is known as the trade winds. The area at the equator where the winds die out is known as the doldrums. The circulations that rise in the tropics, sink at 30° and flow back to the equator are known as Hadley cells. Other circulations continue to move poleward; those that occur between 30 and 60° are called Ferrel cells.
Polar Cells: Cold air at the poles that sink and travels toward the equator before rising and meeting Ferrel cells.
Ferrel Cells: Air from the Hadley cells that continue towards the poles before rising at ~60° N & S.
Hadley Cells: Warm air that rises from the equator and spreads toward the poles before sinking at ~30° N & S.
Jet Stream: Strong, high-altitude, westerly winds caused by strong differences in pressure and temperature in the upper-levels of the atmosphere. In winter, when there are greater temperature contrasts, the jets are more pronounced and move toward the equator; in summer they weaken and move poleward. While jet stream winds are very fast, they are relatively narrow: they can be thousands of miles long, hundreds of miles wide, and just a mile or so deep. Long lines of clouds often indicate the presence of a jet stream; the cloud forms when air moves upward and rotates around the jet stream. A knowledge of the location and strength of jet streams is essential to aviation; pilots can reduce flight time by "hitching a ride" on jet streams.
Doldrums: The windless area at the equator.
Westerlies: Warm, moist winds that blow from the west.
Polar Easterlies: Cold easterly winds that blow from the poles to 60°.
Northeast Trade Winds: Winds that blow toward the equator.
Anabatic: Upslope winds.
Katabatic: Downslope winds.
Frontal Systems: There are three main types of fronts- warm fronts, cold fronts and occluded fronts. The stronger the various fronts are, the more severe the weather tends to be, with a greater temperature change across cold and warm fronts. Fronts rarely occur in the tropics where the temperature differences are minor.
Diurnal Heating/Cooling: The diurnal heating and cooling cycle of Earth's surface has a marked effect on the weather. As the surface heats up, thermals form and the air just above the ground becomes more turbulent. This brings stronger winds from the lower atmosphere down to the surface, with winds likely to become gusty at times. Sea breezes are a feature of coastal locations. Overnight, Earth's surface rapidly loses heat, particularly if the skies are clear. Temperature inversions can form, with winds below the inversion becoming light and variable. Soon after sunrise, the heating of the ground begins the cycle again, thermals generate, inversions dissipate, and upper-level winds drop to the surface. The rising Sun heats hills more rapidly than valleys. This causes thermals to form over the hilltops. Air from the valleys flows up the slope to replace the rising air in the thermals. Overnight, this cool air flows down the hillsides into the valley.
Coastal Winds: Coastal locations commonly experience a reversal of winds during the day and again overnight. Sea breezes form when the land heats up more quickly than the ocean, causing pressures to fall over the land and the air to rise. Cooler air rushes in from the water to replace the rising, warmer air, creating a sea breeze. Sea breezes normally reach peak strength in the late afternoon. Overnight, the land cools more rapidly than the ocean, so its surface air, cooler than the ocean air, drains off the land and onto the sea.
Monsoons (‘mausim’- Arabic for season): Seasonal changes in wind, often rain bearing, that are experienced in many tropical regions. Originally used by seafarers crossing the Arabian Sea to describe the six- month reversal of the winds from the NE to the SW. Although monsoons affect the continents of Asia, Africa and Australia, nowhere are the monsoonal winds and rainfalls so dominant as over S. and SE Asia. In India more than 75% of the annual rains fall during the SW monsoon. Half the world's population relies on monsoon rains for vital water supplies. Indeed, India, Bangladesh and Pakistan are together described as the monsoonal subcontinent.
The monsoons of S. and SE Asia are driven by seasonal changes in the global weather patterns. During the N. hemisphere winter, the Siberian high-pressure system intensifies, to become the world's strongest. This produces strong northeasterly winds that blow toward the equator. These winds remain dry until they pick up moisture over the South China Sea and become the NE monsoon of SE Asia. The SW monsoon of the summer months forms when an intense heat low forms over central Asia. This draws in the SE trade winds that become southwesterly after they cross the equator. These winds can bring heavy rains to south Asia. The convergent zone between the trade winds and the monsoon is called the ITCZ. The rains produced by monsoons are essential for the survival of billions of people. With the arrival of the monsoonal rains, fields that have been barren for months become fertile acres. Although the monsoon winds are an annual occurrence, there are times when the rains are late, or lighter or more sporadic than normal. Crops fail and millions face starvation.
Ocean Currents: Have a huge impact on the weather experienced over nearby lands. The air above cold currents carry little moisture, which helps to create deserts on the west coasts of continents at midlatitudes around the world. The contrast between cold ocean temperatures and the warm land also produces regular sea breezes along these coasts. Warm currents transfer abundant moisture to the winds that blow over them, feeding the weather systems that can bring rain and thunderstorms to eastern coasts.
___________________________________________________________________________
Weather in Action
Water Cycle: Water evaporates from the various liquid reservoirs on Earth's surface- oceans, rivers and lakes-into the atmosphere where it condenses into clouds. Eventually the water leaves these clouds in a process known as precipitation, returning to the surface as rain and snow. More water evaporates from the oceans than is replenished over them by precipitation, but more precipitation falls on the world's landmasses than is lost by evaporation from the land. This endless process maintains a balance: the excess water that falls on the land either runs off in rivers and streams that eventually flow back to the oceans, or it percolates into the ground where it takes a slow, subterranean route toward the oceans. Once there, the cycle of evaporation and precipitation begins again, and the cycle continues. A balanced water cycle is critical to the health of planet Earth.
Humidity: The amount of water vapor- a colorless gas- in the atmosphere. Vapor levels depend upon temperature and vary considerably across the planet, from barely discernible amounts in polar latitudes to nearly 4% of the air by volume in tropical regions. As the air temperature increases, more water molecules evaporate, some of them eventually condensing into liquid. When an equilibrium occurs between the evaporation rate and the condensation rate, the air has reached saturation point and can hold no more vapor.
Saturation Point: Occurs when air contains a high level of water vapor- when a dynamic equilibrium is attained between the rates of evaporation and condensation.
Dewpoint: The air temperature at which saturation occurs.
Cloud Formation: A cloud is an aggregation of water droplets or ice crystals that have become dense enough to be visible. The formation of these droplets or crystals usually requires the presence of tiny airborne nuclei in the atmosphere to serve as sites for the condensation or deposition to occur. Clouds usually form when moist air is cooled to saturation, followed either by condensation to form water droplets or by deposition to produce ice crystals. Temperature normally decreases with altitude. Therefore, clouds forming at high levels of the troposphere tend to be ice-crystal clouds, while those at lower levels are more typically composed of water droplets. The rising motion of air to produce a cloud can be associated with convection or mechanical or dynamic uplift. Convection occurs when warm air becomes more buoyant than its surroundings and moves upward. Mechanical, or orographic, uplift happens when air moves over mountain barriers. Dynamic uplift is associated with large-scale air movement into surface low-pressure systems or along frontal surfaces where the air density is uneven. In brief, the basic processes for cloud formation are: lifting, cooling and condensation.
An air mass will continue to rise as long as its temperature is higher than that of the air around it. If this situation persists as the air mass rises, conditions are said to be unstable. But if an air mass quickly reaches the temperature of the surrounding air, and stops rising, conditions are said to be stable. A rising air mass cools at a rate of 9.8° C per km. Therefore, if we know the temperature of a rising air mass at ground level, and the temperature of the air at different levels of the troposphere, we can calculate how far the air will rise.
Convection: Occurs when warmed, near-surface air becomes buoyant and moves upward to form "puffy" clouds on the updrafts.
Orographic Uplift: Results from wind ascending along a mountain to form clouds on the upwind slopes.
Frontal Activity: Occurs when warm air ascends along a frontal boundary over cooler air.
High Clouds: There are three main types of high cloud, all varieties of cirrus (‘filament’, ‘hair’)- cirrus itself and its two major forms, cirrocumulus and cirrostratus. All three types, usually floating at heights >6100m, are composed of many millions of ice crystals, because temperatures at this altitude are usually well below freezing, and a saturated air mass will produce ice rather than water droplets. Winds are often strong, and help produce the characteristic elongated wispy appearance of cirrus formations. Cirrus clouds sometimes form in isolated patches.
Cirrostratus: A combination of cirrus and stratus; generally recognizable by a transparent thin white sheet or veil of ice crystals forming high-level clouds that appear as layered streamers.
Cirrocumulus: A combination of cirrus and cumulus; high-level ice crystal clouds consisting of a layer of small white puffs or ripples.
Middle Clouds: There are two main types of middle-level cloud: altocumulus and altostratus (‘alto’- high). Despite the ‘alto’ in their name, these clouds are found below cirrus clouds but well above low-level clouds. They float at heights between 2000-6100m, and are normally composed of water droplets, which give them a sharp outline. But they can also be made up of ice crystals, as temperatures at this altitude may fall below the freezing level.
Altostratus: Stratiform clouds that consist primarily of water droplets that appear as a relatively uniform white or gray layered sheet.
Altocumulus: A middle-level cloud type that has some vertical development as indicated by the suffix “cumulus”; generally, have a layered appearance but they also consist of white to gray puffs or waves.
Low Clouds: There are five common low-cloud types. Cumulus clouds have a cauliflower-shaped top and a flat base; they generally form when localized pockets of warm air rise. Stratus clouds have a layered appearance, and occur when relatively large areas of moist air rise gently to a level where condensation occurs. A mix of the two types are stratocumulus clouds-layered clouds with convective elements that have very little vertical development. Then there are thunderstorm clouds- the heavy shower- producing cumulonimbus with a fibrous top, often anvil shaped. Finally, there are the heavy rain-producing nimbostratus clouds. These have a base that is generally of a ragged nature. Low clouds typically have bases below 2000 m and are made up mainly of water droplets, although "tall" clouds with substantial vertical development contain ice and snow and, in cumulonimbus formations, hail. There are minor variations, too: cumulus humilis is broader than it is long, and cumulus mediocris is as tall is it is wide.
Stratus (‘stratum, layer’): Low-level clouds.
Stratocumulus: Low-level layered clouds that have some vertical development as indicated by the suffix "cumulus." Stratocumulus clouds consist of a layer of large rolls or merged puffs.
Vertical Clouds: Many middle- and high-level cloud formations have great horizontal extent but are not developed vertically- that is, they are not very tall or thick. But there are other types of cloud, usually formed by convection, that attain great vertical development; some of these actually extend from low levels to the top of the troposphere, which is effectively the upper limit for most cloud formation. These giant clouds that literally fill the visible sky are called cumulonimbus or thunderstorm clouds, and can attain heights nearly double that of Mount Everest, almost 18,000 m into the upper atmosphere. Fully formed, they are crowned with a huge, wedgelike anvil-shaped mass of cloud. Considerable vertical development can also occur with smaller clouds such as the so-called towering cumulus that frequently develop into cumulonimbus clouds later in their lifecycle.
Cumulus (‘pile, heap’): A tall cloud of great height.
Nimbus (‘rain’): Rain-bearing clouds (commonly used as a suffix).
Cumulonimbus: Vertically developed (cumulus) clouds that are also rain producers (nimbus). These "tall," high clouds usually extend up to the troposphere and have a puffy lower portion and a characteristic smooth or flattened anvil-shaped top. These clouds usually produce heavy rain or hail.
Nimbostratus: Rain-producing (nimbus) layered (stratus) clouds; low- to mid-level clouds that have the appearance of a uniform gray layer.
Fog: Cloud that forms near the ground and, like cloud, forms as a result of condensation. As it condenses, water vapor in contact with the ground adheres to atmospheric particles such as dust specks. While most fog consists of water droplets, ice fog often forms as a collection of ice crystals in polar regions where temperatures may fall below -30°C. Fog droplets form either by the addition of water vapor or the cooling of air; as a result, there are several types of fog-radiation fog, upslope fog, advection fog and steam fog. Fog can be eerie and mysterious, or tranquil and calming. As it reduces visibility, it can also be dangerous for motorists, mariners and pilots. Thick fogs can develop in cities, where there are millions of tiny particles on which water vapor can condense. Fog combined with dust or smoke is known as smog. Fog is often associated with valleys it is usually the result of overnight radiational cooling on already cool air that has drained to the lowest place in the landscape, deep in a valley.
San Francisco Fog: Occurs due to moist onshore winds flowing over the cold waters of the Pacific Ocean that form advection fog during the summer months.
Mist: (Often confused with fog), a suspension of tiny droplets that does not reduce visibility to the same extent.
Von Karman Vortices: The horizontal flow of air in a stable environment, such as over the ocean, carries clouds around an island barrier. The peaks on the island extend upward through the temperature inversion that caps low-level vertical motion and cloud formation, thereby forcing the flow of air around the island barrier rather than over it. This barrier produces eddies in the wind downwind of the island. When viewed from above, these vortices appear in the cloud pattern in the wake of the barrier. This arrangement often develops multiple eddies with opposite rotation produced by an oscillating flow downwind of the island barrier.
Precipitation: Meteorologists group precipitation into liquid (rain and drizzle), solid (snow, ice pellets and hail) and freezing (freezing rain and freezing drizzle). Rain is liquid precipitation that falls primarily from nimbostratus or cumulonimbus clouds with drops at least 0.5 mm in diameter. Drizzle is numerous small liquid drops with diameters between 0.25-0.5 mm that come from stratus clouds. Snow is frozen precipitation that consists of white ice crystals arranged in a variety of branched and hexagonal forms, often forming snowflakes. Precipitation can also be classified as either steady or intermittent. Steady rain or snow usually results from frontal activity, while intermittent precipitation is a combination of convection and atmospheric instability.
Virga: Evaporation of rain before it hits the ground; occurs due to a layer of dry air beneath a cloud. Because virga does not reach the ground, it cannot technically be classified as precipitation. However, the evaporation that produces virga increases the water vapor content in the layer of dry air and thus makes it more likely that subsequent falls will reach the ground.
Rainbow: Occurs when white sunlight, a mixture of multiple colored light, passes through a prism- such as some form of water-and is bent, dispersing into its component colors. Sunlight passing into and out of a spherical raindrop is bent and dispersed twice into its component colors. If the Sun is within ~42° of the horizon and there is nearby falling rain, an observer facing away from the Sun and toward the rain will see a rainbow with a series of colored bands ranging from blue on the inside to red on the outside.
___________________________________________________________________________
Extreme Weather
Thunderstorms: Form when cumulus clouds continue to grow until they extend throughout the troposphere, forming mountains of moisture that can reach up to 15 km in height. The conditions required to produce this cloud growth can be provided by a cold front. The wedge of cold air associated with the advancing cold front drives under the existing air mass, producing an upward motion in the air. As the thunderstorm develops, updrafts and downdrafts form, and the cloud flattens into an anvil as it reaches the top of the troposphere. So-called unstable conditions, in which the temperature of the atmosphere decreases rapidly with height can also result in the development of thunderstorms. A typical thunderstorm will last from 1-2 hrs, before it is slowed by downdrafts that are assisted by the accompanying rain. Occasionally more intense, severe storms last much longer than 2 hrs. They may produce intense bursts of lightning and thunder, heavy rain, hail and strong winds; their impact can be devastating, particularly in urban environments. Thunderstorms are sometimes arranged along a line of low pressure, known as a squall line because the downdrafts cause gusty winds at the surface. A fully formed squall line is constantly regenerated by the cooler downdrafts that lift warmer, moister air in its path. Most thunderstorms have a three-phase lifecycle.
Developing Stage: Occurs as cumulus clouds grow. Strong updrafts prevent rain from falling, and there is no lightning.
Mature Stage: Occurs when ice particles grow in the upper cloud and become sufficiently large to create precipitation. Downdrafts form; the air becomes colder, more turbulent and electrically charged. Lightning occurs and rain, or perhaps hail, falls.
Dissipating Stage: The storm dissipates as the precipitation creates weak downdrafts that deprive the cloud of its energy supply. The cloud evaporates and the storm subsides. This final stage may last for up to an hour.
Lightning: Occurs when areas of opposite electrical charge build up within cumulonimbus clouds, with a positive charge tending to gather along cloud tops and a negative charge nearer the base of the cloud. Because air is a poor conductor of electricity, these charges continue to accumulate until enormous electrical differences are generated. The imbalance in the electrical charges is corrected abruptly by a gigantic discharge- lightning. Lightning heats the air to >30,000°C, producing an explosive expansion of air- thunder.
We hear thunder as a loud crack if it is close or as a low, rumbling sound if it is more distant. Because light travels at 299,792 km/s, we see a lightning flash almost as soon as it occurs. But because thunder takes five 3 seconds/km, it is often many seconds before we hear its sound. To estimate how far away lightning is, count the number of seconds between seeing the lightning and hearing the thunder. Then divide the number of seconds by three to calculate the distance in kilometers (five for miles).
Lightning tends to strike where the positive charge is greatest on the ground below the cloud. This may be a tall object such as a tree or high building, or, more favorably, a lightning conductor such as a metal rod. The popular belief that lightning never strikes twice is untrue. Skyscrapers can be struck several times a year, and the Empire State Building in New York was once famously hit 15x in 15 minutes.
Microburst: Typically begins 5 km above the ground. Precipitation falls inside a towering cloud, dragging nearby air with it. As the air hits the ground, it spreads rapidly from its touchdown point, bringing a burst of very strong winds. Rain evaporating in mid-air forms a dark fringe known as virga. Wet microbursts are formed in the same way as dry, but in this case, precipitation reaches the ground.
Blizzards: During the winter months, blizzards and ice storms occur over large areas of Europe and North America. They can seriously disrupt everyday life by paralyzing transport and creating a dramatic increase in the demand for power, as well as exposing those who have to venture outside to dangerous conditions. A blizzard is a storm with winds >56 km/h and heavy snowfall, along with very cold temperatures. The combination of these elements creates blowing snow, with near zero visibility, deep snow drifts, and potentially lethal wind-chill.
Ice Storm: Occurs when rain falls from a layer of air warmer than 0°C into a layer of air close to the ground with a temperature cooler than 0°C. This can cause pellets of ice— called sleet—to form, and sometimes results in all outdoor surfaces being coated with a layer of ice. Ice storms make any outdoor activity hazardous: sidewalks become slippery, and treacherously icy roads usually result in a spate of highway accidents.
Tornadoes: Commonly associated with a severe type of storm known as a supercell thunderstorm and characterized by extremely powerful updrafts that sometimes extend to the top of the cloud, producing a bulge in the classic anvil shape, called an overshoot. As the wind speed increases rapidly with height, and the wind changes direction, the updraft near the storm's center rotates rapidly—a phenomenon called wind shear. This rotational characteristic is one of the main forces behind the savage, spinning energy of the tornado. The power of a tornado is emphasized by the deafening roar that usually accompanies it; the sound of the wind can be heard several miles away and is at its peak when the tornado is touching down to the ground. Large hailstones and intense lightning activity sometimes accompany tornadoes, producing even more damage.
Supercell Thunderstorm: A severe storm that contains a strong rotating updraft called a mesocyclone. Under the right conditions this system extends downward to become more compact, causing it to rotate faster, finally reaching the ground as a tornado. Two signs are useful in assessing whether a storm will develop into a tornado. The first is the overshoot phenomenon, when the usual flat top of the anvil develops an ominous bulge. This indicates that the rush of air near the center of the storm is so powerful that it has pushed through the troposphere into the stratosphere. The second is the development of mammatus clouds.
Tornado Alley: The area of the US Midwest that includes the lowlands of Mississippi, the Ohio and the lower Missouri river valleys. Depending on the time of year, the borders of Tornado Alley extend from lowa and Nebraska in the N. to central Texas in the South. As the Great Plains heat up during summer, the air expands and rises, sucking more air in to take its place. Tropical moist air from the Gulf of Mexico blows into the Plains and collides with cold, dry air from the Rockies to the west. A unique combination of moisture supply, and a drying and cooling middle atmosphere provides the ideal conditions for tornadoes.
Fujita Scale
F0: 64-117 kph winds; light damage.
F1: 118-180 kph; moderate damage.
F2: 181-251 kph; considerable damage.
F3: 252-330 kph; severe damage.
F4: 331-417 kph; devastating damage.
F5: >417 kph; incredible damage.
Tornado Watch: Issued when meteorological conditions indicate that tornadoes are possible in a given area. Residents should remain alert for approaching storms and be aware of changing weather conditions.
Tornado Warning: Issued when a tornado has been sighted or indicated by weather radar and indicates imminent danger to life and property in the path of the storm.
Hailstones: One of the most destructive products of a severe thunderstorm- they can damage automobiles, roofs and even injure livestock and people. Hail begins high in a thunderstorm as ice crystals, which grow as more ice forms about each "stone." After falling within the cloud, the ice particles are swept aloft again in the updraft, and gather more ice. After several such cycles, large hailstones composed of different layers of ice eventually fall to the ground. The size of a hailstone depends upon the number of layers of ice it contains, which itself is based on how long the hailstone remains within the thunderstorm— hailstones consisting of 25 ice layers have been recorded. Hailstorms are usually short lived.
Dust Devils: Small whirlwinds comprised of swirling columns of rotating wind in the world's hot regions that occur when high temperatures at ground level produce an extremely vigorous uplift of air. The spiraling rotation that scoops up dust and debris from the ground is usually generated when the prevailing wind meets and is deflected by natural obstacles in the landscape such as hills, ridges and sand dunes. Dust devils are most common in arid regions.
Hurricanes: Characterized by extreme rainfall, high waves, and high velocity winds. “Hurricane” is used for systems that develop over the Atlantic or E. Pacific oceans. In the NW Pacific and Philippines, they are called typhoons while in the Indian and south Pacific oceans, they are called cyclones. The most favorable conditions for their development are found between 5-15 degrees of latitude, slightly away from the equator where the Coriolis force is strong enough to help spin up the hurricanes and sea temperatures are >26°C. Once formed, they may last for days or even weeks before they sweep poleward or cross over the land with potentially disastrous results. Around 80 hurricanes form each year, with ~35 forming near SE or S. Asia, 25 near the Americas and the remainder shared across the S. Indian and Pacific oceans. The hurricane season runs from June to November in the N. hemisphere and from November to May in the S. hemisphere.
The practice of naming storms has a long history. In the 1800s, hurricanes in the West Indies were named according to the saint's day on which the storm occurred. For example, Hurricane San Felipe struck Puerto Rico on September 13, 1876. Another storm struck Puerto Rico on the same day in 1928, and this storm was named Hurricane San Felipe the Second. Later, forecasters started using latitude-longitude positions to describe hurricanes, but soon realized it was quicker and easier to use short, distinctive names, which are less subject to error than the older, more cumbersome identification methods. These advantages are especially important in exchanging detailed storm information between hundreds of widely scattered stations, coastal bases and ships at sea. Using women's names became the practice during WWII, following the use of a woman's name for a storm in the 1941 novel Storm by George R. Stewart. Since 1953, tropical storms have been named from lists originated by the US National Hurricane Center and now maintained and updated by an international committee of the World Meteorological Organization. The lists featured only women's names until 1979, when men's and women's names were alternated. Six lists are used in rotation. Thus, the 2004 list will be used again in 2010. The name lists have a French, Spanish, Dutch and English flavor because hurricanes affect many nations and are tracked by the weather services of several countries. The letters Q, U, X, and Y are not included because of the scarcity of names beginning with these letters. The only time that there is a change in the list is if a storm is so deadly or costly that the future use of its name for a different storm would be inappropriate for reasons of sensitivity. If that occurs, the offending name is struck from the list and another name is selected to replace it. Over time, several names have been changed. On the 2002 list, for example, Cristobal has replaced Cesar, Fay has replaced Fran, and Hanna has replaced Hortense. On the 2004 list, Gaston has replaced Georges and Matthew has replaced Mitch. On the 2006 list, Kirk has replaced Keith.
Floods: Occur following heavy rain and snow fall and account for 40% of casualties from natural disasters. Communities most at risk are those nestled in valleys against the sides of steep terrain; settlements that lie in broad river valleys and river deltas are also vulnerable to flooding. In some areas of the world, floods are part of the natural weather cycle. In the Nile valley, for example, regular flooding sustained agriculture for thousands of years. Today, many tropical regions depend on the floods that follow monsoonal rains to nourish crops.
Flash Floods: Occur when intense, short-term rainfall cannot be dispersed by soil absorption, runoff or drainage.
Slab Avalanche: A varied combination of events can culminate in an avalanche above the snowline. Initially a good depth of snow is required to establish a substantial base. Heavy follow-up snowfalls can lead to huge accumulations of snow on the upper reaches of the mountain slopes. If there is heavy rain over the lower slopes, the support for the large snow mass farther upslope is weakened. Sometimes, if the slope is steep enough and the snow deep enough, an avalanche can occur even without the downslope rain. The unsupported mass of snow can spontaneously break free and come cascading down the mountainside. This may be triggered by the smallest of vibrations: a loud noise or the sound of a skier can be sufficient. A strong wind gust or a temperature rise are other common triggers.
Drought: Occurs when there is a sustained and abnormal deficit of rain, based on the expected rainfall of an area at a given time of year.
Wildfire: Occur most often when there is an annual buildup of dry vegetation following winter rainfall and summer drought. When strong winds and high temperatures coincide, a wildfire outbreak is likely. The intense heat generates strong local thermals which send burning embers far up into the sky, igniting new fires well ahead of the main fires. In addition, fire-induced localized wind patterns, in combination with local topography, may generate rotating "fire tornadoes" that spin ahead of the main firefront.
___________________________________________________________________________
Watching the Weather
The first scientific attempts to understand weather date back to Meteorologica by the Greek scholar Aristotle (384-322 BCE). The treatise was an ambitious attempt to describe the physical world, and its title gave rise to the term meteorology. Aristotle's pupil Theophrastus (c 372-287 BC) continued his work with On Weather Signs, which listed 50 signs of storms, 80 of rain and 45 of wind. Like Aristotle, his observations were mixed, with some shrewd deductions and some misguided premises. Roman scholars, too, showed interest in meteorology. Pliny the Elder's (23-79) monumental work Historia Naturalis drew together records, observations and superstitions from Egypt and Babylon, Greece and Rome-some accurate, others perpetuating the myths of earlier times. When the Roman Empire collapsed in the 5c, scientific endeavor was confined to the Islamic world.
The discoveries of the Renaissance ushered in a new age of reason and scientific discovery in the late 17-18c. French scientist Blaise Pascal discovered that air pressure changed with height and could also be related to changes in the weather itself. In England, a young scientist called Isaac Newton, born in 1642, the year of Galileo's death, devised laws of physics and motion that form the basis of today's computer models of the weather. Swedish astronomer Anders Celsius in 1742 invented a scale that divided the temperature difference between the boiling and freezing points of water into an even hundred degrees. Developments flowed rapidly, and by the turn of the 18c permanent weather stations were being established. By the end of the century, learned societies were sharing information gleaned from their embryonic meteorological observations.
In the 1870s, the Signal Service of the US Army began to compile and distribute detailed weather maps and weather statistics from across the nation on a daily basis. The regular production and public display of these maps by pioneering meteorologists paved the way for the development of modern weather forecasting. Weather maps have become increasingly sophisticated and accurate.
Weather RADAR: Emits pulses of radio waves that bounce back from various bodies, or "scatterers" in the air. Although smoke and dust can scatter radar signals back, the strongest echoes come from raindrops, snow and hail. Doppler RADAR measures the change in frequency of the returned echoes, enabling wind speed to be calculated. These devices have a very narrow beam which scans the skies at different angles so that a 3D picture of the weather is produced. Radar can peer deep into approaching thunderstorms.
___________________________________________________________________________
Global Climate
Tropical Climates: Found in the equatorial regions that lie between the tropics of Capricorn and Cancer, and feature high temperatures and substantial precipitation throughout much of the year. With little seasonal variation in the intensity of overhead sunlight, temperatures remain high, with the lowest monthly temperature no lower than 18°C. While there can be rainy and dry seasons, rainfall is typically high, with at least 100 mm of rain every month. The seasonally constant light, warmth and rainfall of tropical zones provide ideal conditions for life, giving rise to lush rain forests and producing the greatest species diversity on Earth. Tropical rain forests occupy only 7% of Earth's landmass but contain 50% of the world's plant and animal species. Tropical rain forests support a broad diversity of fauna in four habitats- 1) the emergent layer with the tallest trees, 2) the canopy forming a dense cover of tree crowns, 3) the dim understory and 4) the moist forest floor.
Subtropical Climates: Found across tropical and subtropical latitudes, the subtropical climate features a distinct wet season/dry season cycle with relatively high temperatures throughout the year. The difference between the warmest and coolest months may amount to only three or four degrees, but the wet season is characterized by high humidity’s. The seasons of the subtropics are produced by the shifting of high-pressure cells, which move poleward in summer and back toward the equator in winter; variations in the ITCZ, a belt of rainshowers and thunderstorms circling the equator; and monsoonal winds, which blow clouds and rain out to sea in winter. In some subtropical regions, the transition from tropical areas is marked by monsoon forests and cloud forests.
Arid Climates: Typically create deserts, where the annual precipitation is <250 mm, and high temperatures ensure that evaporation exceeds this precipitation. Deserts are subject to huge daily temperature fluctuations. The lack of cloud cover allows temperatures to soar during the daytime but fall rapidly after the Sun sets. Many arid zones lie under constantly sinking air such as near subtropical high-pressure cells or downwind of mountains, resulting in cloud-free sky and dry conditions. Some deserts occur in continental interiors, where little moisture arrives from the oceans; coastal deserts are located beside cold ocean currents that suppress precipitation. Despite minimal rainfall, high temperatures and drying winds, many plants and animals have adapted to these harsh environments.
Semiarid Climates: Feature large expanses of grasslands and savannas because the annual precipitation ranges from 250-760 mm- enough water to support some vegetation but too little to sustain full forests. Semiarid regions extend from the tropics into the middle latitudes, wherever passing weather systems supply some moisture. Periods of severe drought also regularly occur. With few trees, these flat, exposed regions are very windy. The grasslands are maintained by sweeping seasonal fires and grazing by large herd animals, which clear the buildup of thatch and remove competing woody plants. Most of the grasslands' living mass lies underground. The thick roots of grasses store nutrients and collect moisture from the soil. After fire or grazing, the root system colonizes new areas.
Mediterranean-type Climate: Characterized by warm, dry summers and mild, wet winters. It is created by seasonal variations in the position of subtropical high-pressure cells over western sections of the major continents. During summer, these cells drift poleward and their eastern flanks keep the regions dry and warm with sunny skies. In winter, however, the highs drift back toward the equator, permitting rain-bearing midlatitude storms to traverse the regions. Scrublands dominate in Mediterranean climates. This vegetation can survive the drought like conditions and wildfires that are a feature of the warm, dry summer months.
Temperate Climates: Found in midlatitudes where nearly half the months have temperatures >10°C. These regions all experience four distinct seasons, but the severity of the winter varies according to their proximity to the sea. Along the western edges of the continents, the prevailing ocean winds tend to create a temperate oceanic climate where the lowest monthly temperature rarely falls below 0°C), with essentially no winter snow cover. Elsewhere, temperate continental climates may have one or two months of persistent snow. Annual precipitation is generally adequate, though freezing winter temperatures can lock up moisture as snow and ice. The dominant vegetation is deciduous forest. Animals need to survive cold winters and seasonal variations in their food supply. Deciduous forests are the dominant vegetation in temperate regions. Before winter's onset, leaves wither and fall as protection from the cold and often droughtlike conditions.
Northern Temperate: Since continental landmasses with extreme seasonal temperature contrasts are found primarily in the N. hemisphere, such regions are known as the northern temperate (or boreal) zone. With their large tracts of coniferous forest, they mark a transition between the Arctic tundra to the north and the temperate forest to the south. Typically, the northern temperate zone is marked by a relatively low annual average temperature, with strong seasonal contrasts provided by long, cold winters and short, cool summers. At least one month has an average temperature exceeding 10°C. Annual precipitation is small, with snow in winter and relatively heavy summer rainfall. The soils become waterlogged in the spring thaw and often remain soggy because of an underlying layer of permanently frozen soil. Northern temperate forests are dominated by conifers such as spruce which have a conical shape that allows snow to slide off, while their needlelike leaves withstand cold, dry winters.
Polar Climates: Experience extended intervals of darkness and light throughout the year. With long winter darkness and relatively weak summer sunshine, the average temperature of the warmest month typically is <10°C. Precipitation, mostly snow, is also relatively light, with annual totals usually <250 mm. This harsh climate results in a landscape covered by low-growing tundra vegetation or barren ice cap. The equatorward boundary of the polar zone is defined by the timberline, where the last hardy trees grow before the treeless plains of the tundra begin. Arctic plants include hardy grasses and sedges; animals include large warm-blooded wolves and bears. The Antarctic supports lichens and mosses, whales, seals and birds.
Tundra: A cold arctic desert of the northern polar zone where the only plants are perennials, such as grasses and sedges, that survive a very short growing season with little precipitation.
Mountain Climates: Intercept and alter moving air masses, creating their own weather patterns. While no single set of characteristics apply to all mountain climates, they differ significantly from nearby valley climates, experiencing lower temperatures, higher winds, greater precipitation, reduced O and greater exposure to UV sunlight. The altitude marking the start of the mountain zone depends on latitude. In the Himalayas, the zone begins at 2700m, while in the Alps, the zone lies above 900 m and, in the Sierra Nevada, above 1200 m. Conifers such as pines and firs are superbly adapted to these conditions.
Coastal Climates: Because the temperature of the near-surface water changes quite slowly throughout the year, temperature variations along the coast are delayed, producing a relatively stable climate with few fluctuations in temperature. Sea breezes have a moderating effect, lowering temperatures on summer days. In spite of their stable temperatures, coastal regions can be harsh environments. Plants and animals have to survive strong onshore winds, waves, salt spray and windblown sand. Sandy shores and dunes can become desert-like as water percolates quickly through the sand. Rocky shores are exposed to pounding surf and constant soaking and drying.
Venus Climate: Strong sunlight filters through the clouds and heats the surface, but the clouds and the CO2 in the atmosphere prevent the heat from escaping. As a result, it is as hot at Venus' poles as at the equator, and the night side is no cooler than the day side. After filtering through the clouds, the light is colored orange and appears to be about as bright as an overcast day on Earth.
___________________________________________________________________________
Changing Climate
The UN’s IPCC has issued projections that indicate human activities are likely to produce noticeable warming in coming decades. Earth's climate has been erratic in the past and, to some extent, current warming could be part of natural fluctuations in ocean circulation, atmospheric patterns or solar activity. There seems to be little doubt, however, that humans have influenced recent and current warming. Action has been, and is being, taken. In 1992 the Rio Summit determined to stabilize greenhouse gas emissions at 1990 levels; in 1997 the Kyoto protocol set a target of reducing emissions by an average of 5.2% below 1990 levels by 2012. Translating these objectives into action poses challenges for both the developed and developing world. Energy must be conserved; low-pollution industrial plants, automobiles and public transport systems must be developed. Renewable energy sources such as the Sun, wind and water must become more efficient and cost-effective. Changes like these are technically possible; commitment by the international community, national governments and individuals can bring them to fruition.
Scientific evidence points to a gradual increase in solar luminosity of around 20-30% since Earth formed about 4600 Ma. This luminosity increase occurs as H in the Sun's core is converted to He. Lesser variations are closely related to the periodic change in sunspot activity- the well-known 11-yr sunspot cycle. Solar output will typically change by around 2% from a time of minimum sunspot activity to the corresponding sunspot maximum. There are also cycles within cycles; the very high to very low activity cycle takes about 80 yrs. Magnetic field fluctuations triggered by sunspot activity have a cycle of approximately 22 yrs, while lunar tide patterns recur every 19 yrs.
Milankovitch Theory: Links fluctuations of the ice ages with three variations in Earth's position relative to the Sun. These variations cause radiation changes of up to 15% at high latitudes- changes that influence the expansion and melting of polar ice sheets. The tilt of Earth's axis fluctuates back and forth between 22-24.5 degrees every 41,000 yrs. Earth's axis also wobbles like a gyroscope and traces a complete circle in about 23,000 yrs. Moreover, Earth's orbit also pulsates, becoming more and less elliptical every 100,000 and 433,000 yrs.
Stromatolites: Cushion-like masses composed of layers of blue-green algae; among the few organisms that survive from the pre-dinosaur age. The oldest stromatolite fossils date back approximately 3500 million years. They lived in the shallows of saline, sunlit waters.
Volcanic Eruptions & Global Cooling: Massive volcanic eruptions can send vast amounts of ash and SO2 into the upper atmosphere. The SO2 reacts with stratospheric water vapor to produce a dense haze that can stay in the stratosphere for years. This haze absorbs some incoming solar radiation and reflects more back out to space, thus raising the temperature of the stratosphere and cooling the lower levels of the troposphere. If the eruptions are large enough, such as the massive Mt Pinatubo eruption in 1991, the effects can last for years.
It is now thought that SO2 emissions, rather than ash, have the most dramatic effect on temperature. The eruption at El Chichón, Mexico, which occurred two years after that of Mt St Helens in the US, released a similar amount of ash, but the SO2 released was far greater. These emissions were not measured until the 1970s, but it is believed that the emissions from Tambora, south of Borneo, in 1815, must have been immense. A drop in temperature of 2-3°C after the eruption of Tambora resulted in drastic food shortages, with accompanying riots in France, famine in Switzerland and crop failures in America. The following year, 1816, was known as "the year without a summer."
Ice Ages: Periods of glaciation that have occurred on Earth ~ every 200 My, and have lasted for millions, or even tens of millions of years. During an ice age, the polar regions are cold, extensive glaciers cover much of the planet, and temperature variations between the poles and the equator are strongly differentiated.
Southern Oscillation Index (SOI): The mean sea-level pressure difference between Tahiti and Darwin. When the SOl is strongly negative for several months, an El Niño is said to occur; when it is strongly positive, a La Niña occurs.
The best way to identify whether an El Niño or a La Niña is developing is to monitor the sea-surface temperature patterns across the tropical Pacific Ocean. During strong El Niño’s, sea-surface temperatures become unusually warm over the equatorial E. Pacific Ocean, particularly around coastal Peru. This movement of warm water along the coastline is dreaded by local fishermen because the water lacks nutrients and results in decimation of fish across the area, including one of the main harvests- the anchovies. Sea birds that feed off the anchovies also die during these periods. The opposite effect occurs during a La Niña, with colder than normal waters making a return to the tropical E. Pacific and warmer waters being carried on the current N. of Australia. A significant La Niña event occurred in 1989. La Niña produces the opposite climate variations from El Niño. For example, the parts of Australia and Indonesia that are prone to drought during El Niño are typically wetter than normal during La Niña.
Thermohaline Circulation (‘Great Ocean Conveyor Belt’): Cold, salty water, sinks into the deep ocean in the N. Atlantic, flows south and then east around S. Asia to resurface and be warmed in the Indian and N. Pacific oceans. Surface currents carry warmer water back through the Pacific and south Atlantic. The round trip takes between 500-2000 yrs. Studies suggest that the strength of this transport can easily change speed or direction. Recent changes in ocean-water temperature may have contributed to climatic fluctuations, such as the sustained drought in the Sahel since the late 1960s, reduced hurricane activity in the Atlantic and a rise in ENSO events in the tropical Pacific.
___________________________________________________________________________
Terminology
Deciduous: Trees that shed their leaves during the long dry season, and produce new leaves and flowers when the annual wet season brings rain.
Lichens: Small, slow-growing composites of fungi and algae that can survive in harsh environments. Attaching themselves to the surface with tiny hairlike growths, the lichens obtain all their nutrients from the surrounding air and bare rock.
___________________________________________________________________________
Chronology
1992: Hurricane Andrew strikes Florida, resulting in ~$30B in damage (Weather by Buckley).
Jun, 1991: The volcanic eruption of Mt. Pinatubo, Philippines, ejects 10.5 km3 and 20M tonnes of SO2. Once in the stratosphere, the SO2 spread around the world as sulfuric acid, cooling Earth’s temperature by ~.5C. High levels of aerosols remained in the atmosphere for at least two years, possibly masking the effects of global warming (Weather by Buckley).
12 Oct, 1979: The lowest air pressure in Earth’s history, 870 hP, is recorded during Typhoon Tip, West of Guam in the Pacific Ocean (Weather by Buckley).
25 Dec, 1974: Cyclone Tracy strikes Darwin, Australia, killing 49 people (Weather by Buckley).
31 Dec, 1968: The highest air pressure in Earth’s history, 1083.5 hP, is recorded in Agata, Siberia (Weather by Buckley).
1966: The first high-altitude (geo-stationary) satellites are launched, which orbit the Earth above the equator at 35,800 km (Weather by Buckley).
1963: The World Weather Watch (WWW) is launched by 150 countries to exchange data on a regular basis and facilitate the preparation of global weather charts (Weather by Buckley).
Apr, 1960: The USG launches the Television IR Observation Satellite (TIROS), the first weather satellite, which provides weather forecasters with a broad picture of cloud formations. By 1963 photographs could be obtained directly from satellites as they passed overhead. Three years later the first geostationary satellite, hovering over the equator, is launched (Weather by Buckley).
15-16 Mar, 1952: Chilaos, La Reunion on the Indian Ocean experiences 1870mm of rain, the greatest 24-hr period of rainfall in Earth’s recorded history (Weather by Buckley).
1951: The World Meteorological Organization (WMO) is established as a specialized agency of the UN. The WMO acts as the world's "umbrella" meteorological organization, providing "the authoritative scientific voice on the state and behavior of the Earth's atmosphere and climate” (Weather by Buckley).
23 Mar, 1925: An F5 tornado strikes the US Tri-State area (Missouri, Illinois, Indiana), killing 695 and injuring >2000 (Weather by Buckley).
13 Sep, 1922: The hottest temperature recorded in Earth’s history, 57.8C, occurs at Al’ Aziziyah, Libya (Weather by Buckley).
1922: British mathematician Lewis Fry Richardson first applies mathematical techniques to produce a crude 24-hr forecast, a forerunner of numerical prediction techniques (Weather by Buckley).
1909: Earl Douglas of the Carnegie Museum, Pittsburgh, USA, notices a dinosaur skeleton eroding out of an exposed sandstone ledge in UT. Several years of excavation work brought to light a seemingly endless array of dinosaur fossils. Today the site is part of the Dinosaur National Monument. Ten genera have been found at the site. They inhabited an extensive, lowland, alluvial plain and were probably swept away and drowned by large-scale floods. The carcasses would have been dumped at the river bends, where the currents slowed (Weather by Buckley).
1896: An F4 tornado strikes St. Louis, Missouri, killing 255 (Weather by Buckley).
11-14 Mar, 1888: The Great White Hurricane; a massive blizzard engulfs much of the east coast of the USA, killing 400 people (Weather by Buckley).
Oct, 1871: A drought formed wildfire in Wisconsin destroys a 96.6 km stretch of land and kills >1200 people (Weather by Buckley).
1856: William Ferrel first identifies Ferrel cells- are from Hadley cells that continue towards the poles before rising at ~60° N & S (Weather by Buckley).
1840: A tornado strikes Natchez, Mississippi, killing 317 (Weather by Buckley).
1803: Luke Howard, a London pharmacist, writes “On the Modifications of Clouds,” which first systematically attempts to classify the clouds (Weather by Buckley).
1753: English scientist George Hadley first describes Hadley cells- warm air that rises from the equator and spreads towards the poles before sinking at ~30° N & S (Weather by Buckley).
1752: US statesman and philosopher, Benjamin Franklin (1706-1790), attaches a wire to a kite and flies it during an electrical storm. In an experiment that he was lucky to survive, Franklin proved that lightning was actually an electrical discharge. He is also credited with inventing the lightning rod, which protects buildings from direct hits by lightning (Weather by Buckley).
1743: US statesman and philosopher Benjamin Franklin (1706-1790) provides the first analysis of a storm system, based on contemporary newspaper reports (Weather by Buckley).
1742: Swedish astronomer Anders Celsius invents the Celsius scale, dividing the temperatures between the boiling and freezing points of water into an even hundred degrees (Weather by Buckley).
985-1410: The Norse colony in Greenland in established during a warm period with ~300-400 colonists from Iceland. At first, the settlement prospered and by the early 12c, it supported ~5000 people. Supplies were brought in from Iceland. But then the weather cooled, storms intensified and the pack ice expanded. Visits from Iceland became less frequent. The last contact was in 1410 and the settlement died out later that century (Weather by Buckley).
300 BCE: Chinese inventors develops a calendar that divides the year into 24 “festivals” and describing the weather associated with each one (Weather by Buckley).
18 Ka: Dating of the Cosquer cave petroglyphs (now submerged under the Mediterranean) (Weather by Buckley).
25-14 Ka: Eastern Siberia is connected to Alaska by a land bridge, allowing humans to cross by land (Weather by Buckley).
50 Ka: Australia’s aboriginal people migrate from SE Asia (Weather by Buckley).
3 Ma: The Earth experiences an Ice Age (Weather by Buckley).
15 Ma: The Earth experiences an Ice Age (Weather by Buckley).
36 Ma: The Earth experiences an Ice Age (Weather by Buckley).
330-245 Ma: The Big Freeze; a drop in temperatures causes the Permo-Carboniferous glaciation period (Weather by Buckley).
3.5 Ga: Stromatolites, cushion-like masses composed of layers of blue-green algae, first appear (Weather by Buckley).
4.6 Ga: Formation of Earth (Weather by Buckley).
___________________________________________________________________________