Weather, state of the atmosphere at a particular time and place. The elements of weather include temperature, humidity, cloudiness, precipitation, wind, and pressure. These elements are organized into various weather systems, such as monsoons, areas of high and low pressure, thunderstorms, and tornadoes. All weather systems have well-defined cycles and structural features and are governed by the laws of heat and motion. These conditions are studied in meteorology, the science of weather and weather forecasting.

Weather differs from climate, which is the weather that a particular region experiences over a long period of time. Climate includes the averages and variations of all weather elements.


Temperature is a measure of the degree of hotness of the air. Three different scales are used for measuring temperature. Scientists use the Kelvin, or absolute, scale and the Celsius, or centigrade, scale. Most nations use the Celsius scale, although the United States continues to use the Fahrenheit scale.

Temperature on Earth averages 15°C (59°F) at sea level but varies according to latitude, elevation, season, and time of day, ranging from a record high of 58°C (140°F) to a record low of -88°C (-130°F). Temperature is generally highest in the Tropics and lowest near the poles. Each day it is usually warmest during midafternoon and coldest around dawn. Seasonal variations of temperature are generally more pronounced at higher latitudes. Along the equator, all months are equally warm, but away from the equator, it is generally warmest about a month after the summer solstice (usually June 21 or 22 in the Northern Hemisphere and December 21 or 22 in the Southern Hemisphere) and coldest about a month after the winter solstice (usually December 21 or 22 in the Northern Hemisphere and June 21 or 22 in the Southern Hemisphere). Temperature can change abruptly when fronts (boundaries between two air masses with different temperatures or densities) or thunderstorms pass overhead.

Temperature decreases with increasing elevation at an average rate of about 6.5°C per km (about 19°F per mi). As a result, temperatures in the mountains are generally much lower than at sea level. Temperature continues to decrease throughout the atmosphere’s lowest layer, the troposphere, where almost all weather occurs. The troposphere extends to a height of 16 km (10 mi) above sea level over the equator and about 8 km (about 5 mi) above sea level over the poles. Above the troposphere is the stratosphere, where temperature levels off and then begins to increase with height. Almost no weather occurs in the stratosphere.


Humidity is a measure of the amount of water vapor in the air. The air’s capacity to hold vapor is limited but increases dramatically as the air warms, roughly doubling for each temperature increase of 10°C (18°F). There are several different measures of humidity. The specific humidity is the fraction of the mass of air that consists of water vapor, usually given as parts per thousand. Even the warmest, most humid air seldom has a specific humidity greater than 20 parts per thousand. The most common measure of humidity is the relative humidity, or the amount of vapor in the air divided by the air’s vapor-holding capacity at that temperature. If the amount of water vapor in the air remains the same, the relative humidity decreases as the air is heated and increases as the air is cooled. As a result, relative humidity is usually highest around dawn, when the temperature is lowest, and lowest in midafternoon, when the temperature is highest.


Most clouds and almost all precipitation are produced by the cooling of air as it rises. When air temperature is reduced, excess water vapor in the air condenses into liquid droplets or ice crystals to form clouds or fog. A cloud can take any of several different forms—including cumulus, cirrus, and stratus—reflecting the pattern of air motions that formed it. Fluffy cumulus clouds form from rising masses of air, called thermals. A cumulus cloud often has a flat base, corresponding to the level at which the water vapor first condenses. If a cumulus cloud grows large, it transforms into a cumulonimbus cloud or a thunderstorm. Fibrous cirrus clouds consist of trails of falling ice crystals twisted by the winds. Cirrus clouds usually form high in the troposphere, and their crystals almost never reach the ground. Stratus clouds form when an entire layer of air cools or ascends obliquely. A stratus cloud often extends for hundreds of miles.

Fog is a cloud that touches the ground. In dense fogs, the visibility may drop below 50 m (55 yd). Fog occurs most frequently when Earth’s surface is much colder than the air directly above it, such as around dawn and over cold ocean currents. Fog is thickened and acidified when the air is filled with sulfur-laden soot particles produced by the burning of coal. Dense acid fogs that killed thousands of people in London up to 1956 led to legislation that prohibited coal burning in cities.

Optical phenomena, such as rainbows and halos, occur when light shines through cloud particles. Rainbows are seen when sunlight from behind the observer strikes the raindrops falling from cumulonimbus clouds. The raindrops act as tiny prisms, bending and reflecting the different colors of light back to the observer’s eye at different angles and creating bands of color. Halos are seen when sunlight or moonlight in front of the observer strikes ice crystals and then passes through high, thin cirrostratus clouds.



Blizzard Aftermath
A snow blizzard that struck the East Coast of the United States in January 1996, left the Boston area with about 46 cm (about 18 in) of snow. A snowstorm is called a blizzard when visibility is less than 0.40 km (0.25 mi) and the snowfall is accompanied by strong winds.
Lisa Quinones/Black Star

Precipitation is produced when the droplets and crystals in clouds grow large enough to fall to the ground. Clouds do not usually produce precipitation until they are more than 1 km (0.6 mi) thick. Precipitation takes a variety of forms, including rain, drizzle, freezing rain, snow, hail, and ice pellets, or sleet. Raindrops have diameters larger than 0.5 mm (0.02 in), whereas drizzle drops are smaller. Few raindrops are larger than about 6 mm (about 0.2 in), because such large drops are unstable and break up easily. Ice pellets are raindrops that have frozen in midair. Freezing rain is rain that freezes on contact with any surface. It often produces a layer of ice that can be very slippery.

Snowflakes are either single ice crystals or clusters of ice crystals. Large snowflakes generally form when the temperature is near 0°C (32°F), because at this temperature the flakes are partly melted and stick together when they collide. Hailstones are balls of ice about 6 to 150 mm (about 0.2 to 6 in) in diameter. They consist of clusters of raindrops that have collided and frozen together. Large hailstones only occur in violent thunderstorms, in which strong updrafts keep the hailstones suspended in the atmosphere long enough to grow large.

Precipitation amounts are usually given in terms of depth. A well-developed winter storm can produce 10 to 30 mm (0.4 to 1.2 in) of rain over a large area in 12 to 24 hours. An intense thunderstorm may produce more than 20 mm (0.8 in) of rain in 10 minutes and cause flash floods (floods in which the water rises suddenly). Hurricanes sometimes produce over 250 mm (10 in) of rain and lead to extensive flooding.

Snow depths are usually much greater than rain depths because of snow’s low density. During intense winter storms, more than 250 mm (10 in) of snow may fall in 24 hours, and the snow can be much deeper in places where the wind piles it up in drifts. Extraordinarily deep snows sometimes accumulate on the upwind side of mountain slopes during severe winter storms or on the downwind shores of large lakes during outbreaks of polar air.



A tornado that ripped through Wichita Falls, Texas, was caught on film by several observers. Stirred up dust and water droplets turned the funnel-shaped cloud a dark color. The funnel winds of a tornado can reach speeds of more than 480 km/h (300 mph).
Archive Films

Wind is the horizontal movement of air. It is named for the direction from which it comes—for example, a north wind comes from the north. In most places near the ground, the wind speed averages from 8 to 24 km/h (from 5 to 15 mph), but it can be much higher during intense storms. Wind speeds in hurricanes and typhoons exceed 120 km/h (75 mph) near the storm’s center and may approach 320 km/h (200 mph). The highest wind speeds at the surface of the Earth—as high as 480 km/h (300 mph)—occur in tornadoes. Except for these storms, wind speed usually increases with height to the top of the troposphere.


Pressure plays a vital role in all weather systems. Pressure is the force of the air on a given surface divided by the area of that surface. In most weather systems the air pressure is equal to the weight of the air column divided by the area of the column. Pressure decreases rapidly with height, halving about every 5.5 km (3.4 mi).

Sea-level pressure varies by only a few percent. Large regions in the atmosphere that have higher pressure than the surroundings are called high-pressure areas. Regions with lower pressure than the surroundings are called low-pressure areas. Most storms occur in low-pressure areas. Rapidly falling pressure usually means a storm is approaching, whereas rapidly rising pressure usually indicates that skies will clear.


Weather systems occur on a wide range of scales. Monsoons occur on a global scale and are among the largest weather systems, extending for thousands of miles. Thunderstorms are much smaller, typically 10 to 20 km (6 to 12 mi) across. Tornadoes, which extend from the bases of thunderstorms, range from less than 50 m (55 yd) across to as much as 2 km (1.2 mi) across.

The vertical scale of weather systems is much more limited. Because pressure decreases so rapidly with height and because temperature stops decreasing in the stratosphere, weather systems are confined to the troposphere. Only the tallest thunderstorms reach the stratosphere, which is otherwise almost always clear.


All weather is due to heating from the Sun. The Sun emits energy at an almost constant rate, but a region receives more heat when the Sun is higher in the sky and when there are more hours of sunlight in a day. The high Sun of the Tropics makes this area much warmer than the poles, and in summer the high Sun and long days make the region much warmer than in winter. In the Northern Hemisphere, the Sun climbs high in the sky and the days are long in summer, around July, when the northern end of Earth’s axis is tilted toward the Sun. At the same time, it is winter in the Southern Hemisphere. The southern end of Earth’s axis is tilted away from the Sun, so the Sun is low in the sky and the days are short.

The temperature differences produced by inequalities in heating cause differences in air density and pressure that propel the winds. Vertical air motions are propelled by buoyancy: A region of air that is warmer and less dense than the surroundings is buoyant and rises. Air is also forced from regions of higher pressure to regions of lower pressure. Once the air begins moving, it is deflected by the Coriolis force, which results from Earth’s rotation. The Coriolis force deflects the wind and all moving objects toward their right in the Northern Hemisphere and toward their left in the Southern Hemisphere. It is so gentle that it has little effect on small-scale winds that last less than a few hours, but it has a profound effect on winds that blow for many hours and move over large distances.



Hurricane Destruction
Hurricanes cause most of their damage with strong winds, but they can also spawn tornadoes and cause sudden storm surges, or rises in water level. Hurricanes are defined as tropical storms that have winds of 119 km/h (74 mph) or more and are given ratings from 1 (minimal) to 5 (catastrophic) based on wind speed, height of storm surge, and barometric pressure.
Herman Kokojan/Black Star

In both hemispheres, the speed of the west wind increases with height up to the top of the troposphere. The core of most rapid winds at the top of the troposphere forms a wavy river of air called the jet stream. Near the ground, where the winds are slowed by friction, the air blows at an acute angle toward areas of low pressure, forming great gyres called cyclones and anticyclones. In the Northern Hemisphere, the Coriolis force causes air in low-pressure areas to spiral counterclockwise and inward, forming a cyclone, whereas air in high-pressure areas spirals clockwise and outward, forming an anticyclone. In the Southern Hemisphere, cyclones turn clockwise and anticyclones, counterclockwise.

The air spreading from anticyclones is replaced by sinking air from above. As a result, skies in anticyclones are often fair, and large regions of air called air masses form; these have reasonably uniform temperature and humidity. In cyclones, on the other hand, as air converges to the center, it rises to form extensive clouds and precipitation.

During summer and fall, tropical cyclones, called hurricanes or typhoons, form over warm waters of the oceans in bands parallel to the equator, between about latitude 5°and latitude 30°north and south. Wind speed in hurricanes increases as the air spirals inward. The air either rises in a series of rain bands before reaching the center or proceeds inward and then turns sharply upward in a doughnut-shaped region called the eye wall, where the most intense winds and rain occur. The eye wall surrounds the core, or eye, of the hurricane, which is marked by partly clear skies and gentle winds.

In the middle and high latitudes, polar and tropical air masses are brought together in low-pressure areas called extratropical cyclones, forming narrow zones of sharply changing temperature called fronts. Intense extratropical cyclones can produce blizzard conditions in their northern reaches while at the same time producing warm weather with possible severe thunderstorms and tornadoes in their southern reaches.

Thunderstorms are small, intense convective storms that are produced by buoyant, rapidly rising air. As thunderstorms mature, strong downdrafts of rain- or hail-filled cool air plunge toward the ground, bringing intense showers. However, because thunderstorms are only about 16 km (about 10 mi) wide, they pass over quickly, usually lasting less than an hour. Severe thunderstorms sometimes produce large hail. They may also rotate slowly and spout rapidly rotating tornadoes from their bases.


Thunder and Lightning
Thunderstorms are violent rain storms that produce thunder, lightning, and frequently hail. They form when violent updrafts push hot, humid air high into the cold, upper atmosphere. The subsequent rapid cooling of the air produces an intense, but short-lived, storm.
BBC Worldwide Americas, Inc./Hot Shots Cool Cuts Inc.

Most convective weather systems are gentler than thunderstorms. Often, organized circulation cells develop, in which cooler and denser air from the surroundings sinks and blows along the ground to replace the rising heated air. Circulation cells occur on many different scales. On a local scale, along the seashore during sunny spring and summer days, air over the land grows hot while air over the sea remains cool. As the heated air rises, the cooler and denser air from the sea rushes in. This movement of air is popularly called a sea breeze. At night, when the air over the land grows cooler than the air over the sea, the wind reverses and is known as a land breeze.

On a global scale, hot, humid air near the equator rises and is replaced by denser air that sinks in the subtropics and blows back to the equator along the ground. The winds that blow toward the equator are called the trade winds. The trade winds are among the most steady, reliable winds on Earth. They approach the equator obliquely from the northeast and southeast because of the Coriolis force.

The tropical circulation cell is called the Hadley cell. It shifts north and south with the seasons and causes tropical monsoons in India. For example, around July the warm, rising air of the Hadley cell is located over India, and humid winds blow in from the Indian Ocean. Around January the cooler, sinking air of the Hadley cell is located over India, and the winds blow in the opposite direction.

A variable circulation cell called the Walker Circulation exists over the tropical Pacific Ocean. Normally, air rises over the warm waters of the western Pacific Ocean over the Malay Archipelago and sinks over the cold waters in the eastern Pacific Ocean off the coast of Ecuador and Peru. Most years around late December this circulation weakens, and the cold waters off the coast of South America warm up slightly. Because it occurs around Christmas, the phenomenon is called El Niño (The Child). Once every two to five years, the waters of the eastern Pacific Ocean warm profoundly. The Walker Circulation then weakens drastically or even reverses, so that air rises and brings torrential rains to normally dry sections of Ecuador and Peru and hurricanes to Tahiti. On the other side of the Pacific Ocean, air sinks and brings drought to Australia. El Niño can now be predicted with reasonable accuracy several months in advance.



Hurricane Katrina Observed from Space
A satellite image shows Hurricane Katrina as it crossed Florida, heading into the Gulf of Mexico. The storm made landfall on August 29, 2005, in Plaquemines Parish southeast of New Orleans. The storm surge breached the city’s levees, and Hurricane Katrina became the costliest natural disaster in U.S. history.

Since the early 20th century, great strides have been made in weather prediction, largely as a result of computer development but also because of instrumentation such as satellites and radar. Weather data from around the world are collected by the World Meteorological Organization, the National Weather Service, and other agencies and entered into computer models that apply the laws of motion and of the conservation of energy and mass to produce forecasts. In some cases, these forecasts have provided warning of major storms as much as a week in advance. However, because the behavior of weather systems is chaotic, it is impossible to forecast the details of weather more than about two weeks in advance.

Intense small-scale storms, such as thunderstorms and tornadoes, are much more difficult to forecast than are larger weather systems. In areas in which thunderstorms are common, general forecasts can be made several days in advance, but the exact time and location of the storms, as well as of flash floods and tornadoes, can only be forecast about an hour in advance. (For a discussion of weather forecasting methods and technologies, see Meteorology.)


Human beings can change weather and climate. Water-droplet clouds with tops colder than about -5°C (about 23°F) can be made to produce rain by seeding them with substances such as silver iodide. Cloud seeding causes ice crystals to form and grow large enough to fall out of a cloud. However, although cloud seeding has been proven effective in individual clouds, its effect over large areas is still unproven.

Weather near the ground is routinely modified for agricultural purposes. For example, soil is darkened to raise its temperature, and fans are turned on during clear, cold nights to stir warmer air down to the ground and help prevent frost damage.

Human activities have also produced inadvertent effects on weather and climate. Adding gases such as carbon dioxide and methane to the atmosphere has increased the greenhouse effect and contributed to global warming by raising the mean temperature of the Earth by about 0.5°C (about 0.9°F) since the beginning of the 20th century. More recently, chlorofluorocarbons (CFCs), which are used as refrigerants and in aerosol propellants, have been released into the atmosphere, reducing the amount of ozone worldwide and causing a thinning of the ozone layer over Antarctica each spring (around October). The potential consequences of these changes are vast. Global warming may cause sea level to rise, and the incidence of skin cancer may increase as a result of the reduction of ozone. In an effort to prevent such consequences, production of CFCs has been curtailed and many measures have been suggested to control emission of greenhouse gases, including the development of more efficient engines and the use of alternative energy sources such as solar energy and wind energy.


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