Meteorology

A

Radiosonde

A radiosonde measures air temperature, air pressure, and humidity from Earth’s surface up to an altitude of about 30,000 m (about 100,000 ft). The radiosonde consists of a small box attached to a gas-filled balloon. As the balloon rises, a barometer measures air pressure, a thermometer measures temperature, and a hygrometer measures humidity. All of this information is transmitted by radio back to the ground. Special tracking equipment monitors the movement of the radiosonde, and this tracking information is then converted into wind speed and wind direction. When the balloon bursts, the radiosonde descends to earth by parachute.

B

Doppler Radar

Radar provides meteorologists with information about precipitation and storms. A radar unit sends out a pulse of microwaves. When the microwaves strike objects, such as falling precipitation, some of the microwaves are reflected back to the radar unit, where they are detected by an antenna and displayed on a screen. The elapsed time between transmission and return indicates how far away the precipitation is. Doppler radar can determine wind speed by measuring the speed at which precipitation is moving horizontally toward or away from the radar antenna. It does this by measuring the change in frequency of the returning microwaves—the frequency of the returning waves decreases if the rain is moving away from the radar unit and increases if the rain is moving toward it. This change in frequency is called the Doppler effect. Meteorologists also use Doppler radar to peer into severe thunderstorms and locate tornadoes. Presently, there is a network of about 150 Doppler radar units at selected sites within the continental United States.

C

Weather Satellites

A weather satellite is a cloud-observing platform in space. Satellites provide cloud observations day and night over vast regions. There are two main types of weather satellites: geostationary satellites and polar orbiting satellites. Geostationary satellites orbit Earth at the same rate that Earth spins. Hence, they remain about 36,000 km (about 22,000 mi) above a fixed spot on the equator and constantly monitor a specific region below them. Successive cloud photographs from geostationary satellites provide meteorologists with valuable information about the development, movement, and dissipation of weather fronts, storms, and clouds. Polar orbiting satellites, situated about 850 km (about 530 mi) above Earth’s surface, pass over the North and South poles on each orbit photographing the clouds directly beneath them. Because Earth rotates beneath the satellite, each orbit enables the satellite to monitor an area that is west of its previous pass. Thus, the satellite photographs the entire surface of Earth every 12 hours. Since polar orbiting satellites observe clouds at a much lower altitude than geostationary satellites, they provide more photographic detail of cloud systems.

IV

Structure of the Atmosphere

By studying the atmosphere, meteorologists have discovered that it can be divided into a series of layers. Based on a vertical profile of temperature, the layers consist of the troposphere, stratosphere, mesosphere, and thermosphere.

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The lowest layer, the troposphere, is warmed by Earth. Sunlight warms Earth’s surface, and the surface warms the air. Therefore, the warmest air is next to the ground and air temperature normally decreases with height. This pattern of decreasing air temperature with altitude occurs usually up to an altitude of between about 8,000 m (about 26,000 ft) at the poles and 16,000 m (about 52,000 ft) at the equator. This region of the lower atmosphere where air temperature normally decreases with height is called the troposphere. The troposphere is kept well stirred by rising and descending air currents.

The layer of atmosphere above the troposphere is called the stratosphere. In the stratosphere, air temperature begins to increase with height, mainly because ozone (a type of oxygen) in the stratosphere absorbs energy from the Sun, principally ultraviolet radiation. Although the amount of ozone in the stratosphere is quite small, it is important because it protects living things on Earth by absorbing the Sun’s harmful ultraviolet radiation. However, chemicals emitted near the earth, such as chlorofluorocarbons (CFCs), can be injected into the stratosphere by the updrafts in thunderstorms. In the stratosphere these chemicals can help to destroy ozone. Occasionally, concentrations of CFCs in the stratosphere are high enough to destroy nearly all of the ozone over large regions, producing a hole in the ozone layer. In recent years, such holes have occurred every spring over Antarctica. Many regions in the temperate latitudes have occasionally experienced a significant thinning of the ozone layer.

Above the ozone-rich stratosphere lies the mesosphere, where air temperature, again, decreases with height. The mesosphere is the coldest layer of the atmosphere and extends from an altitude of about 50 km to about 85 km (about 30 mi to 50 mi). Above the mesosphere lies the hot thermosphere, where air temperatures can exceed 1000°C (1800°F), primarily due to oxygen absorbing the Sun’s energetic rays.

V

Energy Flow and Global Circulation

The average amount of energy Earth absorbs from the Sun each year is equal to the average amount of energy Earth loses to space. This energy balance, however, is not maintained for each latitude. Annually, tropical regions gain more energy from the Sun than they lose, while polar regions lose more energy to space than they gain. The tropics do not continuously grow warmer and the polar regions do not continuously grow colder, however, because the atmosphere transports warm air toward the poles and cold air toward the equator. The oceans do the same with water. The wind pattern generated by the unequal heating of Earth’s surface produces the major wind belts over the globe. This average wind flow is called the general circulation of the atmosphere.

The average wind flow of the general circulation is much less complex than the actual global circulation at any given instant. Winds tend to develop into rotating eddies, called high and low pressure areas, that move across the middle latitudes and complicate the wind flow pattern.

Models make simplifying assumptions about Earth, its atmosphere, and its winds, and meteorologists have developed a simple model, called the three-cell model, to describe the average wind flow of the general circulation. The three-cell model assumes that Earth is covered with water and that the Sun is always over the equator. With these assumptions, the model makes the following predictions, each of which matches the average surface wind patterns observed, but not the winds aloft.

In the tropics, the intense sunlight heats the surface, which warms the air, causing it to rise. This reduces the air pressure at the surface, forming a broad region of low pressure. As the warm, humid air rises, it often condenses into huge thunderstorms that provide the tropics with ample rainfall.

Near the top of the troposphere, the rising air branches and moves toward the North and South poles. As the air aloft moves toward the poles, it gradually cools. At the same time, the air slowly squeezes together and becomes more dense.

Near 30° latitude, the air aloft becomes dense enough to produce high-pressure areas at the surface, called the subtropical highs. As the surface air moves outward from the surface highs, the air aloft sinks to replace it and warms by compression, which tends to evaporate any clouds in it. Cloudless skies and little rainfall characterize the region around 30° latitude. Many of the world’s deserts are found near this latitude.

At the surface, some of the sinking air moves back toward the lower pressure at the equator. This flow of air toward the equator is known as the trade winds. Due to the Coriolis force, a force that results from the rotation of Earth, the trade winds are deflected to the west. In the northern hemisphere, the trade winds blow from the northeast, and in the southern hemisphere, they blow from the southeast. The trade winds complete a thermally driven convection cell that begins with the Sun warming the tropics, air rising above the equator, flowing toward the poles, then sinking near 30° latitude and returning to the equator. At the equator, the trade winds from the northern hemisphere meet the trade winds from the southern hemisphere forming a boundary called the intertropical convergence zone (ITCZ).

Another cell occurs in the polar latitudes. At the poles, cold air sinks into an area of surface high pressure. As the cold surface air flows toward the equator, it meets milder middle latitude air near 50° to 60° latitude. Here, the converging air rises for the return trip to the poles. The rising air also cools and often condenses into clouds. Hence, plentiful rainfall characterizes the region between 50° and 60° latitude. In this region, where the rising air moves toward the poles, regions of surface low pressure often form.

The third cell of the three-cell model occupies the mid-latitudes between the other two cells. Some of the rising air between 50° and 60° latitude begins the journey aloft back toward the equator. At about 30° latitude, this air begins to sink in the vicinity of the subtropical highs. The surface winds tend to blow from the high-pressure region at about 30° latitude toward the low-pressure region between 50° and 60° latitude. The Coriolis force deflects these surface winds, producing the prevailing westerlies of the middle latitudes.

Meanwhile, poleward of the prevailing westerlies, cold, polar air moves toward the equator, and the Coriolis force deflects this air, producing a polar wind belt called the polar easterlies. Near 50° to 60° latitude, the polar easterlies meet the prevailing westerlies. Here the winds are blowing in opposite directions along a boundary called the polar front. It is along the polar front that middle latitude storms often develop.

The three-cell model of the general circulation assumes that the Sun is always above the equator. In the real world, the zone of maximum surface heating shifts seasonally. Because the Sun is overhead in the northern hemisphere in July and overhead in the southern hemisphere in January, the major surface wind belts and pressure systems shift northward in July and southward in January.

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