ATMOSPHERIC PROPERTIES

Atmospheric Temperature and Layers

The atmosphere, composed mainly of nitrogen and oxygen with traces of carbon dioxide, water vapor, and other gases, acts as a buffer between the Earth and the Sun. The layers (troposphere, stratosphere, mesosphere, thermosphere, and exosphere) vary around the globe and in response to seasonal changes.

The troposphere is the lowest layer of the Earth's atmosphere, extending to a height of 8-15 km, depending on latitude. The stratosphere, warmer than the upper troposphere, is the next layer and rises to a height of 50 km. Temperatures in the mesosphere, 50 to 80 km above the Earth, decline with altitude to -70 degrees to -140 degrees Celsius, depending upon latitude and season. Temperatures increase again with altitude in the thermosphere, which begins about 80 km above the Earth. They can rise to 2,000 degrees C. The exosphere begins at 500 to 1,000 km and the few particles of gas there can reach 2,500 degrees C during the day.

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Cloud Properties

When looking at Earth from space, one of the most distinct features we see is its cloud cover. Clouds are visible aggregates of water droplets, ice particles, or a mixture of both that occur in the atmosphere above the Earth's surface.

The study of clouds, where they occur, and their characteristics, may well be the key to understanding climate change. Low, thick clouds primarily reflect solar radiation and cool the surface of the Earth. High, thin clouds primarily transmit incoming solar radiation; at the same time, they trap some of the outgoing infrared radiation emitted by the Earth and radiate it back downward, thereby warming the surface of the Earth.

Research on global cloud cover and cloud properties will be performed using data from several instruments scheduled for flight on the Tropical Rainfall Measuring Mission (TRMM) and on the EOS satellites.



Cloud Types

Clouds fall into two general categories: sheet-like or layer-looking stratus clouds (stratus means layer) and cumulus clouds (cumulus means piled up). These two cloud types are divided into four more groups that describe the cloud's altitude.

High clouds form above 20,000 feet in the cold region of the troposphere, and are denoted by the prefix CIRRO or CIRRUS. At this altitude water almost always freezes so clouds are composed of ice crystals. The clouds tend to be wispy, are often transparent, and include cirrus, cirrocumulus, and cirrostratus.

Middle clouds form between 6,500 and 20,000 feet and are denoted by the prefix ALTO. They are made of water droplets and include altostratus and altocumulus.

Low clouds form from the surface up to approximately 6500 ft. and are denoted by the prefix (or suffix) strato or stratus (meaning layered). They are composed of water droplets and include fog, stratus, stratocumulus, and nimbostratus.

Vertical clouds, such as cumulus, rise far above their bases and can form at many heights. Cumulonimbus clouds, or thunderheads, can start near the ground and soar up to 75,000 feet.

Clouds provide an overall cooling effect on the Earth's surface.

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Global Cloud Cover

Clouds play an important role in the Earth's climate system by affecting the amount of heat (in the form of electromagnetic radiation) that is allowed to pass into or out of the system.

This image shows the global distribution of annual mean cloud amount. The lowest percentage of cloud amount is shown by red areas and the highest is represented by blue areas. The numbers on the color bar at the left represent percentage of cloud cover.

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Global Cloud Cover

Global cloudiness is organized roughly into three latitudinal bands: one near the equator and one in each hemisphere at higher latitudes. The higher latitude cloud bands are composed of clouds forming in the cyclonic storm systems that dominate the mid-latitude circulation. The tropical cloud band is formed by moving clusters of deep convective storms that exhibit complex structure, strongly affected by the influence of the arrangement of land and water on the tropical circulation.

The above graphic is a composite image of the global distribution of cloud and land/ocean surface temperatures measured by various meteorological satellites on July 4, 1983. The lowest temperatures are indicated on the left side of the individual color scales and the highest temperatures are found on the right side.

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Cloud Albedo

Energy goes back to space from the Earth system in two ways: reflection and emission. Part of the solar energy that comes to Earth is reflected back out to space in the same, short wavelengths in which it came to Earth. The fraction of solar energy that is reflected back to space is called the albedo. Over the whole surface of the Earth, about 30 percent of incoming solar energy is reflected back to space. Because a cloud usually has a higher albedo than the surface beneath it, the cloud reflects more shortwave radiation back to space than the surface would in the absence of the cloud, thus leaving less solar energy available to heat the surface and atmosphere. Hence, this "cloud albedo forcing," taken by itself, tends to cause a cooling or "negative forcing" of the Earth's climate.

Different parts of the Earth have different albedos. For example, ocean surfaces and rain forests have low albedos, which means that they reflect only a small portion of the Sun's energy. Deserts, snow, and clouds, however, have high albedos; they reflect a large portion of the Sun's energy.

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Aerosol Properties

Atmospheric aerosols consist of small particles of liquid and solid material suspended in the air. Both natural and human-produced materials occur in the aerosol particles. Aerosol particles larger than about 1 micrometer in size are produced by processes such as windblown dust and sea salt from sea spray and bursting bubbles. Aerosols smaller than 1 micrometer are mostly formed by condensation processes such as conversion of sulfur dioxide gas to sulfate particles and by formation of soot and smoke during burning processes. After formation, the aerosols are mixed and transported by atmospheric motions and are primarily removed by cloud and precipitation processes.

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Global Distribution of Aerosols

The figure shows the global evolution and dispersion of the Pinatubo aerosol cloud over nearly a two-year period. During this period, the aerosol cloud completely encircled the globe in a narrow zone, then the cloud dispersed outward over the globe.

The Mount Pinatubo eruption had major impacts on the Earth system. The resulting aerosols are believed to have contributed to reduced ozone concentrations over the polar region of the Southern Hemisphere. This was the lowest observed level ever.

The "Before" image shows low levels of sulfuric acid/water aerosol before the eruption of Pinatubo. The "During" image shows the 40-days following the eruption, with reds and yellows being higher concentrations of the aerosols. Twenty months later, the aerosol cloud can be seen dipsersed over the globe in the "After" image.

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Global Distribution of Aerosols Over The Oceans

A July map of atmospheric aerosol patterns over the oceans estimated from polar-orbiting meteorological satellites is shown. The largest arerosol amounts are indicated by red colors, with decreasing aerosol amounts going from yellow to green to blue; the lowest amounts are shown in black.

Major aerosol plumes originate from West Africa, Southwest Africa, Indonesia, East Asia, Central America, and eastern North America. Regions of enhanced aerosol in the Austral summer over the southern and equatorial Pacific ocean probably originate from natural marine sources. The summer and spring hemispheres show generally higher aerosol backscattering than during the winter and fall. (Courtesy of Rudolf Husar, Washington University, St. Louis)

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Example: Kuwait Oil Fires

At the end of the Persian Gulf War in the spring of 1991, 732 oil wells were set ablaze in Kuwait. Approximately 550 wells were still burning in May and June, when a group of scientists began to study the properties and climatic impact of the dense smoke that rose from them.

Significant environmental pollution and changes in the regional weather resulted from the fires. Air temperatures below the plumes were reported to be about 7 degrees C lower than in adjacent areas without smoke. Smoke plumes covered tens of thousands of square kilometers, contributing to Bahrain experiencing that year its coldest May in 35 years. Average temperatures were about 4·C lower than normal. Also, the oil well fires produced carbon dioxide at an estimated level of about 1.5% of the global annual production.

This illustrates cooling of the region below the smoke due to reflection of solar radiation by aerosol particles in the smoke plume. Instruments such as the Moderate-Resolution Imaging Spectroradiometer (MODIS) and the Multi-angle Imaging SpectroRadiometer (MISR) planned as part of the EOS Program, will provide observations of fires and their smoke on a global scale.

In the image at the right, heavy smoke plumes can be seen moving from the fires in the center toward the bottom right.

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