What You Should Know about the Atmosphere for the General Science Subtest of the ASVAB

By Rod Powers

The General Science subtest of the ASVAB will expect you to know a few things about the Earth’s atmosphere. The atmosphere contains many layers of air surrounding Earth’s surface. Starting with the layer closest to Earth and extending outward, the information below tells you about those layers.

Layer Name Location Details
Troposphere Extends about 8 miles above the Earth This layer is where the jet stream is located and where almost all weather changes occur.
Stratosphere Extends about 30 miles A major reported cause of ozone depletion is the presence of chlorofluorocarbons (CFCs) in the Earth’s stratosphere. CFCs undergo a series of chain reactions, which ultimately lead to the destruction of the ozone layer.
Mesosphere Extends about 50 miles Millions of meteors burn up daily in the mesosphere as a result of collisions with the gas particles contained there.
Ionosphere Extends about 70 miles This layer reflects most radio waves, making it important to communications.
Note: Scientists disagree among themselves as to whether the ionosphere is a separate atmospheric layer or whether it’s part of the thermosphere.
Thermosphere Extends about 350 miles The International Space Station has a stable orbit within the upper part of the thermosphere, between 208 and 285 miles.
Exosphere Extends about 6,200 miles It’s only from the exosphere that atmospheric gases, atoms, and molecules can escape into outer space. No boundary exists between the exosphere and space; therefore, exosphere is sometimes used synonymously with outer space.

Warming up to cold fronts

Temperature affects air density (how closely packed the air molecules are). When the sun shines, land and water absorb its warmth. Land warms up more quickly than water, so air over land is warmer than air over water during most of the day. At night, the air over land cools more quickly than air over water. The angle of the sun also affects air density (the sun shines directly over the equator but not the poles).

Cold air is denser than warm air. Because it’s denser, cold air has high pressure, compared to warm air’s low pressure. (A barometer measures atmospheric pressure.) Air moves from areas of high pressure to areas of low pressure, creating wind.

Air masses have certain characteristics depending on where they form:

  • If an air mass forms over land, it’s dry, and if it forms over water, it’s wet.
  • Air masses formed in Earth’s northern and southern regions are cold, and those formed at the equator are warm.

When two different air masses meet, they don’t mix. They form a boundary called a front. When cold air meets warm air, a cold front develops. The warm air may be pushed up to form clouds, causing heavy rain. When a warm air mass meets a cold air mass, a warm front develops. The warm air passes over the cold air, forming a different kind of cloud, which causes light rain.

Classifying clouds

Clouds are made of small droplets of water or bits of ice that are spread out from each other. Rain (or snow) falls when the drops get too big and heavy to stay in the cloud. Clouds have three main types, and the ASVAB may ask you a question or two about their characteristics, which are detailed here.

Cloud Type Description What It Forecasts
Cirrus Thin, wispy, high clouds Generally indicate rain or snow
Cumulus White, puffy pillows, often flat-bottomed with rounded tops Common during fair weather, but when they gather, they cause heavy rains
Stratus Broad, flat, and low-hanging (gray blanket) If close to the ground, they may produce drizzle

Additionally, a prefix or suffix is frequently given to the cloud name to indicate which level of the atmosphere it’s in or whether it’s producing precipitation (rain, sleet, snow, and the like):

  • Cirro- is the prefix given to high clouds (base above 20,000 feet).
  • Alto- is the prefix given to midlevel clouds (base between 6,000 and 20,000 feet).
  • Nimbo- added to the beginning of a cloud name or -nimbus added to the end means the cloud is producing precipitation.

Therefore, a cirrocumulus cloud is a white, puffy, flat-bottomed, rounded-topped cloud at high altitude. Altostratus clouds are gray, broad, flat clouds at mid-altitude.