Layers Of The Atmosphere

The lowest layer of the atmosphere, rising from the surface to approximately 18 km (11 mi) altitude, is the troposphere. This is where all weather occurs; most of the clouds are found here. In the upper parts of the troposphere, high-speed "rivers of air" travel around the planet. The strongest of these "rivers," called jet streams, blow in a generally west-to-east direction between 30 degrees north latitude (30°N) and 60 degrees north latitude (60°N), and between 30 degrees south latitude (30°S) and 60 degrees south latitude (60°S).

Above the troposphere lies the stratosphere, extending up to approximately 50 km (30 mi) altitude. Near the upper reaches of this level, ultraviolet (UV) radiation from the sun causes oxygen atoms to group together in triplets (O3), rather than in pairs (O2) as is the case nearer the surface. An oxygen triplet is a molecule known as ozone. This gas is opaque to most UV rays. Oxygen atoms form a self-regulating mechanism that keeps the surface from receiving too much UV radiation from the sun. Certain gases are produced by industrial processes carried on by humans; these gases rise into the stratosphere and cause the ozone molecules to break apart into their individual atoms. This makes the upper stratosphere more transparent to UV. Some scientists contend that if this process continues, it could have an adverse effect on all life on the planet.

Above the stratosphere lies the mesosphere, extending from 50 km (30 mi) to an altitude of 80 km (50 mi). In this layer, UV radiation from the sun causes electrons to be stripped away from atoms of atmospheric gas. The result is that the mesosphere contains a large proportion of charged atoms, or ions. This occurs in a layer that engineers and scientists call the D layer of the ionosphere.

Above the mesosphere lies the highest layer of the atmosphere, known as the thermosphere. It extends from 80 km (50 mi) up to more than 600 km (370 mi) altitude. This layer gets its name from the fact that the temperature is extremely high. Ionization takes place at three levels within the thermosphere, called the E layer, the F1 layer, and the F2 layer. Fig. 2-2 is a cross-sectional drawing of the earth's atmosphere, showing the various layers and the ionized regions.

Atmospheric Layers
Fig. 2-2. The atmosphere is hundreds of kilometers thick. Weather affecting the surface occurs below approximately 16 km (10 mi) altitude.

HEAT TRANSPORT

The sun warms the earth to a greater extent in some areas than in others, and the air seeks to equalize this imbalance by convection. This is what makes the winds and clouds, and is ultimately responsible for all weather.

The equatorial regions receive more heat from the sun than they can radiate back into space. The polar areas are the opposite: They radiate more energy than they get from the sun. The polar/equatorial temperature difference is the result of the astronomical fact that the sun's average angle is more direct at the equator than at the poles. Thus the polar regions have become ice shrouded, and the equatorial zone has not. The different surface characteristics increase the temperature differential still further: Snow and ice reflect much solar energy and absorb little, while vegetation and dark soil reflect little energy and absorb much. If the air did not act to equalize this temperature discrepancy, at least to some extent, the tropics would be boiling hot, and the poles would be incredibly cold.

Fig. 2-3 shows a simplified model that explains how the atmosphere can transport heat on a large scale. The air over the tropics, especially near the equator, is heated by contact with the earth. Therefore, it rises, because warm air always rises. The air over the arctic and antarctic is cold, so it descends, because cold air always descends. The result is that air flows from the poles toward the equator along the planet's surface, and from the equator toward the poles at high altitudes.

According to the model of Fig. 2-3, we should expect to have a prevailing northerly surface wind in the northern hemisphere, and a prevailing southerly surface wind in the southern hemisphere, with no surface winds at the equator or at either pole. This is not what we observe, because Fig. 2-3 is an oversimplification. The air flow around the earth is affected not only by temperature differences, but by geography and the fact that the earth rotates on its axis.

In the actual atmosphere, there are three major convection regions, called Hadley cells, in the northern hemisphere, and three "mirror-image" ones in the

North pole

Equat

North pole

Equat

Equator

South pole

Fig. 2-3. Simplified model showing how heat can be transported from warm zones to cold zones by convection.

Equator

South pole

Fig. 2-3. Simplified model showing how heat can be transported from warm zones to cold zones by convection.

North pole: air descends

air descends

Equator: air ascends

air descends

North pole: air descends

air descends

Equator: air ascends

air descends

Atmosphere Layers

air descends

Equator: air ascends

air descends

South pole: air descends

Fig. 2-4. Simplified rendition of convection in the real atmosphere of our planet, showing approximate locations of the Hadley cells.

air descends

Equator: air ascends

air descends

South pole: air descends

Fig. 2-4. Simplified rendition of convection in the real atmosphere of our planet, showing approximate locations of the Hadley cells.

southern hemisphere. Ascending air is found at or near the equator and at about 60°N and 60°S. These areas are semipermanent low pressure systems, and the weather in and around them is characterized by abundant cloudiness and rainfall. Descending air is found at or near the poles and at about 30°N and 30°S. The descending air is associated with semipermanent high-pressure systems, with predominantly fair weather. This convection pattern, neglecting the effects of the earth's rotation, is shown in Fig. 2-4. But even this is an oversimplification.

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Responses

  • odovacar
    What is the importance of the elayer of the atmosphere?
    7 years ago

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