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3. Troposphere

The troposphere is the layer of air that begins at the surface of the earth and extends upward
to an average altitude of 7 miles. Its height varies from the equator to the poles, and from

season to season. It is higher over the equator than over the poles, and it also has a greater

height in the summer. The troposphere contains about three quarters (75%) of the earth's

atmosphere by weight and almost all of the weather. The border between the troposphere
and the stratosphere is called the tropopause. It is important because high-speed winds are

often found near the tropopause. I

4. Stratosphere

The stratosphere is the layer just above the tropopause. The stratosphere begins (base) at an

average height of seven miles and extends upward to an average height of 22 miles (top).

The characteristics of the layer are a slight increase of temperature with height and almost

no humidity. Occasionally, however, a strong thunderstorm in the troposphere will cross the
tropopause and enter into the lower stratosphere.