Wind
During hot summer afternoons, the ground heats up in the hot
sun. Remember from lesson 3 that air near the erath's surface
is warmed by heat from the ground. This means that the sun heats
the ground, the ground then warms the air just above it. This
warm air then rises.
The rising air will continue to move upward
until it reaches a level where the surrounding air is just
as warm or warmer. Sometimes this rising air reaches a level
called the Tropopause. The Tropopause acts as a "lid"
to this rising air. Most of the time the air cools before it reaches
the Tropopause, and begins to sink back down. Whether a section
of air is rising or sinking, wind is created by air from another
section flowing to fill the void. What is the Tropopause?
In order to understand the Tropopause and why it acts as a
"ceiling" to rising air from the lower atmosphere,
we must look at the structure of the atmosphere.  The
lowest 10-12 miles of our atmosphere is called the Troposphere.
From the figure above it can be seen that temperature declines
with height in the Troposphere. At 10-12 miles above the ground
an amazing thing happens. The atmosphere stops cooling with
higher altitudes and actually begins to warm instead!
The height at which the atmosphere stops cooling
with altitude is called the Tropopause. This invisible ceiling
is where the ozone layer begins, which is what stops the air
from rising any higher. From the Tropopause to about 50 miles
above the surface of the Earth is a layer called the Stratosphere.
The Stratosphere actually warms with altitude. Want to see wind
circulation first hand? Give this experiment a whirl!
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