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UNIT 1

Introduction To Weather
SECTION 1. EARTH GEOGRAPHY

1. Movement Of The Earth - Rotation
The earth is a sphere that revolves around the sun. At the same time that the earth revolves
around the sun, it also rotates around its own axis. The sun radiates light/heat that is
received by the earth. When the earth rotates on its axis, some parts of the earth receive
light/heat from the sun (day); while parts on the opposite side of the earth do not (night).
Since the earth completes one rotation every 24 hours, each part of the earth has one "day"
period and one "night" period every 24 hours. The rotation of the earth produces the
sequence of nights and days. See figure 1.

2. Movement Of The Earth - Revolution

While the earth is rotating around its own axis, it is also revolving around the sun in a circular
path. The earth completes one revolution around the sun every 365% days. Each revolution
the earth makes around the sun takes one year. See figure 1.

3. The Angle Of The Earth's Axis
If the earth's axis were straight up and down, as in figure 2, the amount of radiation from the
sun that each part receives would not change as the earth made its revolution around the
sun. But, the earth's axis is not straight up and down, it is at an angle as is shown in figure 3.
According to where the earth is in its revolution around the sun, one hemisphere will be
closer to the sun than the other. See figure 3.

4. The Seasons
The revolution of the earth around the sun and the angle of the axis produce the general
changes in temperature that are called the seasons: winter, spring, summer, and fall. When
the areas above the equator in the northern hemisphere have winter because they are
angled away from the sun, the areas below the equator in the southern hemisphere have
summer because they are angled toward the sun. See figure 3, position 1. Six months later,
when the earth has made half of a revolution around the sun, the positions will be reversed.
The northern hemisphere will be having summer while the southern hemisphere will be
having winter. See figure 3, position 2. As the earth continues its revolution, the northern
hemisphere will start to be away from the sun