Air Pressure
Weather is the condition of the air around the Earth. It develops in the
atmosphere that surrounds the Earth. Weather changes from day to
day, even from hour to hour. One of the factors that determines
weather is air pressure.
Air is matter because it has mass and takes up space. Air is what
takes up space in a blown up balloon. Gravity acts on air by pulling it
and holding it as close to Earth’s surface as possible. Air pressure is the weight of the
air being pulled down on an area of the Earth’s surface. Earth’s atmosphere is pushing
down on each square inch with 15 pounds of pressure!
Two things affect air pressure: the amount of air above it and temperature.
Air pressure is affected by temperature. Warm air has less density than cold air
because the particles of air are farther away from each other. Cold air has more density
than warm air because the particles of air are closer to each other. Because warm air is
lighter, warm air has less air pressure. Because cold air is heavier than warm air, cold
air has more air pressure.
Air pressure is also affected by the amount of air above it. Imagine that you are lying
on the floor. Someone puts a book on your stomach. You feel the weight of the book,
“book pressure”. If someone adds ten more books, you will feel a lot more “book
pressure”. The same idea is true for air pressure. If you are at the top of a mountain,
there is not much air above you, so the air pressure is low. If you move down into a
valley, there is a lot more air above you, so the air pressure higher.
Air pressure is greatest in the troposphere. In the upper layers of the
atmosphere, there are fewer molecules exerting less pressure. Even
within the troposphere, air pressure gets lower as you go higher. That is
why your eardrums pop when you go up a steep mountain in a car. The
popping is your eardrum trying to balance the higher air pressure inside
your ear with the lower air pressure outside your ear.
Changes in air pressure can affect humans. If you are climbing a
mountain, it may become harder to breathe the higher you go. This is because the air is
less dense and there are fewer oxygen molecules.
Meteorologists use a barometer to air pressure. A barometer measures air pressure in
inches of mercury. If the barometer rises, the high pressure indicates fair weather or no
rain. If the barometric pressure drops, the low pressure indicates rain or storms are on
their way.
1. The weight of the air being pulled down on Earth is known as __________________
2. What force is pulling on air and keeping the atmosphere close to Earth?
________________________________________
3. What are 2 factors that effect air pressure?
__________________________________________________________________
4. In which temperature of air are the molecules closest together? ________________
5. Why does cold air have more air pressure than warm air?
6. Where would you find a greater number of air molecules – at the bottom of a
mountain or the top of a mountain? ___________________________
7. A general rule for air pressure is that it
higher in the atmosphere.
increases
decreases
as you go
8. Where is air pressure greater: at a high altitude or a low altitude? Why?
9. Why do mountain climbers have a hard time breathing when they get near the top of
the mountain?
10. Draw a picture that illustrates the decrease in air pressure as you go higher in the
atmosphere.
11. What instrument measures air pressure? _______________________________
12. Yesterday the barometer read 29.97”. Today it reads 30.02”. What kind of weather
can we expect?