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 and even from
hour to hour. There are several factors that determine the weather. Let’s take a look at
one of these factors: air pressure.
Remember that air is matter. All matter has mass and takes up space, so air has
mass and takes up space. Weight is the measure of gravity acting on matter. Gravity
acts on air by pulling it and holding it as close to the Earth’s surface as possible. The
weight of the air being pulled down on an area of the Earth’s surface is called air
pressure.
Air pressure is affected by two things. First, it is 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. We will call that “book pressure.” If someone adds two more books, you feel more “book pressure.” If
someone adds ten more books, you 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 (a low amount of pressure).
If you move down into a valley, there is a lot more air above you, so the air pressure is greater, or there is high
pressure.
High pressure is associated with cooler, dryer air. Why do you think that is? Pause and discuss with your
group. Well, it’s because the cooler air is sinking, pushing down and creating more of a push, more pressure.
Because the air is cooler, there tends to be less moisture in the air, therefore there is less possibility for clouds
and/or precipitation. Since the air is cooler and dryer, high pressure tends to be associated with more clear skies, or
“happy” weather.
Air pressure is also affected by temperature. Warmer air weighs less than the same amount of colder air
because the particles of air are farther away from each other, or less dense. Colder air weighs more than the same
amount of warmer air because the particles of air are closer to each other, or more dense. Because warm air weighs
less, warm air has less air pressure. Because cold air weighs more, cold air has greater air pressure.
Low pressure is associated with warmer, moist air. Why do you think that is? Pause and discuss with your
group. Well, it’s because the warmer air is rising, creating less of a push, less pressure. Because the air is warmer,
there tends to be more moisture in the air, therefore there is more possibility for clouds and/or precipitation. Since
the air is warmer and moist, low pressure tends to be associated with more cloudy skies and/or precipitation, or
“lousy” weather.
Air pressure changes, sometimes very quickly. Imagine that you are in a very small room with many
classmates. You are all standing elbow-to-elbows, pushed against each other and against the walls. You are in an
area of high pressure (a high amount of pressure). Suddenly, someone opens a door to a very large room. You
would want to move into the larger room where you would have enough space to be comfortable. You would want
to move to an area with less pressure, and the sooner you could get there, the better! The same idea is true for air
particles. Air wants to move from an area of high pressure to an area of low pressure, and it will do so as quickly as
possible. When the air moves from areas of high pressure to areas of low pressure, this creates what we feel as
wind.
Based on what you and your group just read, use the provided sheet to cut out descriptors to help you explain what
is happening in the image below. Please also shade in the arrows already on the picture to represent cool air with
blue and warm air with red.
Low
Pressure
Cool air
sinks.
Warm
air rises.
High
Pressure
Wind is created when air moves from
HIGH to LOW pressure.
Let’s relate this to another example of wind that happens at a beach or a shore. Look at the two images below and
compare each. Using the images, make a bulleted list in the T-chart below to explain what’s happening.
Here are some
guiding
questions to
help you fill in
your chart:
1. What
differences do
you notice?
2. Where is there
high pressure?
Low pressure?
3. What do you
notice about
land and ocean
temperatures?
4. When do they
each happen?
5. Where is the
air rising?
Sinking?





Sea
Day time
Breeze
Land temperature is higher than water
Low pressure over land
High pressure over water
Warm air is rising over the land—cool
air is sinking over the water





Land
Night time
Breeze
Water temperature is higher than land
Low pressure over water
High pressure over land
Warm air is rising over the water—cool
air is sinking over the land