Air Pressure and Weather

Contributor: Lindsey Congalosi. Lesson ID: 13049

"Why must it rain when I'm planning a picnic or washing my car?" It may seem like those events cause the rain, but there are other reasons our skies are clear or cloudy, so the pressure's not on you!


Earth Science, Practical Life Skills

learning style
personality style
Grade Level
Middle School (6-8), High School (9-12)
Lesson Type
Dig Deeper

Lesson Plan - Get It!


Look outside. What is the weather like? Do you see clear or overcast skies? Windy or calm? Rainy or dry? Why is our weather the way it is, and how can it change so quickly? Most importantly, when do you need to bring an umbrella?! Read on to find out!

dog running with umbrella

  • Have you ever watched the weather on the nightly news?

You may have checked to see what you should wear tomorrow, if you should expect snow, or if you need to grab an umbrella. Clearly, the weather forecast is useful in our everyday lives, but did you know that we can get a lot more information out of the weather forecast if we know what to listen for?

Take a look at this map:

air pressure map

  • What does this map tell you?
  • What do the lines mean?
  • What are those things that look like festive worms?
  • Why are there giant Hs and Ls seemingly floating above us?

So many questions!

First, let's talk about the Hs and Ls. These refer to high air pressure and low air pressure systems. H is for high, and L is for low, in case you haven't figured that out yet.

  • Wait, what's air pressure?

You are about to find out!

Above Earth, there are layers of air that we call our atmosphere. Most of the air is found in the lower atmosphere because gravity is pulling the air molecules towards Earth. That's why mountain climbers need to bring oxygen tanks on high-altitude climbs. Most air is pulled to the ground, so there's less air up high; meaning there's less oxygen for the climbers as they get to the top.

Try this: Hold out your hand. Picture one square inch on your palm.

one square inch on a hand

In that one square inch, you are holding a tall column of air. If we were to measure the weight of this column of air — all of the air from your palm to the top of the atmosphere — it would weigh 14.7 pounds! There are 14.7 pounds of air pushing down on every square inch of you (and everything else on Earth) all the time. We don't notice or feel this pressure because we're used to it, just like a fish doesn't notice the water it's been swimming in all of its life.


Earth is heated by the sun, but not every place gets the same amount or intensity of sunlight. This uneven heating causes differences in air density and pressure. Some areas have higher air pressure; some areas have lower. These high- and low-pressure systems are found everywhere on Earth. These systems are constantly moving, generally from west to east.

sun on the earth

A high-pressure system is one with, as you might guess, lots of air pressure.

Imagine a giant hand pushing the air down onto us. Terrifying, right? This has a couple of effects. The moisture that would normally be kept in the sky as clouds is instead pushed down to Earth, where it spreads out as invisible water vapor. This results in clear, cloud-free skies. The air that is being pushed down onto us from above is cooler than the air usually at Earth's surface. This means slightly cooler temperatures. Therefore a high-pressure system means cooler-than-usual (not cold) temperatures and clear skies.

Sounds like the perfect day, doesn't it? Weather associated with a High-pressure system would make a lot of people Happy.

dog on the beach

A low-pressure system is the opposite of all of that.

Reduced air pressure means water vapor is able to rise, cool, and condense into clouds. Low-pressure weather is associated with very cloudy, overcast skies. This build-up of atmospheric moisture often causes precipitation and severe storms. A hurricane is the result of a low-pressure system forming over the ocean. Lots of water evaporates, rises up because of the low pressure, and then turns into a huge rotating system of clouds, wind, and rain.

A Low-pressure system sounds pretty Lousy, doesn't it?

dog at rainy window

Take a moment to compare and contrast these two types of pressure systems.

  1. Print the Graphic Organizer - Venn Diagram, found in Downloadable Resources in the right-hand sandbar.
  2. Label one circle "High-pressure" and the other "Low-pressure."
  3. Take a moment to review what you just learned and add anything that is unique to each pressure system on the correct side and anything they have in common in the middle space.

Now, watch Scott Dorval, from Fox 9 Now KNIN, explain why high pressure is associated with clear skies and low pressure is associated with cloudy skies. Add any new information to your Venn diagram.

Why is high pressure associated with fair, clear skies while low pressure systems are associated with dark clouds and precipitation?:


Now that you've heard the explanation, take another look at the air pressure map.

air pressure map

See those white lines? These are isobars that connect points of equal pressure. In other words, every point on a single line has the same air pressure. As you move towards a high-pressure area, the air pressure increases, and when you move towards a low-pressure system, it decreases.

  • iso- = equal
  • -bar = pressure
  • isobar = lines of equal air pressure

air pressure

In the Northern Hemisphere, the air in a high-pressure system is pushing down and out in a clockwise motion, and the air in a low-pressure system is moving inwards and upwards counterclockwise. Everything is the same in the Southern Hemisphere except the direction of the circulation. High-pressure systems in the Southern Hemisphere cause winds to move in a counterclockwise motion and vice versa. This difference is caused by Earth's rotation, and it is called the Coriolis effect.

Coriolis effect

The pushing down of air that occurs in high-pressure zones and rising up of air in low-pressure zones causes air to move from high- towards low-pressure areas. The closer the isobars are together, the faster the pressure is changing, causing the winds to go faster and faster!

Now that you have learned about pressure systems, test your knowledge in the Got It? section.

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