The Effects of Semi-Permanent Pressure Cells

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Lesson Transcript
Instructor: Julie Zundel

Julie has taught high school Zoology, Biology, Physical Science and Chem Tech. She has a Bachelor of Science in Biology and a Master of Education.

Although semi-permanent pressure cells don't sound particularly exciting, they actually are. This lesson will describe how pressure systems develop and examine four semi-permanent pressure cells.

What Is a Semi-Permanent Pressure Cell?

When you look up into the sky, you probably aren't thinking about weather patterns or pressure cells. Maybe you notice a nice blue sky, or puffy clouds, or even a bird flying, but semi-permanent pressure cells? No, that's probably not what pops into your mind. But it should! So, why don't you climb into your hot air balloon, and we will check out everything you've ever wanted to know about pressure systems!

As you ascend in your hot air balloon, you may notice the pressure decreases. Before we get too high, let me quickly go over some things you should know about pressure. Air is made up of particles that are bouncing around. The more particles, the greater the pressure they exert on their surroundings. So, as you ascend, you'll notice there are fewer particles, so less air pressure!

Temperature also affects air pressure. If air is warm, the particles are farther apart, so they exert less pressure compared to cold, dense air. Your hot air balloon, believe it or not, floats because of temperature. The hot air inside your balloon is less dense than the surrounding cold air, so the balloon rises.

While you're floating around up there, let me briefly fill you in on semi-permanent pressure cells, which are areas where high or low-pressure systems exist for much of the year. In the Northern Hemisphere, high-pressure systems are typically associated with air that moves in a clockwise direction. High-pressure systems usually create dry, clear weather. Contrast this with low-pressure systems, which are associated with air that moves in a counterclockwise direction and creates increased precipitation and storms.

Why don't you head out to the cold ocean in your hot air balloon? You are going to be inside a parcel of air, which just means a clump of air that shares similar properties. Because of your proximity to the cold ocean, you may notice that parcel of air is getting colder and colder. This takes longer than seconds, but it's a short lesson, so you'll have to use your imagination again.

Now, think back to what I said before. As air cools, the molecules that make up the air slow down and get closer together. As a result, the parcel of air takes up less room than it did before it cooled. This can happen in your balloon, too. If the air inside your balloon cools, the particles get closer together and take up less space. Your balloon might even seem to have deflated a little!

Since the parcel of air takes up less room, other parcels of air rush in to take up the space once held by your parcel of air. Now you have more air in the same volume than you did before the parcel cooled. And, since there is more air, you have more pressure being exerted, or a high-pressure system.

The opposite happens in a low-pressure system. Let's use the same parcel of air as before. In this scenario, why don't you travel to a warm place where the heat radiates from the land and warms the air parcel?

Since the parcel is warm, the molecules move faster and get further apart, thus taking up more space and pushing out other parcels of air. This can happen to your balloon, too! As air heats up inside of your balloon, the air takes up more space, and your balloon may seem to inflate.

But back to the parcel of air. If the parcel takes up a lot of room and pushes out other parcels of air, there is less air in the given volume, so less pressure is exerted, hence the low-pressure system. In reality, the formation of high and low-pressure systems is slightly more complicated, but this explanation allows you to get your foot in the door.

Semi-permanent pressure systems are these highs and lows that last a really long time. So, now that you have some background, let's check out some semi-permanent pressure systems and see how they impact weather patterns.

Examples and Effects

Why don't you head towards the Azores, which are a group of islands located in the North Atlantic Ocean? This semi-permanent pressure system is called the Azores High. Can you guess how it got that name? Yep, because it's near the Azores. In fact, you'll notice the semi-permanent highs and lows get their names based on where they are located. I guess that makes things easier to remember!

Diagram of the Azores High

Okay, back to high-pressure systems. Remember, they are often associated with clear and dry conditions, and the Azores High is no exception. High-pressure systems bring dry conditions to the Sahara Desert, droughts to the Mediterranean, and warm and dry weather to parts of Europe and the Eastern United States.

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