Prioritize…
After completing this section, you should be able to:
- describe the main force that creates winds in the atmosphere (the pressure gradient force).
- qualitatively relate the strength of the pressure gradient force to the speed of the wind.
Read…
The first step in understanding atmospheric circulation is to identify the forces responsible for moving air. So, let's begin with a simple but essential question: what causes air to move horizontally? In other words, what makes the wind blow? The answer lies in atmospheric pressure, which refers to the force exerted by the weight of the air above a given area. This pressure isn’t the same everywhere; it varies from place to place. Specifically, differences in pressure across different regions of the globe create a force known as the “pressure gradient force,” which sets the air in motion. Let’s take a closer look.
You’ve probably heard the terms “low-pressure” and “high-pressure” systems mentioned during weather forecasts. These refer to differences in atmospheric pressure, which is the force exerted by the weight of air molecules above a given area, caused by gravity. Atmospheric pressure varies with height (the higher you are, the lower the pressure, which is why you may get lightheaded when you fly to a ski resort), but near sea level, it usually measures around 1013.25 millibars, or about 14.7 pounds per square inch. So, what does it mean in a climate context when we talk about “low” and “high” pressure? In simple terms, low-pressure systems occur when the air in a column above a particular region weighs less than the air in surrounding areas. High-pressure systems form when the air column is heavier.
Video: Gravity and Air Pressure (2:10)
Gravity and Air Pressure
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The force of gravity keeps you standing on the ground by giving you weight. But it also keeps air close to the ground as well. Just in the same way our feet press down on the ground when we are standing up, air presses its weight down on the ground too. This is what we call air pressure.
We cannot feel air pressure because our bodies are naturally able to cope with the weight of the air above us. In fact, air is so light it weighs as much as just one bottle of water. We are able to measure the air pressure using a barometer, which looks like this. The barometer displays the pressure on a screen to show where the pressure is high, low, or changing.
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Sometimes there is more air pressing down on the ground. This is called high pressure. High pressure is caused by air sinking and being squashed together. When air is rising, we get low pressure, and so less air is pressing down on the ground. Air moves from being squashed under areas of high pressure to areas of low pressure where it is less squashed. This movement of air from high to low pressure creates wind.
In the UK, if there is high air pressure during the summer months, then we usually get hot and sunny days, ideal for trips to the seaside. High air pressure in the winter means cold weather and normally a frost in the mornings, so you need to wear warm clothes. If there is low pressure, then it’s normally cloudy, wet, and windy, so it’s a good time to put on a raincoat and wear your wellies.
While the pressure differences between these systems are often subtle—typically just a few percent at sea level—they have a significant influence on climate patterns. These pressure gradients drive wind and atmospheric circulation, playing a key role in redistributing moisture and energy across the globe. For example, regions of persistent low pressure can lead to more rainfall and cooler temperatures, while high-pressure areas are frequently associated with dry, stable weather. Over long periods, these pressure systems help shape broader climate zones. They are the primary influencers of everything from tropical monsoons to polar wind patterns. The movement of air due to these pressure differences is one of the fundamental processes that governs climate dynamics on Earth.
To see what I mean, let's perform a simple experiment. A Plexiglass container (pictured above) has two sections, separated by a removable barrier. The left section has more water (colored blue for easy visualization!) in it than the right, which means the water on the left is heavier – a full glass of water is heavier than an empty glass. Because of this, the water pressure at the bottom of the left section is higher than in the right section. Now, if I take away the barrier, the water will flow from the higher-pressure side (left) to the lower-pressure side (right). So, when the barrier is removed, the water, which was still before, starts moving because of the difference in pressure.
Let’s go back to a basic idea from physics: for something that is sitting still to start moving, a force must be applied to it. Since the water in our experiment was still at the beginning, there must have been a force that caused it to start moving. In this case, that force is called the pressure-gradient force. The key thing to remember about the pressure-gradient force is that it always pushes from areas of higher pressure to areas of lower pressure.
If the amount of water in each compartment is almost the same, the pressure-gradient force (PGF) will be much weaker because the water weights are nearly equal. With a smaller pressure-gradient force, the water will flow much more slowly. This brings us to the second important point: the size of the pressure-gradient force (which in this experiment is represented by the difference in water pressure between the two compartments) determines how fast the water will flow.
Explore Further…
Experiment! You can easily create your own pressure gradient right now!
Take an empty soda or water bottle and loosen the cap until it's barely hanging on by the last thread. Point away from you, your friends, pets, anything breakable… and squeeze!
When you squeeze the bottle, you increase the pressure of the air inside. This air becomes “high pressure,” while the air outside remains at “low pressure.” As the air tries to flow from high to low pressure because of this pressure gradient, the cap will pop off. The harder you squeeze, the greater the pressure difference, the faster the “wind” you create is, and the further the cap will fly!
In the atmosphere, the pressure-gradient force operates similarly to the water flow in our experiment, only on a much larger scale. Just as water moves from areas of higher pressure to lower pressure, air does the same, creating wind. This process is central to the Earth's atmospheric circulation, where pressure gradients between the equator and the poles drive large-scale wind patterns. These winds help to redistribute energy from the warm tropics to the cooler polar regions and – as we’ll see in a little while – influence Earth’s climate zones.