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When your friend squeezes your arm, you feel pressure!

That's because molecules collide with each other and things like your arm, the ground, or a tree. They exert a force on those surfaces.

Molecules in Earth’s atmosphere constantly bounce off each other and everything else around them. The force exerted by these air molecules is called air pressure.



When it's warmer, molecules move faster.
What happens to air pressure when the temperature goes up?


(Thanks to Ian Littlewood for supplying this applet.)



Adjust the slider beneath the box above to see how the pressure changes. What do you see?
The pressure decreases
The pressure doesn't change
The pressure increases


What's a high pressure system?

Sometimes, high in the sky, air slows its forward motion and piles up. This is called convergence. The piled-up air sinks. When the sinking air reaches the Earth’s surface, it spreads out. This is called divergence. In the Northern Hemisphere it spreads out with clockwise surface rotation and is called a high pressure system.

What's a low pressure system?

High in the sky, air moves away from an area faster than air enters an area. When air spreads out (divergence) at high altitudes, air from below flows upward to fill the space, making a low pressure system. The system has surface convergence and anti-clockwise rotation in the Northern Hemisphere.



Now that you understand what causes air pressure, check out the other ingredients of weather!

[Temperature] [Pressure] [Volume] [Density] [Combine Ingredients]

[Thunderstorms & Tornadoes] [Hurricanes] [Blizzards & Winter Weather] [Clouds]

[Predict the Weather!]

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