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What is Air? | Pressure | Stable & Unstable | Volume | Weather Front | Layers
Air Pressure
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. Molecules that are packed closely are at higher pressure than molecules that are more spread out. The molecules inside this balloon are at a higher pressure than the molecules outside the balloon.
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.
