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Air Pressure

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Air Pressure

- An object’s velocity remains constant unless acted upon by an external force.
- An object’s acceleration (change in velocity) is directly proportional to the force and inversely proportional to its mass.
- For every action there is an equal and opposite reaction (forces come in pairs).

- Consequence of Newton’s Laws
- Called Kinetic Energy (KE)
- KE directly proportional to mass
- KE directly proportional to velocity squared

- Gas molecule hits wall and bounces back
- Its velocity has changed direction – there must have been a force (Newton’s 1st Law)
- Wall exerted a force on the molecule (Newton’s 2nd Law)
- Molecule exerted equal and opposite force on wall (Newton’s 3rd Law)

v

F

-F

-v

Wall has area A

Fewer molecules hitting wall give lower pressure

More molecules hitting wall give higher pressure

Main influence is density of air:

- Number of gas molecules per unit volume
- Average separation of molecules increases with temperature

- Average mass of the gas molecules
- 78% N2 and 21% O2 29 g/mol
- H2O 18 g/mol
Number has greater affect on density

Gravity compresses atmosphere giving maximum number density (pressure) at surface.

- Density variations due to temperature variation (thermodynamics) and H2O concentrations
- Net convergence or divergence of winds in a circulation pattern
- Horizontal winds blow toward a location (Low, cyclone) or away from a location (High, anticyclone)
- Wind speed changes down wind (later)

- Internal energy (kinetic energy) of a gas is directly proportional to its temperature
- Velocity of gas molecules is directly proportional to temperature
- If gas is enclosed in container, pressure increases with increasing temperature
- If gas is not enclosed (like atmosphere) density decreases with increasing temperature giving lower pressures

- If surface air divergence greater than aloft convergence, the air density and surface air pressure decrease
- If surface air divergence less than aloft convergence, air density and surface pressure increase