Air Pressure

Newton’s Laws of Motion1. An object’s velocity remains constant unless

acted upon by an external force.2. An object’s acceleration (change in velocity)

is directly proportional to the force and inversely proportional to its mass.

3. For every action there is an equal and opposite reaction (forces come in pairs).

maFm

Fa

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

221 mvKE

Origin of Pressure, P• 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

-v-F

F

Wall has area A

A

FP

Pressure Depends on Number of Molecules

Fewer molecules hitting wall give lower pressure

More molecules hitting wall give higher pressure

Atmospheric Pressure at Earth’s SurfaceMain 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

Average Variation of air pressure with altitude

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

Horizontal Pressure Variations• 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)

Thermodynamics• 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

Air Mass Densities

Temperature

Cool Warm

Humidity

Low Most Dense Less Dense

High More Dense Least Dense

Net Divergence in a Low System

• 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