1. Air Pressure

2. Newton’s Laws of Motion
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).

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

4. Origin of Pressure, P v F -F -v
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

5. Pressure Depends on Number of Molecules
Fewer molecules hitting wall give lower pressure
More molecules hitting wall give higher pressure

6. Atmospheric Pressure at Earth’s Surface
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

7. Average Variation of air pressure with altitude
Gravity compresses atmosphere giving maximum number density (pressure) at surface.

8. 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)

9. 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

10. Air Mass Densities Temperature Cool Warm Humidity Low Most Dense
Less Dense
High
More Dense
Least Dense

11. 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