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# Air Pressure PowerPoint PPT Presentation

Air Pressure. 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.

Air Pressure

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## Air Pressure

### 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).

### Energy of Motion

• Consequence of Newton’s Laws

• Called Kinetic Energy (KE)

• KE directly proportional to mass

• KE directly proportional to velocity squared

### 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

F

-F

-v

Wall has area A

### 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 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

### 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

### 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