Structure of Earth's Atmosphere

Composition

Temperature

Humidity

Pressure

Wind

Pressure is a force per unit area exerted by the weight of air above -- about 1 kg/cm² or 14.7 lb/in² at the surface of Earth (equivalent to a 10 meter column of water)

Units of presure are N/m², where N=Newton=force required to accelerate a1 kg mass 1 m/s² . The SI unit for pressure is the Pascal (Pa).

1 Pa = 1 N/m2

A standard atmosphere (and average typical of a mid latitude location) exerts a pressure of 101,325 Pa at sea level

The unit adopted by the National Weather Service is the millibar (mb)

1 mb = 100 Pa --> standard sea level pressure = 1013.25 mb

1. gas molecules are able to fill space available
2. gas molecules bounce off one another when they collide (and off the wall if in a container)
3. the atmosphere is bounded above (gravity) and below (ground)

Air pressure is the force per unit area exerted against a surface by continuous collision of gas molecules

First, consider a change in temperature while holding density (volume) constant

Temperature increase --> speed of molecules increases (force increases, higher pressure

Second, consider a change in density (volume) while holding temperature constant

Density (# molecules/volume) increase -->increase in # of collisions (higher pressure)

Boyle's Law - at constant temperature, the volume of gas varies inversely with pressure

--> p₁V₁ = p₂V₂

Jacques Charles (1787)

Charles' Law - at constant pressure, the volume of a given mass is directly proportional to absolute temperature

--> increase in temperature results in increase in volume

--> V₁ / V₂ = T₁ / T₂

Combine these two laws ==> Ideal Gas Law

where p=pressure, r=density, R=gas constant, and T=temperature

Factors affecting wind

Wind is the result of horizontal differences in air pressure - air flows from higher to lower pressure

If Earth did not rotate, and if there was no friction, air would flow directly from high to low pressure

On a rotating Earth with friction wind is controlled by combination of forces:

1. pressure gradient force
2. coriolis force
3. fricton

Newton's first law of motion - an object at rest will remain at rest, an object in motion will remain in motion (at constant velocity) as long as no force is exerted on the object

Coriolis force - apparent force due to the rotation of Earth

To an observer on Earth, objects moving in any direction are deflected to the right in the northern hemisphere, and to the left in the southern hemisphere

The magnitude of the coriolis force is a function of the:

1. rotation rate of Earth
2. latitude
3. object's speed

   coriolis force = 2 m W v sinf

where m=mass, W=rotation rate, v=speed, f=latitude