MET 02 - Atmospheric Pressure

What is atmosphere pressure? Average atmospheric pressure at sea level

The force per unit area exerted by the atmosphere on any surface contact with it.

Pressure = Force / Area

Average pressure (MSL)

What are the two types of Barometers?

Mercury Barometer and Aneroid Barometer

An altimeter is a barometer that shows an increase in height with a decrease in pressure, and vice versa.

What are isobars?

• Lines joining points of equal atmospheric pressure on a surface pressure chart

• Isobars build up patterns of low or high pressure

  • Isobars close together => Wind is strong

  • Isobars far apart => Wind is light

  

What is a Depression?

A region of comparatively low pressure shown by more or less circular and concentric isobars surrounding the centre, where the pressure is lowest.

Also called a “Low”

Non-Frontal (small) depression can occur virtually anywhere

Buy Ballot’s Law

Wind will move around a low-pressure system in an anti-clock direction in the northern hemisphere.

“Wind on your back, Low is on your left”

Pressure and Altitude: ISA Values

Pressure is inversely proportional to altitude.

What is the Pressure Lapse Rate?

Lapse rate - 27ft / hPA

What is the effect of temperature on Pressure?

Warm air

• Pressure falls slowly with height               

• Expands pressure levels

Cold air

• Pressure falls rapidly with height

• Compacts pressure levels

Actual Pressure Lapse Rate formula