Air Pressure

Objectives:

- determine the effect altitude has on air pressure

-describe the weather conditions associated with high and low

pressure

Who has heard of the expression “light as air”?

What is air pressure? The weight of the atmosphere as it

pushes down on the Earth’s surface Air pressure decreases as altitude

increases. Ex: airplane Air pressure is most noticeable when

you are rapidly ascending (going up) or when you are rapidly descending

Air pressure decreases as altitude increases

Air Pressure in Weather Changes in elevation are not the only

changes in air pressure In general, air pressure decreases as

temperature increases As air gets warmer, the molecules move

faster and get farther apart and there are less molecules than in an equal volume of cold air

Air Pressure in Weather Changes in humidity also cause

changes in air pressure The more water there is in the air

(Humidity), the lighter the air is, which causes lower air pressure

Air Pressure in Weather Air pressure changes give us a simple

way of forecasting weather A decrease in air pressure means

warmer more humid air is coming, along with rain or snow

An increase in air pressure means cooler air, drier, and more fair weather is coming

Analyzing Changes in Air Pressure Meteorologists analyze pressure

changes using isobars on weather maps

An isobar is a line that joins points that have the same air pressure

The area defined by an isobar is called a high pressure area (high) or a low pressure area (low)

Air Pressure Wind always moves from high pressure

to low pressure. (think bicycle tire with a hole-wind moves from inside (high), out (low)

Coriolis Effect Because of the

earth’s ROTATION, wind traveling from the north/south pole gets DEFLECTED (put off course).