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).