Earth’s Climate
Defined by average temperature, precipitation, humidity, air pressure, and wind over
time in a particular place
 varies from location to location
What is air pressure
weight of the Earth’s atmosphere pressing down on everything at the surface
Also called: atmospheric pressure
How is air pressure measured
With a weather instrument called a barometer
Lines on a weather map that display equal air pressure
Each isobar represents a particular value of air pressure at sea level
What is responsible for weather?
Air Pressure, because air pressure varies over the Earth’s surface and these variations
cause weather
What influences weather?
Global wind systems
Atmospheric pressure changes
Many other factors
Types of pressure systems
Low pressure: region of warm moist air
associated with stormy weather
High pressure: sinking cool air
associated with fair (sunny) weather
Which direction do air pressure
systems always flow?
From high (H) to low (L) systems
As you climb up a mountain
Air pressure decreases
Why? The higher the altitude the less pull gravity exerts on the air molecules and
consequently, the air pressure decreases
The equator vs. the poles
Equator  air is warmer
Why? Warm air is less dense than cool air when temperatures are higher, so pressure is
Poles  air is cooler
Why? Cool air is more dense than warm air when temperature is cold, so pressure is higher
Uneven heating of land & water
(equation vs. poles)
Causes a rising and sinking of warm and cool air masses creating convection currents and
Examples of major atmospheric