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Essential Question:
Air Pressure and Wind (Chapter 19)
Air Pressure
 the weight of the atmosphere pushing down on the Earth exerting a force per
unit area
 is exerted in all directions because air molecules move in all directions
 dependent on the weight of the air above any point on the Earth’s surface
 decrease with eleveation
Measuring Air Pressure
 Barometers measure air pressure
 Two types:
o Mercury
o Aneroid
 Units:
o inches & millimeters (mercury readings)
o millibars (used on weather maps)
Factors Affecting Air Pressure
1. Temperature
o as temperature increases, air pressure decreases
o molecules move farther apart
2. Humidity
o as humidity increases, air pressure decreases
o water vapor has less mass than air
Essential Question:
Air Pressure on Weather Map
 Isobars are lines that connect areas of equal air pressure
 connected areas form patterns and allow us to view weather patterns
 closely spaced isobars indicate rapid pressure change
Highs and Lows
 when air pressure is greater at the center than the outside we have a high;
 when air pressure is lower at the center than the outside we have a low
The Air in Highs & Lows
High Pressure
Sinking air
Rising air
Usually colder; dry air
Warmer; humid air
Brings fair weather
Usually brings storms
Clockwise away from center
Counterclockwise towards center
Low Pressure
moderates temperature, distributes moisture, cleanses the atmosphere
Named for the direction in which they came
Created by differences in air pressure
Blows from high pressure to low pressure
Essential Question:
Measuring Wind
 Anemometer: measures wind speed in mph or kph
 Wind Vane: measures wind direction
Air Pressure & Wind
 Pressure Gradient is the change in the air pressure divided by the distance
over which the air pressure changes
 the greater the pressure gradient the stronger the wind blows between two
Sea Breeze
 land heats up faster than the ocean
 air over land rises (creating a local low pressure)
 cooler, denser air from over the ocean blows in to replace rising air
 breezes blow from water onto land
Land Breeze
 Reverses at night
 Ocean holds heat from day, land loses heat quickly
 Breeze switches (blows from land onto water)
Sea and Land Breezes
Essential Question:
1. The Coriolis Effect
 the tendency of an object moving over the Earth’s surface to curve away from
its path of travel
 caused by Earth’s rotation
 greater near the poles
 increases as speed on an object increases
 Noticeable for objects traveling over great distances (i.e. planes, winds, ocean
 Northern hemisphere
deflected to the right
 Southern hemisphere
deflected to the left
The Coriolis Effect & Wind
o Winds blow clockwise OUT of a high pressure center
o Wind blow counterclockwise INTO a low pressure center
o This is reverse for the Southern hemisphere
Essential Question:
2. Friction
smoother the surface, faster the winds
faster the winds, the stronger the Coriolis Effect
This means the Coriolis Effect has a lesser effect on surface winds than upper
atmosphere winds (there’s less friction in the upper atmosphere)
3. Time of Year [remember air flows from high to low pressure]
High and Low pressure have temperature differences
temperatures varies throughout the year, base on the amount of insolation
winds change seasonally
The Jet Stream
 band of very strong winds in the upper troposphere
 thousands of kilometers long, hundreds wind and one kilometer tall
 polar-front jet stream controls our weather and brings cold air from the poles
 faster in the winter
Convection Cells
 continuously heat atmosphere
 create global winds as air moves away from highs and towards lows
Global Wind Patterns
if the Earth did not rotate, we’d
have one large convection cell
Hot air rises at the equator cools,
then sinks at the poles and flows
back down to the equator
Essential Question:
Because the Earth does rotate the Coriolis Effect changes the convection
air cools long before it reaches the poles
3 - celled circulation model is used
3-Celled Circulation Model
 Circulation cells are caused by
alternating high and low pressure
bands at the surface
 also called Hadley cells
 winds deflect to the right in the
Northern hemisphere
Intertropical Convergent Zone (ITCZ)
 Area where the surface winds of
the two hemispheres converge
 hot, humid, little to no wind present
 also called the doldrums
Trade Winds
 area between 30o and 0o in each
 steady winds
 Location where merchants wanted to
sail their ships
Effect of Global Wind Patterns
 Precipitation patterns are effected
 Deserts at mid-latitudes & on leeward
side of mountains