Atmospheric Pressure and Winds

Chapter 19

Air Pressure

• The weight of the atmosphere per unit area.– 1kg per cm² at sea level– 14.7 lbs/in² at sea level

• Directed equally in all directions• Barometer

– 76cm / 30in at sea level– given in metric unit of millibars (mb)

Barometer

• Instrument that measures air pressure

• Mercury Barometer– Requires a tall

column of mercury

• Aneroid Barometer– Uses no liquid

Mercury Barometer

Air Pressure Records

Why Air Pressure Changes

• Temperature Changes– warm air is lighter than cold air

• Changing Humidity – the more water vapor, the lighter the

air– water vapor pushes out heavier O2 and

N2 of the dry air

What Makes Wind Blow

• Pressure Gradient Force– the difference in air pressures that

cause winds

• Local Winds– Sea Breeze: daytime wind from sea,

land heats faster than water and causes air to rise

– Land Breeze: nighttime wind from land to sea, water is warmer than land

Local Winds: Sea Breeze

Local Winds: Land Breeze

Earth’s Rotation and Weather Systems

• Coriolis Effect– Turns wind to the

right

• Clockwise – Out of HIGH

pressure

• Counterclockwise– Into LOW pressure

High Pressure Center

Low Pressure Center

Global Air Currents

Air Masses and Fronts

Chapter 20

Air Masses

• Huge section of air that has the same weather throughout

• Take on the characteristics of the area they originated over

• maritime tropical – mT• continental tropical –

cT• maritime polar – mP• continental polar - cP

Air Mass Source Regions

Fronts

• Boundary between 2 air masses

• Causes warm air to rise– Where

temperatures and humidity change

Warm Front: Map View

• Slow moving• Long lasting rain• Cirrus to stratus

clouds, then nimbostratus

Warm Front: Side View

Warm Front: Side View

Cold Front: Map View

• Fast moving• Unstable air• Cumulus –

cumulonimbus clouds

• Thunderstorms• Clears quickly and

leaves clear skies

Cold Front

Cold Front

Occluded Front: Map View

• When a cold front overtakes a warm front

• Form low pressure systems

Occluded Front: Side View

Warm Air

Cold AirCold Air