GEOG 1112: Weather and Climate

Atmospheric Pressure, Wind, & Global Circulation

Air Pressure

• Weight of air exerting pressure on surface of Earth

• Air pressure decreases with altitude

• Warm air is lighter than cool air, so it tends to rise – cool air sinks

Vertical and horizontal pressure differences that lead to

atmospheric motions.

Air Pressure & Altitude

Lower Density,Lower Pressure

Higher Density,Higher Pressure

Pattern of pressure: a non-linear decrease with height

Why?

Vertical Changes in Pressure

Atmospheric Pressure Systems

High-pressure system-also called anticyclone-circulating body of air-descending air-clockwise circulation innorthern hemisphere

Low-pressure system-also called cyclone-circulating body of air-rising air-counterclockwise circ. in northern hemisphere

Atmospheric Pressure Map

Isobars – lines of equal air pressure on a map

Direction of Air Flow• Unequal heating of land surfaces

• Pressure gradient force – air flows from high to low

• Coriolis force – deflection or change in direction caused by Earth’s rotation

• Frictional forces – places a drag on that air flow

Isobars and Pressure Gradient

• The closeness of isobars decides the gradient between high and low pressure

• The closer the isobar, the higher the winds and vice versa

• Winds at the surface always blow at right angles to isobars

Pressure Gradient

H L

isobars

1028

mb

1024

mb

1020

mb

1016

mb

1012

mb

1008

mb

1004

mb

1000

mb

Pressure Gradient Force

Coriolis Force•Due to Earth’s rotation

•Pulls wind to right in northern hemisphere

•Pulls wind to left in southern hemisphere

•Strongest at poles

•None at equator

Geostrophic Wind• Horizontal wind in the upper atmosphere that

moves parallel to isobars. Results from a balance between pressure gradient force and Coriolis force.

Low Pressure

High Pressure

500 mb

504 mb

508 mb

512 mbPGF

Frictional Forces

• Near surface, friction (F) works against pressure gradient force (PGF), so resulting wind direction is between pressure gradient force and coriolis force (CF)

Three Forces Combined

Rossby Waves

Figure 6.17

Jet Stream

Strong boundaries often occur between warm and cold air. In

the mid-latitudes, the polar front marks this thermal discontinuity

at the surface.

The Polar Front and Jet Streams

L L

H

H

Global Pressure & Atmospheric Circulation

• Unequal heating of tropics and poles

• Global circulation on non-rotating, uniform Earth

Global Circulation Model

Seasonal Changes in Circulation• ITCZ migrates with subsolar point

• Trade winds, STHs, westerlies, all follow the ITCZ north & south with seasons

Seasonal Changes in Circulation

• Monsoon

– Seasonal shift of prevailing wind dir. due to land/water contrasts

– Happens throughout subtropical regions

– Most significant in South & SE Asia

South Asian MonsoonWinter Monsoon•Cold air over Asia

•Sinking air/High pressure

•Cool, dry NE winds

•ITCZ far south

Summer Monsoon•Warm air over Asia

•Rising air/Low pressure

•Warm, moist wind – rain

•ITCZ swings north

Local Wind SystemsSea Breeze•Sun-heated air over land rises – Low pressure

•Cooler air over water sinks – High pressure

Land Breeze•Night air over land cools fast – High pressure

•Air over water stays warmer – Low pressure

Local Wind Systems – Mountain-Valley Breeze

Subtropical High-pressure Cells

Global Oceanic Circulation

Oceanic Circulation• Thermohaline Circulation – warm water in gulf stream

cools and evaporates as it heads north making it heavier – downwelling, then a long trip to the Pacific where upwelling occurs

ENSONormal El Niño