GEOG 1112: Weather and Climate Atmospheric Pressure, Wind, & Global Circulation

Air Pressure Weight of air exerting pressure on surface of Earth Vertical and horizontal pressure differences that lead to atmospheric motions. 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

Air Pressure & Altitude Lower Density, Lower Pressure Higher Density, Higher Pressure

Vertical Changes in Pressure Pattern of pressure: a non-linear decrease with height Why?

Atmospheric Pressure Systems High-pressure system -also called anticyclone -circulating body of air -descending air -clockwise circulation in northern hemisphere Low-pressure system -also called cyclone -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

H L Pressure Gradient isobars 1028 mb 1024 mb 1020 mb 1016 mb 1012 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 mb PGF

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

The Polar Front and Jet Streams Strong boundaries often occur between warm and cold air. In the mid-latitudes, the polar front marks this thermal discontinuity at the surface.

Global Pressure & Atmospheric Circulation Unequal heating of tropics and poles Global circulation on non-rotating, uniform Earth L L H

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 Monsoon Summer Monsoon Winter Monsoon Warm air over Asia 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 Systems Sea 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

ENSO Normal El Niño