Visualizing Physical Geography Copyright © 2008 John Wiley and Sons Publishers Inc. Chapter 5 Winds and Global Circulation

Visualizing Physical Geography Copyright © 2008 John Wiley and Sons Publishers Inc. Atmospheric Pressure Atmospheric pressure: pressure exerted by the atmosphere because of the force of gravity acting upon the overlying column of air Atmospheric pressure at a single location varies slightly from day to day Barometer: instrument that measures atmospheric pressure

Visualizing Physical Geography Copyright © 2008 John Wiley and Sons Publishers Inc. Atmospheric Pressure Air Pressure and Altitude Atmospheric pressure decreases with altitude

Visualizing Physical Geography Copyright © 2008 John Wiley and Sons Publishers Inc. Local Wind Patterns Winds are identified by the direction from which the wind comes Wind vane? Anemometer?

Visualizing Physical Geography Copyright © 2008 John Wiley and Sons Publishers Inc. Local Wind Patterns Pressure Gradients Wind is caused by differences in atmospheric pressure from one place to another Air tends to move from regions of high pressure to regions of low pressure Pressure gradient: change of atmospheric pressure measured along a line at right angles to the isobars Isobars ?

Pressure Gradients develop because of unequal heating in the atmosphere

Local Wind Patterns Local Winds Santa Ana winds Sea and land Breezes Mountain and valley winds

Cyclones and Anticyclones Coriolis Effect: effect of the Earth’s rotation that acts like a force to deflect a moving object on the Earth’s surface to the right in the northern hemisphere and to the left in the southern hemisphere Acts at right angles to direction of motion Deflects to right in the northern hemisphere, left in the southern hemisphere No deflection at equator, maximum deflection at poles

Cyclones and Anticyclones Coriolis Effect Balance of Forces on a parcel of surface air: Pressure Gradient Coriolis Effect Strength increases with speed of motion Strength decreases with latitude Friction Exerted by ground surface Proportional to wind speed Acts opposite to direction of motion

Cyclones and Anticyclones Cyclone: center of low atmospheric pressure  Air spirals inward (convergence) and upward  Associated with cloudy, rainy weather Anticyclone: center of high atmospheric pressure  Air spirals downward and outward (divergence)  Associated with fair weather

Visualizing Physical Geography Copyright © 2008 John Wiley and Sons Publishers Inc. Global Wind and Pressure Patterns Hadley Cell: low- latitude atmospheric circulation cell with rising air over the equatorial trough and sinking air over the subtropical high- pressure belts Intertropical Convergence Zone (ITCZ): zone of convergence of air masses along the equatorial trough Subtropical high-pressure belts: belts of persistent high atmospheric pressure centered at about lat. 30º N and S Polar Front: front lying between cold polar air masses and warm tropical air masses

Visualizing Physical Geography Copyright © 2008 John Wiley and Sons Publishers Inc. Global Wind and Pressure Patterns ITCZ and Monsoon Circulation ITCZ and Hadley cells shift with the seasons Shift is very large in Asia

ITCZ and Monsoon Circulation Monsoon: seasonal reversal of winds

Winds Aloft  Pressure decreases less rapidly with height in warmer air than in colder air  There is a temperature gradient from the equator to the poles  So, at high altitudes there is a pressure gradient from the equator to the poles  strong winds at high altitudes

Winds Aloft The Geostrophic Wind Forces acting on an upper air parcel: 1. Pressure gradient force:  Moves from high pressure to low pressure 2. Coriolis force  Deflects to right in NH, to left in SH Pressure gradient force balances coriolis force  Wind blows parallel to isobars Geostrophic Wind: wind at high levels above the Earth’s surface blowing parallel with a system of straight parallel isobars

Winds Aloft Global Circulation at Upper Levels:

Winds Aloft Rossby Waves, Jet Streams, and the Polar Front Rossby waves: horizontal undulations in the flow path of the upper-westerlies; upper-air waves Polar front: zone where cold polar air meets warm tropical air Rossby waves: Arise in the polar front Contribute to variable weather in mid-latitudes

Flow of air along front smooth for days or weeks. Undulations begin and Become stronger. Rossby Wave forms. Warm air Pushed poleward, cold air south Waves stronger. Tongue of cold air Brough south and warm north. Tongue pinched off. Pool of cold air farther south than originally. Become cyclones of cold air Persist for days of weeks.

Visualizing Physical Geography Copyright © 2008 John Wiley and Sons Publishers Inc. Winds Aloft Jet streams: high-speed air flow in narrow bands within the upper-air westerlies and along certain other global latitude zones at high levels

Polar-front jet stream Westerly; associated with Rossby waves Subtropical jet stream Westerly; above subtropical high- pressure cells Tropical easterly jet stream Runs east to west Summer only Develops in Asia

Ocean Currents Oceanic circulation: Currents exchange heat between high and low latitudes Ocean current: persistent, dominantly horizontal flow of water

Visualizing Physical Geography Copyright © 2008 John Wiley and Sons Publishers Inc. Ocean Currents Large-Scale Circulation of Ocean Waters

Ocean Currents Current Patterns During an El Niño event: Upwelling along Peruvian coast ceases Trade winds weaken Weak equatorial eastward current develops

Moisture and temperature patterns alter