Visualizing Physical Geography Copyright © 2008 John Wiley and Sons Publishers Inc. Chapter 5 Winds and Global Circulation Visualizing Physical Geography by Alan Strahler and Zeeya Merali

Visualizing Physical Geography Copyright © 2008 John Wiley and Sons Publishers Inc. Chapter Overview Atmospheric Pressure Local Wind Patterns Cyclones and Anticyclones Global Wind and Pressure Patterns Winds Aloft Ocean Currents

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 Measured with a barometer Average pressure at sea level can be expressed in different ways: 1 kilogram/cm 2 =15 pounds/in 2 = 101,320 Pa (pascals)=76 cm Hg (mercury)=29.92 in Hg= mb (millibars) 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 direction tracked with a wind vane Wind speed measured with an 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 Isobars: lines on a map drawn through all points having the same atmospheric pressure Pressure gradient: change of atmospheric pressure measured along a line at right angles to the isobars

Visualizing Physical Geography Copyright © 2008 John Wiley and Sons Publishers Inc. Local Wind Patterns Pressure Gradients develop because of unequal heating in the atmosphere Convection loop: circuit of moving fluid, such as air or water, created by unequal heating of the fluid

Visualizing Physical Geography Copyright © 2008 John Wiley and Sons Publishers Inc. Local Wind Patterns Local Winds Santa Ana winds Sea and land Breezes Mountain and valley winds

Visualizing Physical Geography Copyright © 2008 John Wiley and Sons Publishers Inc. 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

Visualizing Physical Geography Copyright © 2008 John Wiley and Sons Publishers Inc. 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

Visualizing Physical Geography Copyright © 2008 John Wiley and Sons Publishers Inc. 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 Cold temperatures  Siberian High intense in January Warm temperatures  Asiatic Low intense in July

Visualizing Physical Geography Copyright © 2008 John Wiley and Sons Publishers Inc. Global Wind and Pressure Patterns ITCZ and Monsoon Circulation Monsoon: seasonal reversal of winds Summer monsoon: warm, humid air moves onshore: heavy rains Winter monsoon: cold dry air moves offshore; dry conditions

Visualizing Physical Geography Copyright © 2008 John Wiley and Sons Publishers Inc. Global Wind and Pressure Patterns Wind and Pressure Features of Higher Latitudes The north pole is on water, the south pole is on land. Land-water differences influence pressure patterns. January: Northern Hemisphere  Highest pressure in Siberia and Canada Southern Hemisphere  Highest pressure in Antarctica July: Northern Hemisphere  Low pressure over continents Southern Hemisphere  High pressure in Antarctica

Visualizing Physical Geography Copyright © 2008 John Wiley and Sons Publishers Inc. 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

Visualizing Physical Geography Copyright © 2008 John Wiley and Sons Publishers Inc. 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

Visualizing Physical Geography Copyright © 2008 John Wiley and Sons Publishers Inc. Winds Aloft Global Circulation at Upper Levels: Weak easterlies at equator Tropical high-pressure belts Westerlies in the mid- and high- latitudes Low pressure at poles

Visualizing Physical Geography Copyright © 2008 John Wiley and Sons Publishers Inc. 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

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

Visualizing Physical Geography Copyright © 2008 John Wiley and Sons Publishers Inc. Ocean Currents Oceanic circulation: Differences in density and pressure Temperature differences Salinity differences Force of wind on surface water Energy transferred from wind to water by friction Deflected by Coriolis force 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

Visualizing Physical Geography Copyright © 2008 John Wiley and Sons Publishers Inc. 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