Chapter 5: Atmospheric Pressure and Wind

The Impact of Pressure and Wind on the Landscape Atmospheric pressure indirectly affects the landscape Changes manifest primarily by changes in wind and temperature Wind has a visible component to its activity Severe storm winds can drastically affect the landscape Figure 5-2

The Nature of Atmospheric Pressure Gas molecules continuously in motion Force exerted by gas molecules is called atmospheric pressure Force exerted on every surface the gas touches Pressure is approximately 14 lbs per square inch Figure 5-1

The Nature of Atmospheric Pressure Factors influencing atmospheric pressure Density At higher density, particles are closer & collide more frequently, increasing pressure Temperature Warmer particles move faster & collide more frequently, increasing pressure Figure 5-3 Figure 4-15

The Nature of Atmospheric Pressure Figure 5-4 Measurement of Pressure Barometer – device to measure atmospheric pressure Average Sea Level Pressure 1013.25 MILLIBARS 29.92 INCHES OF HG (MERCURY) 14.7 LBS / SQ. INCH 760 MM OF HG No matter which unit of measurement is used, pressure ALWAYS decreases with altitude Mapping pressure with isobars Isobar: line joining points of equal pressure Pressure gradient: horizontal rate of pressure change Directly affects wind speed

The Nature of Wind Wind: horizontal air movement Origination of wind Figure 5-5 Wind: horizontal air movement In theory, why does air move? Energy imbalances on globe Why does air actually move? Differences in horizontal atmospheric pressure Origination of wind Uneven heating of Earth’s surface creates temperature & pressure gradients Direction of wind results from pressure gradient Winds blow from high pressure to low pressure Winds are named for the direction they blow from!!!

The Nature of Wind Forces which govern the wind Pressure gradient force Characterized by wind moving from high to low pressure, always Winds blow at right angles to isobars Coriolis force Turns wind to the right in the N Hemisphere, left in S Hemisphere Only affects wind direction, not speed, but faster winds turn more Friction Wind is slowed by Earth’s surface due to friction, does not affect upper levels Figure 5-7

The Nature of Wind Force balances Geostrophic balance Balance between pressure gradient force and Coriolis Winds blow parallel to isobars Frictional balance Winds blow slightly towards low pressure and slightly away from high pressure Winds slowed by friction weaken Coriolis, so pressure gradient force is stronger and turns the winds Figure 5-6

The Nature of Wind Anticyclones and cyclones Figure 5-8

The Nature of Wind Cyclone: low-pressure cell Air converges & ascends Surface convergence & low pressure indicate rising motion Rising motion results in clouds and storms Anticyclone: high- pressure cell Air descends & diverges Surface divergence & high pressure indicate sinking motion Sinking motion results in sunny skies Figure 5-9

The Nature of Wind Wind speed Determined by pressure gradients Tight pressure gradients (isobars close together) indicate faster wind speeds Wind speeds are gentle on average Figure 5-10 Figure 5-11

The Nature of Wind

Vertical Variations in Pressure & Wind Atmospheric pressure decreases rapidly with height Atmospheric surface pressure centers lean with height Winds aloft are much faster than at the surface Jet streams

The General Circulation of the Atmosphere Atmosphere is in constant motion Major semi-permanent conditions of wind and pressure—general circulation Principal mechanism for longitudinal and latitudinal heat transfer Second only to insolation as a determination for global climate

The General Circulation of the Atmosphere Simple example: Non-rotating Earth Strong solar heating at equator Low pressure forms over equator Ascending air over equator Little heating at poles High pressure forms over poles Descending air over poles Winds blow equatorward at surface, poleward aloft Figure 5-12

The General Circulation of the Atmosphere If Earth didn’t rotate Because Earth rotates

The General Circulation of the Atmosphere Observed general circulation Earth’s rotation increases complexity of circulation Hadley cells Complete vertical circulation cells where warm air rises, cools, moves poleward, & subsides Around 30° N/S Figure 5-14 Figure 5-13

The General Circulation of the Atmosphere 7 Surface Components Intertropical convergence zone (ITCZ) (0°) Trade winds (easterlies) (0° - 30° N/S) Subtropical highs (30° N/S) Westerlies (30° - 60° N/S) Subpolar low (polar front) (60° N/S) Polar easterlies (60° - 90° N/S) Polar highs (90° N/S)

The General Circulation of the Atmosphere Components of the general circulation Subtropical highs (STH’s) Persistent zones of high pressure near 30° N/S Result from descending air in Hadley cells Regions of world’s major deserts No wind AKA horse latitudes Source of 2 of the 3 major wind systems: trade winds & westerlies Figure 5-16

The General Circulation of the Atmosphere Winds are named for the direction they blow from! Trade winds (easterlies) Diverge from equator-ward side of STH’s About 30°N to 30°S SE in S Hemisphere, NE in N Hemisphere Blow east to west Most reliable/consistent of wind systems AKA Winds of commerce Figure 5-17

The General Circulation of the Atmosphere Trade winds (cont.) Heavily laden with moisture Do not produce rain unless forced to rise If they rise, they produce tremendous precipitation and storm conditions Figure 5-20

The General Circulation of the Atmosphere Intertropical Convergence Zone (ITCZ) Zone of rising air from surface heating & convection Zone of low pressure & convergence toward which trade winds blow Near equator Constant rising motion & storminess in this region Position seasonally shifts (more over land than water) AKA Doldrums Figure 5-21

The General Circulation of the Atmosphere Westerlies Form on poleward sides of STH’s Wind system of the midlatitudes Blow west to east 2 cores of high winds – jet streams Subtropical Jet Polar Jet Rossby waves Figure 5-22 Figure 5-24

The General Circulation of the Atmosphere Figure 5-27 Polar highs Develop over poles due to extensive cold conditions Anticyclonic winds Strong subsidence Arctic desert Polar easterlies Regions north of 60°N & south of 60°S Winds blow east to west Cold & dry

The General Circulation of the Atmosphere Polar front Low pressure around 60° N/S Air mass conflict between warm westerlies & cold polar easterlies Rising motion = clouds & precipitation Very strong winds over oceans Polar jet stream position typically coincident with the polar front AKA subpolar low Figure 5-25

The General Circulation of the Atmosphere 7 components of the general circulation Figure 5-26

The General Circulation of the Atmosphere Vertical wind patterns of the general circulation Most dramatic differences in surface and aloft winds is in tropics Antitrade winds Blow from SW in N Hemisphere, & from NW in S Hemisphere Figure 5-28

Modifications of the General Circulation Seasonal modifications 7 general circulation components shift seasonally Components shift northward during Northern Hemisphere summer Components shift southward during Southern Hemisphere summer Figure 5-29

Modifications of the General Circulation Monsoons Seasonal wind shift Most significant disturbance of general circulation Offshore: wind movement from land to water Onshore: wind movement from water to land Seasonal Winds Wet summer Dry winter Develop due to: Shift in position of ITCZ Unequal heating of land & water Figure 5-31 Figure 5-30

Modifications of the General Circulation Major monsoon systems Figure 5-32

Localized Wind Systems Figure 5-34 Convectional circulation from differential heating of land & water Sea breezes Water heats more slowly than land during the day Lower pressure over land, higher pressure over sea Wind blows from sea to land Land breezes At night, land cools faster Higher pressure over land, lower pressure over sea Wind blows from land to sea

Localized Wind Systems Convectional circulation from differential heating of higher vs. lower elevations Valley breeze Mountain top during the day heats faster than valley, creating lower pressure at mountain top Upslope winds out of valley Mountain breeze Mountain top cools faster at night, creating higher spressure at mountain top Winds blow from mountain to valley, downslope Figure 5-35

Localized Wind Systems Katabatic winds Cold winds that originate from cold upland areas Descend quickly down mountain, can be destructive Chinook winds Warm downslope winds H pressure on windward side, L pressure on leeward side Warms adiabatically as it moves down leeward slope of Rocky Mountains Figure 5-36

Localized Wind Systems Santa Ana winds Hot, dry, high speed winds Occur in S. California from Oct to Mar Spread wildfires in dry seasons

El Niño-Southern Oscillation Warming of waters in the eastern equatorial Pacific Associated with numerous changes in weather patterns worldwide Typically occurs every 3 to 7 years for about 18 months El Niño Reference to the Christ Child 1st signs usually appear around Christmas Figure 5-37

El Niño-Southern Oscillation Circulation patterns Normal El Nino Upwelling: winds blowing offshore on W side of continents pushes water westward & brings up cold, nutrient-rich water from below Figure 5-38

Upwelling Phytoplankton shows where upwelling occurs

El Niño-Southern Oscillation Figure 5-40 Patterns associated with El Niño Teleconnections Weather effects La Niña—opposite of El Niño Circulation is similar to “normal” Causes of El Niño Which changes first, the atmosphere or the ocean?

El Niño-Southern Oscillation Sea Levels & Sea Surface Temperatures during normal, El Niño & La Niña years (http://www.pmel.noaa.gov/tao/) Figure 5-39 Figure 5-C Figure 5-D Figure 5-E

El Niño-Southern Oscillation Forecasting El Niño Array of 70 buoys – Galapagos Islands to New Guinea Forecasts? Figure 5-B

Summary Atmospheric pressure and wind affect the geographic landscape in several ways Atmospheric pressure is the force exerted by air molecules on all objects the air is in contact with Pressure is influenced by temperature, density, and dynamic Isobars show areas of high pressure and low pressure Vertical and horizontal atmospheric motions are called wind Wind is affected by many forces Geostrophic balance represents a balance between the Coriolis force and the pressure gradient force Friction slows the wind and turns it towards lower pressure Wind patterns around high and low pressure systems are anticyclonic and cyclonic, respectively Areas of divergence at the surface are associated with sinking motion, convergence at the surface with rising motion Close isobar spacing indicates faster winds

Summary Winds increase rapidly with height, pressure decreases rapidly with height The global atmospheric circulation is called the general circulation There are seven components to the general circulation Each component has associated weather conditions Seasonal modifications to the general circulation exist, including monsoons Localized wind systems affect wind direction locally on diurnal time scales El Niño is a warming of eastern equatorial Pacific water and subsequent switching of the high and low air pressure patterns El Niño is associated with varied weather patterns in different locations globally