19.1

Ch 19 Air Pressure & Wind AIR PRESSURE = the pressure exerted by the weight of the atmosphere above - equal in all directions - measured by a barometer, units are millibars - at sea level, the air pressure is~1kg/cm2 - decreases as you increase altitude - change in mercury level of a barometer = change in weather

Wind Wind is the result of horizontal differences in air pressure air moves from areas of high pressure to low pressure causing wind These different areas of high and low pressure are caused by: unequal heating (solar radiation) of the Earth’s surface

Wind speed is measured by an anemometer Wind direction is measured by a wind vane

WIND DIRECTION – CONTROLLED BY 3 THINGS - Pressure differences Coriolis Effect Friction

WIND DIRECTION- PRESSURE DIFFERENCES the greater the difference in air pressure between 2 regions, the greater the wind speed Specific weather maps indicate pressure differences across a region (Pressure Gradient) Isobars are lines that connect places of equal air pressure

PRESSURE DIFFERENCES cont’d Closely spaced isobars indicate high wind speeds Widely spaced isobars indicate light wind speeds

WIND DIRECTION – CORIOLIS EFFECT - Coriolis Effect describes how Earth’s rotation affects moving objects & wind All free-moving objects are deflected to the RIGHT of their path of motion in the Northern hemisphere and to the LEFT in the Southern hemisphere affects only wind direction the stronger the wind speed, the greater the deflection WIND DIRECTION – CORIOLIS EFFECT

Wind Direction Coriolis Effect cont’d

Wind Direction - cont’d Friction as air travels over rough terrain, the wind slows down changes wind direction only important within a few kilometers of the surface High above friction zone = Jet Stream = fast moving streams of air that travel between 120 to 240 km/hr flow West to East in Northern Hemisphere

19.2

Pressure Centers Most weather predictions are based on locations of high & low air pressure zones

Pressure Centers- LOW - Low Pressure Zones or Cyclones = cloudy conditions and precipitation Pressure decreases toward the center of a low pressure zone Wind blows inward and counterclockwise around a low - Because wind blows inward, air must rise to make room for incoming wind = clouds form

Pressure Centers cont’d - High Pressure Zone or Anticyclones = clear skies and fair weather Pressures increases toward the center Winds blow away and counterclockwise from a high pressure center Air sinks in a high pressure zone = no clouds form

Global Wind Patterns Because more solar radiation is received at the equator than at the poles, global wind patterns act to balance heat on Earth = warm air blows toward high latitudes and cool air flows toward equator Because the Earth rotates, above system is more complex = 3 major wind cells in each hemisphere

GLOBAL WIND PATTERNS 4 Major Wind Cells Equatorial Low Subtropical High Subpolar Low Polar High

Global Wind Patterns cont’d EQUATORIAL LOW- air is heated at equator, it rises (producing abundant moisture) until it reaches the tropopause, then air is deflected toward the poles

Global Wind Patterns cont’d SUBTROPICCAL HIGH – the air that was raised at the equator & deflected descends at 20-30N & S latitude (subtropics), the dry air cools (at altitude)& sinks -creating most of the world’s deserts

Global Wind Patterns Cont… SUBTROPICAL HIGH - some air flows toward the tropics (equator)via trade winds BUT some also travels toward the poles (again its deflected by Coriolis Effect)

Global Wind Patterns… cont’d - The wind traveling toward the poles generate the Prevailing Westerlies of the middle latitudes (Our wind in Indiana almost always blows from West to East because of this wind pattern)

Global Wind Patterns… cont’d - POLAR HIGH 90°= cold polar air sinks and flows along the surface toward 60N&S latitude (Subpolar Low), and is again deflected by Coriolis Effect - creates the Polar Easterlies, that bring cold polar air to Alaska & Canada

Global Wind Patterns… cont’d - SUBPOLAR LOW 60 - 60 latitude is VERY stormy area because of rising air & the collision of warm (prevailing westerlies) and cold air masses (polar easterlies) creating an area known as the POLAR FRONT

LOCAL WINDS Local Wind Patterns Two types of local winds: Land & Sea Breezes Valley & Mountain Breezes Local Wind Patterns local winds are caused either by topographic effects or by variations in surface composition – land and water –in the immediate area

SEA BREEZE – develop shortly before noon & reach greatest intensity during mid to later afternoon Form due to warm air rising over land (lower pressure) causing cooler air over the water (higher pressure) to move in toward the land Results in moderating temperature on land

LAND BREEZE – develop in the evening Form due to land cooling off more rapidly than water. Cool air over the land sinks (higher pressure) causing air to flow toward warmer waer (lower pressure) Can occur near oceans or along the shores of large lakes, such as the Great Lakes

Local Wind Patterns Valley & mountain breezes – similar to land & sea breezes but occur in mountainous regions During the day – air along the slopes of the mountain is heated more intensely than the valley floor Causing warmer air on slopes to rise creating valley breeze

Results in cumulus clouds developing along mountain peaks After sunset pattern is reversed Slopes cool more rapidly causing the more dense air to move down into the valley creating a mountain breeze

Monsoon Continents break up ocean surface, so global wind belts are not continuous = large seasonal temperature differences Large continents like Asia get very cold during winter and create a high pressure zone = air flow goes from continent toward ocean Winter is extremely dry and cold

During summer Asia gets very hot, creates a low pressure zone = air moves from ocean toward continent This complete seasonal wind reversal is called a Monsoon summer brings very wet and hot weather from Indian Ocean Winter is extremely dry and cold

El Nino = unusually warm ocean currents (particularly the central and eastern Pacific Ocean) that creates abnormal weather patterns - Reversal of normally cold currents & their direction - causes very wet winters in Gulf Coast & very warm winters in western US & Canada

La Nina = opposite of El Nino - occurs when surface ocean temperatures are significantly colder than average, creates abnormal weather patterns - Very cold winters to Pacific Northwest & Great Plains, warmer winters for rest of US - Increases hurricane frequency