Atmospheric Pressure and Wind Chapter 5

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Air Pressure

• The force exerted by gas molecules on some area of the Earth’s surface or any other body

• Air pressure is the weight of overlying air. • Compressed by the air above • Pressure is highest at sea

level and decreases as altitude increases

• Pressure is exerted omnidirectional

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The Driving Forces of the Atmosphere• Three key factors • Pressure gradient force

• High and Low pressure areas because the Earths surface is heated unequally

• Air wants to move from higher to lower pressure

• Pressure displayed using isobars

• Wind is the horizontal movement of air

The Coriolis Effect• All things that move on the surface of the Earth or in the

atmosphere appear to drift sideways as a result of the Earth’s rotation

• Objects deflect to the right in the northern hemisphere and to the south in the southern hemisphere

• Deflection strongest at the poles and weakest at Equator • The faster the object, the more it is deflected • Influences direction of movement, not speed Winds and ocean currents are affected by the Coriolis Effect

• Friction – Earth’s surface slows wind

movement – Up to 1000 feet – Reduced wind speed which

reduces coriolis effect

Atmospheric Patterns of Movement

• Rotation of Earth and varied surface causes complex system of circulation

• Basic pattern – Intertropical Convergence zone – Trade winds – Subtropical High – Westerlies – Polar front – Polar easterlies – Polar high

• Intertropical Convergence Zone – Zone where air from north and south meet – Little, low power, erratic wind – Low pressure zone with high rainfall

• Hadley Cells - prominent tropical circulation centered on the equator – Warm air rises and cools, causes thunderstorms – Cooled air then spreads north and south where it

forms bands of high pressure at the surface

• Trade Winds – 25o North and South of Equator – From Subtropical Highs – Predominately Easterly – Most reliable (consistent speed and direction) – Evaporate massive amounts of moisture – Potential for storms and precipitation

• Subtropical Highs – Large semipermanent high pressure cells – Off the west coasts of continents – Weather nearly always calm, warm sunshine – Most of the world’s major deserts – Wind diverges from STH north and south

• Westerlies – West to east wind flow between 30o and 60o

latitude north and south – Less consistent than trade winds

• Jet stream - high altitude, high speed wind – 30,000 to 40,000 alititude – 140 to 700 MPH – Headwind

• Polar Front – Conflict zone for cool polar and warm westerlies – 50-60o latitude – Low pressure zone

• Polar Easterlies – Dry, cold winds – 60o latitude

• Polar High – High pressure cell – More permanent in Antarctic – Anticyclonic – Causes polar easterlies

Seasonal Variations in Locations• Monsoons

– Most significant disturbance in the pattern of general circulation

– Seasonal reversal of winds – Distinctive seasonal precipitation – Heavy summer rains, dry winter season – Caused by movement of the ITCZ

Localized Wind Systems

• Sea and Land breezes – Caused by differential heating of land and sea – Can be strong but rarely travels very far inland – Evening land breeze normally weaker

• Santa Ana Winds – High Pressure cell over Western United States – High, dry, warm winds to the coast – Ideal conditions for wildfires

El Nino/Southern Oscillations• Atmospheric and oceanic phenomena of the

equatorial Pacific • Abnormally warm water off the west coast of

South America • Global impact

– Heavy rainfall, sever droughts, blizzards, tornados,

• La Nina – Opposite of El Nino – Waters off South America unusually cool – Stronger trade winds – Western United States drier than usual