Atmospheric Circulation & Weather
Chap 8
Composition of the Atmosphere
• • • • • Nitrogen – 78.084 % Oxygen – 20.946 % Argon – 0.934 % Carbon Dioxide – 0.033 % Other gases – 0.003 %
GLOBAL HEATING DRIVES ATMOSPHERIC CIRCULATION 1. THE EARTH IS HEATED UNEVENLY Lower latitudes are heated more intensely than higher latitudes, so there is a surplus of heat at low latitudes
2. THE ATMOSPHERE IS HEATED FROM BELOW The atmosphere is warmed by heat radiating from the earth's surface. The atmosphere is transparent to most solar radiation, so little of the sun's energy heats it directly.
3. THE RESULT: THE ATMOSPHERE CIRCULATES IN CONVECTION CELLS. Convection transfers heat from low latitudes (where there is a surplus) to higher latitudes BUT this process is complicated by the fact that the earth rotates.
Global Circulation
• Heat Expand, Cooling Contracts Atmosphere ( 8.1) • 1. Sun Radiation Greater In The Equator Air Heat And Rise • 2. Poles - Air Is Contracting And Descending • 3. Convectional Circulation - Low Pressure Equator, High Poles
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Coriolis Effect
• Deflection Of Object Due To The Earth Rotation (8.11) • 1. Right In The N. Hemisphere (clockwise) • 2. Left In The S. Hemisphere (counterclockwise) • 3. No Deflection In The Equator
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Review the Concepts
• List the most common gases of the atmosphere in order of abundance. Describe what causes uneven heating of the earth. What is the Coriolis effect?
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Atmospheric Circulation
• 1. Six circulation cells ( 8.12) • 2. Equatorial - Doldrums, Low, Cools, Condense, Precipitation • 3. Westerlies - N & S - from West • 4. Trade Winds - N & S • 5. Easterlies - Polar Areas - N & S - from East
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• In between the pressure belts, where atmospheric circulation is parallel to the surface, are THREE wind belts • Note: The wind direction is given as the direction from which the wind is coming. WEST or WESTERLY winds is FROM the west, blowing from west to east. • EAST or EASTERLY wind is FROM the east, blowing from east to west.
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• The Landmasses - Surface Winds Form Elliptical Cells • Seasonal Temperature Variation (8.16) • Warm In The Summer - Low Pressure In Land • Cold In Winter - High Pressure In Land • The uneven distribution of heat and moisture creates the earth biomes
Trade Winds
• 5 ° - 30 ° N and S • moist atmosphere • at least 500 km away from equator • Not affected by Coriolis effect • Northeasterly in the Northern Hemisphere
The Westerlies
• 35 ° - 60 ° N and S
• Related to the divergent flow off of the poleward side of Subtropical High • Can also be predicted by Coriolis Effect on circulation cells
Polar Easterlies
• 65 ° - 85 ° N and S
• Not as persistent as other two wind belts • Direction and speed more variable • Strongly influenced by storms at Polar Front
Review the Concepts
• • Describe what causes the seasons. Name the surface winds of the Earth.
ATMOSPHERE - OCEAN INTERACTIONS
• 1. Hydrologic cycle Evaporation-Condensation-Precipitation involves moisture that originates in the oceans.
2. Surface ocean currents follow the wind pattern above
3. Monsoons(8.15) • seasonal reversal of wind direction caused by difference in thermal properties of land and water • Areas have wet summers and dry winters • Linked to the intertropical convergence zone (ITCZ) • Affects Africa, Asia, India
Land - Sea Breeze
• Atmospheric Circulation - Spatial Variations In Air Temperature (8.16) • 1. Sea Breeze - Land Heat Up, Air Heat Up And Expand - Low pressure • a. High Pressure Over The Water Causes Circulation
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• 2. Land Breeze - Air Over Land Cools Rapidly - Hi Atmospheric Pressure • a. Circulation Moves Air From The Land Toward The Sea
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Hurricanes (Typhoons)
• Large low pressure system with cyclonic circulation (clockwise in the northern hemisphere) • Wind speeds greater than 74 mph (at lower speeds, the low pressure systems are called tropical storms or tropical depressions)
• Necessary conditions for formation • moist atmosphere--high relative humidity • at least 500 km away from equator so that the Coriolis force is strong enough to produce rotation • warm ocean (80 degrees F or above)
• Hurricanes are seasonal (late summer and early fall) because they need heat from ocean to sustain the energy of the storm. • Hurricanes lose energy over land and cold water.
HURRICANES
• Necessary conditions • Formed from thunderstorms • Ocean temperature greater than 80 degrees F • More than 500 km from the Equator
• Related factors • High relative humidity in lower and middle troposphere • Low wind shear • Convection Growth Process
CISK
• Convective Instability of the Second Kind. • A positive feedback mechanism: 1. Surface convergence causes air to rise. 2. Air rises and cools, and condensation occurs.
3. Condensation releases latent heat into atmosphere, thus warming the atmosphere. 4. Warmed air expands. 5. Surface pressure decreases, and a low pressure center develops. 6. Decreased surface pressure increases the winds.
7. Increased winds reinforces surface convergence and the cycle repeats again.
• The storm builds strength with every cycle. What stops the process? • Cool water or Strong wind shear or Land
Hurricane Stages
• 1. Tropical Depression Sustained winds of 23-39 mph One closed isobar Slight organization
2. Tropical Storm Winds 39-73 mph Organized, circular in shape Rotation apparent Assigned a name at this stage Damage: Heavy rainfall (5-10 inches)
3. Hurricane Winds 74 mph or higher; classified by Safir-Simpson scale (1-5) Distinct eye and spiral bands Pronounced rotation
Important features
• 1. Eye 12-30 miles in diameter center of storm lowest pressure not destructive • clear sky, light winds • winds deflected away from the eye by the Coriolis force
• 2. Eye Wall most devastating portion with high winds and heavy rain
• latent heat production maximized
• 3. Spiral Bands 60-120 miles in length radiate outward from eye wall
• rain free areas can occur between bands
Review the Concepts
• • Contrast Land Breeze and Sea Breeze Contrast the following phenomena:
– Hurricanes – Typhoons – Cyclones
Damage
1. Storm surge causes flooding in coastal areas 2. Strong winds 3. Heavy rains cause flooding in inland and coastal areas 4. Tornadoes (occasional)
Movement
• Path depends mainly on the wind belt in which hurricane is located. • 5° to about 30° moves westward, pushed by easterly • Trade Winds Weakens slightly as it crosses the Subtropical High pressure belt • 30° to 60° moves eastward, pushed by west winds of Prevailing Westerlies