• Earth is heated unevenly by the Sun but heat wants to be in equilibrium
• Atmospheric circulation is driven by redistribution of planetary heat
• Atmospheric circulation drives ocean circulation and regional climates

Mechanisms of heat transfer

• Conduction: Transfer of heat via direct contact, heat flows from the warmer to cooler body until both have the same temperature
• Convection: Heat is carried by physical movement of a fluid (air, water etc.) from high temperature region to a low temperature region
• Radiation: Any object with a temperature radiates heat with electromagnetic radiation the Earth loses heat this way -- infrared

Density of air

• Density affects the position the air will have in the layers of the atmosphere i.e. will it rise or sink
• Temperature and water content of air affect its density
• Warm air is less dense than cold air
• Convection cell: Warm air rises, when it is cooled it sinks back down again
• Humid air is less dense than dry air because the water vapor displaces nitrogen and oxygen that are heavier.
• Warm air can contain more water vapour than cold air
• warm, wet air will rise in the atmosphere
• cold, dry air sinks in the atmosphere

• The weight of a 1 inch square column of air stretching from the Earth's surface to the top of the atmosphere is ~ 15 pounds
• atmospheric pressure is ~ 15 pounds per square inch.
• 75% of the atmosphere is below 11 km (~7 miles)
• Higher pressure near the surface of the Earth lower pressure as go higher in the atmosphere.
• When air rises higher in the atmosphere its pressure gets lower (it expands) and it gets colder
• When air sinks in the atmosphere its pressure goes up (it is compressed) and it gets warmer

High pressure zones: air is denser and is descending

Low pressure: air is lighter and is rising

• When warm wet air rises, it cools, cooler air cannot hold as much water vapour => rain
• Large amounts of Latent Heat are released when water vapour turns to rain--milk heater on capuccino maker
• Tropical depressions form over warm (>26°C) ocean water
• Latent heat release causes air to rise faster, storm intensifies-- positive feedback loop -- out of control
• Storm only loses energy when over cold water or land
• When cold, dry air sinks, it gets warmer and dries out drying winds (salt ponds Kauai)

• Most heat at the equator, ocean and land get hot heats the air above it by conduction
• Warm air rises, cools and sinks again -- circulation
• Why does it cool? It expands, it radiates heat, also no longer in contact with the ground, no warming
• Air rises at the Equator -- Inter-Tropical Convergence Zone (ITCZ)
• Equatorial air flows to ~30^o N then sinks to the surface and returns as a surface flow to the tropics -- Hadley cell
• Air sinks at the North Pole because it is cold. It flows S at the surface and is warmed by contact with land/ocean, by ~60^o N it rises into the atmosphere -- Polar cell
• Between 60^o N and 30^o N there is another circulation cell -- Ferrel cell

• Coriolis affects the direction of winds and currents
• Coriolis occurs because the Earth is a sphere rotating around its pole
• Earth rotates about its axis every 24 hours
• Distance around the equator is ~25,000 miles travelling east at ~ 1,000 miles per hour
• Distance around the Earth at 40^o N (Buffalo, NY) ~19,000 miles travelling east at ~800mph
• Stand at the Equator and throw an object due North, as well as northward velocity it is also travelling East at 1,000 mph. This is faster than the Earth is rotating further North, so the object appears to turn to the East, or to the right
• An object thrown due south from 40^o N will appear to turn to the west, or to the right.
• In the Northern hemisphere, Coriolis force deflects movement to the right.
• In the Southern hemisphere, Coriolis force deflects movement to the left
• Combination of atmospheric cells and Coriolis force yield the wind belts
• Wind belts drive surface ocean circulation!