Physical state of atmosphere: Structure

• Pressure: 99% of all air is below ~30 km
• Temperature: Layered structure of the atmosphere
  • Troposphere
  • Stratosphere
  • Mesosphere
  • Thermosphere

Physics of Cloud Formation

• Humidity
  • a measure of the water vapor content of air, depends strongly on temperature.
  • if water vapor capacity ( Saturation Water vapor pressure) exceeded at a given temperature, condensation takes place.
  • relative humidity is the ratio of water vapor pressure to the saturation water pressure at that temperature
• Cloudiness
  • Adiabatic expansion : Rise of warm air (less dense) close to the ground, into low pressure at higher altitudes in the troposhere - cooling
  • Adiabatic compression: Cool dense air sinks to lower regions at higher pressure undergoes compression - heating
  • Dry Adiabatic Lapse Rate: 10 C/km. As air moves up and it gets supersaturated, condensation begins and releases energy (latent heat) which reduces the lapse rate to about 6° C/km.
  • Clouds form when air rises and becomes saturated with water vapor due to adiabatic cooling. All clouds are confined to the troposphere.

Why does the air move?

• Global differential heating : equatorial heating relative to polar regions
• Rising of air at equatorial regions and sinking in polar regions
• General aspects of the global circulation: Hadley cell convection
  • Single Cell convection
  • Three Cell Hadley Convection

Coriolis Effect: Effect of earth’s rotation on moving bodies

• N. Hemisphere - deflection to the right as seen in the direction of the windflow;
• S. Hemisphere - deflection to the left ( anticlockwise).

Global air circulation

• Trade winds
• Westerlies
• Polar Easterlies

Modifications to surface air circulation

• Seasonal temperature differences between oceans and land
• High heat capacity of water
• Summer - land heats up faster, drawing air from oceans -Monsoons
• Winter - oceans warmer, drawing wind from continents