Water vapor is the most important gas in the atmosphere with regard to atmospheric processes. Water vapor is the source of all condensation and precipitation.
Freezing - liquid -> solid
Melting - solid -> liquid
Evaporation - liquid -> gas
Condensation - gas->liquid
Sublimation - solid -> gas
Deposition - gas->solid
Water is unique in that the temperatures at which it changes state occur commonly in the atmosphere and on Earth's surface.
Latent Heat: heat energy not associated with a temperature change.
Humidity: amount of water vapor in the air, two expressions: relative humidity and specific humidity
Saturation: equilibrium is established between two opposite state changes so that they occur at the same rate and the air holds the maximum amount of water possible at that temperature.
Vapor Pressure: contribution to air pressure of the atmosphere that is caused by water vapor.
Specific humidity: weight of water vapor / weight of air
Relative humidity: ratio of air's actual vapor content to its capacity
Factors that affect relative humidity: addition or subtraction of moisture to or from the air, air tempoerature - a decrease in the temperature results in an increase in relative humidity.
Dew point temperature to which air would have to be cooled to reach saturation
Cloupds are not water vapor. They are liquid droplets too tiny to fall to the Earth as rain.
Psychrometer is made of two identical thermometers side by side. A dry bulb measures the actual temperature and a wet bulb (wrapped with water soaked muslin) measures a lower temperature caused by the loss of heat required to evaporate the water. The difference of the two thermometers is interpreted using a data table to determine relative humidity and dew point.
Condensation of water takes three forms: fog, dew and clouds.
Radiation cooling occurs at Earth's surface as the temperature drops and heat is radiated away.
When energy is used to compress air, the motion of the gas molecules increases and the temperature of the air rises.
Expanding gases cool because they do work on the environment and lose energy.
In both these examples heat is not added or subtracted even though the gas changes temperature - this is the meaning of adiabatic.
Dry adiabatic rate: ascending air expands and cools as atmospheric pressure drops. Unsaturated air cools at 10 oC per 1000 meters.
Air is stable or unstable ddepending on its temperature compared to the temperature of the surrounding air. By comparing the environmental lapse rate with the adiabatic rate it is possible to determine the stability of a rising parcel of air.
Stable air resists vertical movement. Nevertheless there are circumstances that can force stable air to rise. This results in thin widespread clouds and possible light to moderate precipitation.
Unstable air forms towering clouds as it rises.
Orographic Lifting: occurs when elevated terrains, such as mountains, act as barriers to flowing air.
Windward mountain slopes are rainy. Leeward mountain slopes are very dry. Rainshadow deserts form on leeward sides of mountains.
Cool air acts as a barrier over which warmer, less dense air rises. This causes forceful lifting of the warm air regardless of its stability. Results of frontal wedging do depend on air stability.
Stable air: thin clouds; light precipitation if any
Unstable air: towering clouds; heavy precipitation
Air masses flow together; intervening air has to go somewhere so it goes up.
For condensation to occur air must be saturated. Condensation nuclei are also required. Tiny droplets form on these nuclei and remain suspended in the air.
(Sears Tower is 443 meters tall)
Fog: cloud with its base at or near the ground.
advection fog: warm moist air blown over a cool surface
radiation fog: Earth cools by radiation and fog settles into low pockets
upslope fog: humid air moving up a mountain and cooling
steam fog: warm water, evaporated into cool air
frontal or precipitation fog: warm humid air liften by frontal wedging, rain enters the air and becomes fog
Millions of cloud droplets must coalesce to produce one rain drop.
Ice Crystal Process - supercooled droplets attach to ice crystals creating snowflakes that melt as the fall.
Collision-Coalescence Process -small drops collide as they fall and join to form larger drops; eventually becoming large enough to produce rain.
A rain gauge is used to measure rain fall quantity