Condensation - water vapor (a gas) in the air turns into liquid water. Condensing water forms clouds in the sky. Water drops that form on the outside of a glass of icy water are condensed water.
Evaporation - liquid water becomes water vapor (a gas). Water vaporizes from the surfaces of oceans and lakes, from the surface of the land, and from melts in snow fields.
Precipitation - water (in the form of rain, snow, sleet, or hail) falls from clouds in the sky.
Infiltration - water soaks into the ground and joins the groundwater. The water is filtered by the ground.
Surface Runoff - rain, snow melt, or other water that flows downhill.
Transpiration - the process in which some water within plants evaporates into the atmosphere. Water is first absorbed by the plant's roots, then later exits by evaporating through pores in the plant. Remember - People Perspire, Trees Transpire!
Sublimation – water changing from solid directly to vapor. You have seen snow ‘disappearing’ even when the temperature is below freezing. It makes ice cubes in your freezer shrink
Specific humidity is the quantity of water vapor in a given volume of air, as grams per cubic meter of air. Since the atmosphere stores energy in water (latent heat of evaporation), specific humidity can predict how violent storms can be. Meteorologists are very interested in absolute humidity.
Dew point is a temperature. When air cools, it can’t hold all of the water it has, and the water condenses. If it is in the air, it forms fog. If it is above freezing and the water comes out on the ground, it is called dew. You can also see that on a cold glass of water in the summer. If it is below freezing, the condensed water is called frost.
The dew point temperature depends on how warm the air is to begin with, and how much water is in the air. If there is a lot of water in the air, then you do not have to cool the air much to have water condense. If the air is saturated (holding as much water as it can at that temperature), then the dew point will be the same as the air temperature. If the air is very dry, then you will have to cool the air a lot before the water condenses. There will be a big difference between the air temperature and the dew point.
The dew point lets us predict how much the air has to cool off at night to get frost or dew or fog. It also lets meteorologists predict how high air must rise before it cools enough to form clouds. “The higher the clouds, the better the weather” means that if air must rise a lot (cool a lot) then the air does not have much water vapor in it. It won’t rain or snow.
Relative humidity compares the amount of water in the air to how much water it could hold at that temperature. The actual temperature and amount of water does not matter. If the air is full (saturated), the relative humidity is 100%. If the air has ½ as much water as it could at that temperature, the relative humidity is 50%.
We care about relative humidity because we care about evaporation. If the relative humidity is close to 100% then the air is saturated. Clothes won’t dry on the line, and your sweat won’t cool you off. Heat Index! If the relative humidity is low, your sweat will evaporate and cool you.
If the air is cold and the relative humidity is close to 100% then the water will steal your heat. We say that the air feels raw and damp. If the air is cold and the relative humidity is low, then the water will evaporate and steal your heat. A warm room in winter will feel cold unless you add moisture.
If the air gets warmer, it can hold more water vapor. Unless you add water, the relative humidity gets lower. That happens when you heat the air in your house. For every increase of 11 ºC, the air can hold twice as much water vapor. So, when you bring cold air inside and warm it to room temperature, unless you add water, it is relatively drier.
Air Temperature Relative Humidity
0 ºC 100%
11 ºC 50%
22 ºC 25%