Humidity is water that has entered the atmosphere through a process of evaporation or transpiration. There are two major sources of humidity in the atmosphere. One is the large amount of is the evaporation through a plant, or transpiration that occurs around forests. For example, the Smokey mountains are named for the large amount of water vapor or transpiration that enters the atmosphere through the action of trees.
Forested areas are distinct from farmland because the roots of trees extend many tens of feet, even hundreds of feet, into the water table below the ground. Because of this, trees and therefor forests continue pumping humidity into the air even then the top layer of the soil is dry. As a result, reasonable humidity may be maintained even during the "dry" season.
Oceans can be a major source of atmospheric humidity. One way to visualize this is to look at an animated weather map for a tropical region such as the Caribbean, South and Central America, or Florida. For most of the year, as the day wears on, clouds are easily visible forming over forested areas of the weather map. Because cloud formation around Caribbean islands, such as Puerto Rico and Cuba is dominated by the land areas, we can easily see for ourselves that terrestrial vegetation is a dominant source of humidity.
An exception to this rule occurs during hurricane season in tropical areas of the world's oceans. When the temperature of the ocean, or any other water body such as the Gulf of Mexico goes above 80 degrees or so, the amount of water that evaporates due to the energy of the sun increases rapidly. These large amounts of humidity that enter the atmosphere are the energy source of hurricanes.
The amount of water that air can hold is dependent upon the air pressure and the temperature of the air. Air can hold more water at higher pressures and higher temperatures. For this reason, humidity is expressed as a percentage of the total that the air can hold at that pressure and temperature. For example, if at 70 degrees a given body of air has humidity at 40 %, and that air is heated, for example by moving over a dry hot un-vegetated area, or merely by the sun as the day goes on, the relative humidity will drop, even though no water has left the air mass. Likewise, if air is rapidly cooled, for example by the onset of the evening, the relative humidity will rise rapidly.
One biome where this phenomenon is particularly evident is the rain forest. The rain forest is so named because essentially every day, with the onset of evening and resultant cooling, the relative humidity goes above 100 percent, and of course condensation or rain begins to fall.
Another important affect related to relative humidity is the adiabatic effect. As an air mass rises, for example by being driven up a mountain by currents, it looses air pressure and temperature as it rises, and as a result the relative humidity increases. As a result, precipitation may, and does occur more often on the windward side of mountains than it does on the leeward side of mountains. This is often evident on the West Coast of the U. S. where rain occurs on the west side of coastal mountain ranges, but little of any may occur on the leeward, or eastern side. As a result, relatively moist coastal areas can occur near very dry areas such as Death Valley.
Humidity is a form of energy in the thermodynamic sense. When water evaporates, it absorbs energy from the environment, and as a result, there is a cooling affect. Conversely, when water condenses, there is energy given off, and there is a warming effect. Humidity helps redistribute the sun's energy more evenly around the globe, and as a result, we are given a more moderate environment. Areas with low humidity, due to lack of coastal areas, or lack of tree growth, tend to have more extreme environments. The climate is, for example, more extreme in both temperature directions in the Dakotas than it is in the Chesapeake region.
Because of this description, global warming may be somewhat of a mis-stated concept. If the normal transfer of energy form tropical areas to polar areas is disrupted by, for example, destruction of the rain forests, it may result in polar areas that are actually colder because water vapor is not transferring energy properly. Even though they are colder, there may be less condensation and snow fall which may contribute to reduction in polar ice cover. For this reason, temperature alone is not a sufficient way to describe or quantify climatic change.