Water in the Atmosphere
Forms of WaterNearly all of the water is located in the lowest layer of the atmosphere, the troposphere. Water is present in variable amounts in the atmosphere, from 0 % to 4 %. Unlike oxygen and nitrogen, the concentration of oxygen depends on local weather conditions and changes greatly from place to place on Earth. Clouds form preferentially over dark vegetation and just downwind of mountain ranges. The water in the atmosphere makes up only a very small percentage of the total water on Earth.
In the atmosphere, water exists as a gas (water vapor from evaporation), as a liquid (droplets of rain and liquid water that coats solid particles), and as a solid (snow and ice). Its structure depends on its state.
Water in the gas phase has a bent structure with an H-O-H angle of 104.5 degrees. In the liquid and solid forms, there are bonds between the hydrogen atoms of one molecule of H2O and oxygen atoms of other molecules. This gives a 3-dimensional structure in which each oxygen atom is surrounded by a tetrahedral array of 4 hydrogen atoms.
HumidityHumidity is the concentration of water in the gas phase that is present in air. In central Illinois summertime, the humidity is very high!
Gas concentration relates the amount of gas molecules in a volume. The amount of gas can be expressed in molecules, moles, or grams. The volume can be in liters, cubic meters, or cubic centimeters. Molar concentration is the number of moles per liter (mol/L or M).
Imagine that you put some water in an empty container and seal it. After a while, some of the water will evaporate. Water will continually evaporate and condense but, after a while, the net amount of water in the gas phase will remain constant. The water(g)-water(l) system is at equilibrium.
The concentration of the water in the gas phase depends on temperature and pressure.