Weather and Water
An understanding of the different phases of water--solid, liquid and gas--is essential to understanding weather. The only difference between these phases is the proximity and motion of the water molecules. When water is in vapor or gas form, the molecules are moving rapidly and are spaced far apart. When water is in liquid for, the water molecules are moving slower and the molecules are closer together. When water is in the solid or ice form, the water molecules hardly move at all and are tightly packed together.
Air has the ability to hold a certain amount of water vapor. The humidity is a reflection of the water vapor content of the air. Warm air has a higher capacity for water vapor. When air cools, the ability for the air to hold water vapor decreases, resulting in a higher humidity. If the humidity reaches 100%, the air can no long hold any more water vapor and condensation takes place.
The change in humidity is very important to understanding weather. When the humidity reaches 100% in the sky clouds will form. When the air reaches 100% humidity at your campsite dew forms. If you are caught sleeping out in the open when dew forms your sleeping bag (or bivy) could be soaked with dew.