Water in a gaseous state is water vapor. The process of evaporation changes water to vapor, and heat speeds up the process. All air contains water vapor, even the seemingly dry desert air. Water vapor is turned back into liquid water through the process of condensation, the opposite process of evaporation. Water goes through continuous cycles of evaporation and condensation, called the water cycle.
Water vapor and droplets in the atmosphere combine with tiny particles of dust and other matter. These droplets coalesce (combine), increase in size and form clouds. The cooler temperatures in the atmosphere speed the condensation process. The particles in the clouds eventually become too heavy to remain suspended in the atmosphere, and fall to Earth as some form of precipitation: rain, freezing rain, hail sleet or snow.
At night, when temperatures drop, water vapor in the air can condense on surfaces close to the ground, leaving visible water droplets called dew. The point at which the change in form takes place is called the dewpoint. Dew is more likely to form on cloudless nights with little or no wind.
Fog is formed of condensed water droplets when the water vapor in the air near the ground cool. Fog can form after rain cools and dampens low-lying air. Fog can form on a cloudless night if the air is humid, especially in the fall when nights grow longer and cooler. Warm humid air masses that blow over a cool surface, such as the surface of the ocean or a blanket of snow, can also produce fog.
Frost occurs in a manner similar to dew; but, instead, the dewpoint is near freezing. The water vapor condenses and freezes on surfaces with temperatures below freezing, producing frost.
Condensation occurs when water vapor cools on the outside of drinking glasses containing cold beverages; on eye glasses when an individual moves from an air-conditioned building outside to hot, humid air; on the inside of car windows from the warm, humid air exhaled by the occupants and on the inside of many windows in the winter.
