The maximum amount of water that can exist as a gas is greater at high temperatures than at low temperatures. Therefore, if the air temperature decreases, the atmosphere can hold less water vapor. After sunset, the temperature often decreases, and so the maximum amount of water vapor that the atmosphere can hold also decreases. The relative humidity rises, although the amount of water vapor in the air remains the same. Eventually, the air may become saturated with water vapor. The temperature at which air becomes saturated is called its dew point. If the temperature drops further, water vapor will condense.
Water vapor typically condenses as dew, frost, clouds, or fog. Dew, shown in Figure 17A, is water vapor that condenses on Earth's surface. Frost, shown in Figure 17B, forms when the dew point of air is below freezing. Frost isn't frozen dew. Rather, frost is formed when water vapor in air changes directly from a gas to solid ice crystals.
When the air warms, as can happen during the day, the amount of water vapor that air can contain increases. As a result, the relative humidity decreases.
What is the dew point of air?
Cloud Formation
The most noticeable features of the atmosphere are clouds. Some are high and wispy; others are thick and dark. A cloud is a dense, visible mass of tiny water droplets or ice crystals that are suspended in the atmosphere. The droplets or crystals that form a cloud are extremely small—one cloud might consist of billions of such droplets and crystals.
Clouds form as warm, moist air rises and water vapor condenses in the atmosphere. Water vapor condenses when air cools to a temperature below its dew point. Such cooling occurs naturally as warm air rises through the atmosphere. As air rises, air pressure decreases. As a result, the air expands and cools, and condensation occurs. Cloud formation, then, is the result of the expansion of air and the resulting drop in temperature.
For a cloud to form, solid particles such as dust and salt must be present for the water vapor to condense upon. When air is cooled below its dew point, water vapor changes into tiny water droplets or ice crystals on these solid particles. This process is similar to the way water vapor condenses onto the bathroom mirror when you take a hot shower.
Figure 17 Dew and frost are two forms of condensation. A Dew can often be seen on spider webs early in the morning after a cool night. B Frost is visible on this pane of glass.
Comparing and Contrasting How are dew and frost similar? How are they different?
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