12. Clouds and precipitation are caused by rising air; clear weather is caused by sinking air. Virtually all clouds as well as precipitation are due to rising air.

The most efficient way to cool a large parcel of air rapidly is to lower its pressure. This occurs when air is lifted (during convection) because as the air rises the pressure decreases. As a result, the temperature decreases and so does the amount of water vapor that the air can hold. The water vapor rapidly condenses into billions of tiny water droplets (or ice crystals), forming clouds (Figure 1). At times, precipitation results as well. On occasion, air can be cooled sufficiently by processes other than convection to form clouds or fog; however, this rarely leads to precipitation.

Conversely, when air sinks its temperature rises so that its capacity for holding water vapor increases (Figure 2). Any cloud droplets would then tend to evaporate and the clouds would disappear.

Figure 1. Cloud formation characteristically results from convection of moist air.	Figure 2. Air overhead that is cooled (in any one of several ways) contracts, increasing air pressure, and causes the air to become denser. As a result, the air subsides, and compressional warming causes a decrease in humidity. Clear skies result.