Clouds are composed of liquid suspended water droplets in about a 100% RH environment. The three primary ways that clouds dissipate is by (1) the temperature increasing, (2) the cloud mixing with drier air, or (3) the air sinking within the cloud. When the temperature increases, the air has a higher capacity to evaporate liquid water. When a cloud's temperature increases, evaporation occurs and reduces the liquid moisture content of the cloud. A cloud can be warmed by solar radiation and longwave emission from the earth's surface. Daytime heating increases the capacity of the air to evaporate liquid water. Low clouds such as fog and low stratus are often dissipated due to daytime heating, especially if a cap exists aloft. Daytime heating's power to erode clouds depends on the sun angle (depends on season), the cloud thickness and the overall stability and lift present in the troposphere. A cloud does not remain perfectly adiabatic. Some environmental air does mix into the cloud mass. If a cloud is no longer developing and not adding additional condensational moisture, the drier environmental air will gradually erode the cloud. Instead of having a sharp very defined appearance, after mixing with environmental air the cloud will look wispy with edges that are not well defined. This process is called entrainment. During entrainment, drier air incorporates itself into the cloud and induces evaporation. When air sinks, it warms adiabatically. Again, warming will induce evaporation and erosion of the cloud. This can occur when dynamic sinking mechanisms instigate or increase over a cloud or cloud field. Dynamic sinking mechanisms include low level CAA, NVA, low level divergence, and downslope flow. Often, more than one of these processes mentioned above occurs simultaneously to erode a cloud or cloud deck. |