Process and Conditions of Condensation
Condensation has been defined earlier as the process in which water vapor is changed to liquid water. It should be noted that when this process occurs at subfreezing temperatures, the vapor often turns directly into the solid form-ice-with no intervening liquid stage. Technically, the transition from vapor to solid is known as sublimation, but we shall use the term condensation to cover either situation, for they are both a part if the same major process.
In the process of condensation in the atmosphere, three general conditions must be satisfied: (1) a sufficient water vapor content must be present, (2) cooling of the air to and below the dew-point temperature is usually required, and (3) nuclei of condensation must exist. These conditions are interrelated in theirs effects.
Forms of Condensation
The forms assumed by condensed moisture in nature are varied and often beautiful in appearance. Depending on the differing conditions under which cooling occurs, the resulting condensation types may be classified under the common headings of dew, frost, clouds and fog.
The Fog
The type of fog that forms depends on exiting conditions and falls into four recognizes categories: radiation fog, advection fog, frontal fog, and upslope fog. The thickness of the fog depends on various factors of humidity, temperature, wind, nuclei, etc.
Dense fog… object not visible at 50 yds
Thick fog… object not visible at 200 yds
Fog… object not visible at 500 yds
Moderate fog…object not visible at ½ nautical mile
Thin fog… object not visible at 1 nautical mile
Forms of Precipitation
All forms of precipitation, regardless of appearance, are often termed collectively hydrometeors. The U.S. Weather Bureau has classified those hydrometers into about fifty specific types.
Rain. Rain is obviously the most familiar example of precipitation. It is so common that in ordinary conversation it usually replaces the more general word. Although the definition of rain is simple, the explanation of its origin is complex, as just the source of rain is simple, the explanation is complex, as just noted. Much rain is the direct result of condensation of water droplets in clouds followed by growth to a size necessary to overcome the buoyant effect of the air. However, a considerable amount of rain is actually melted snow, especially in high latitudes or during the winter in the mid-latitudes.
Snow. When condensation occurs in rising air that has cooled to subfreezing temperatures, typical hexagonal ice crystals tend to form, instead of liquid droplets. It will be remembered that condensation at the earth’s surface yields frost when the dew point is below 32oF. Snow crystals may exist in isolated form or may coalesce to form snowflakes of varying sizes and shapes.
Sleet. Sleet is often true frozen rain. If the waterdrops falling from the clouds encounter a layer of air with freezing temperature, they solidify into small, hard, clear ice pallets. Sleet thus indicates a temperature inversion even though it may be a very slight one. The temperature in the clouds may be very close to freezing and if the air below the clouds is somewhat colder sleet may result. Sleet also develop fro freezing of melted snow as the later falls though a colder layer of air near the ground.
Hail. Hail is a product of the violent convection found in a thunderstorm and occurs only in connection with a thunderstorm. In the thundercloud the strong vertical air swirls the raindrops above and below the freezing level. As a result, the drop freezes when it is carried to a height above the freezing line and grows by the accumulation of snow and water at the different levels. Consequently, when a hailstone is cut apart, it shows a series of concentric shells formed by the successive passages above and below the freezing level.
Glaze. When rain falls on object or on ground having subfreezing temperatures, it freezes into a sheet or coating of ice, known as glaze or freezing rain. If this coating becomes thick, if often has a destructive effect owing to its heavy weight.
Rime. Rime is a freezing fog. It forms as a thick, frosty deposit when objects with subfreezing temperatures encounter a fog. In such a case the minute fog droplets freeze and adhere to the cold surface. Rime is thicker on the windward sides of objects, particularly when forming on hulls, masts, or bulkheads of vessels sailing through fog in cold weather. It also occurs on leading edges of airplanes flying through certain types of clouds.
(Donn, William L.: Meteorology)