Clouds are a visible suspension of water particles, formed as air rises and expands. As air expands, it cools adiabatically. Condensation or sublimation occurs when the air reaches its dew point. Water vapour attaches itself to condensation nuclei (dust particles), and continues to grow until it becomes a visible cloud droplet or ice crystal. Clouds may form through lifting mechanisms, or when moisture is added, such as evaporation from warm bodies of air into cooler air.
Classification is based on two criteria: cloud form, and height in atmosphere. The three basic cloud forms are the following:
High Clouds – Above 6000m. Air is much colder and there is less water vapour. High clouds do not produce precipitation, however they may warn of incoming weather.
Middle Clouds – Between 2000-6000m. Light snow or drizzle possible. ACC clouds are evidence of atmospheric instability, and a high mid-altitude lapse rate. They can develop into cumulonimbus clouds.
Low Clouds – Below 2000m.
Fog. The composition of fog is the same as clouds, however, fog forms without the air mass rising. These processes are:
There are several ways that fog can form:
Fog is associated with calm conditions, which favour surface hoar formation. Surface hoar forms along the upper edge of fog, creating a ring at specific elevations.
Cloud Droplet Growth. All clouds contain ice, water, or water vapour, but not all release precipitation. For a droplet or ice crystal to fall, it must grow approximately 106 times in volume. Cloud droplet growth is through two processes:
Rain. Rain falls either when water droplets formed by the Collision-Coalescence process gain sufficient mass, or when ice crystals formed by the Bergeron process melt during their fall by entering warm air below the cloud. In thick clouds associated with turbulence, droplets and ice crystals have to row larger to overcome up-currents keeping them from leaving the cloud. Light rain or drizzle forms under calmer conditions, where droplets and ice crystals can escape the cloud more easily.
Freezing Rain. Freezing rain is rain that becomes supercooled while passing through cold air near the ground, and when it falls on snow below 0oC, it will freeze into a freezing rain crust. Freezing rain crusts are different from rain crusts, which require re-freezing after falling on moist snow.
Snow. Snow forms exclusively by the Bergeron process. The types of snow crystals that form depend on the degree of superstation of the cloud and the cloud’s temperature.
The type of snow is also dependant on the temperature-supersaturation history of the snow crystals as they fall to the ground. Crystals may go through periods of secondary growth or decay, such as the capped-column crystal. Snow crystals may coalesce to form larger snowflakes, reaching easily 3cm in diameter. Mechanical interlocking, sintering, and electrostatic attraction are the dominant processes to grow snowflakes. The type of new snow will have an impact on snow stability, but the size of the new snow crystals are generally considered more significant.
Rime, Graupel, and Haul. Collisions between supercooled water droplets and snow crystals produce rime. When riming becomes heavy, and the rime particles form dense, round pellets, they become graupel. A thick layer of graupel acts as a weak layer, since the inter-crystal bonds are poor. When further riming produces large, dense pellets with a laminar internal structure, they become hail.
Riming affects meteorological weather stations when the supercooled water droplets freeze instantly on contact with sufficiently cold objects on the ground. This builds a point of ice into the direction of the wind.