Chapter 3: Clouds and Precipitation

Cloud Formation

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.

Cloud Classification

Classification is based on two criteria: cloud form, and height in atmosphere. The three basic cloud forms are the following:

  • Cirrus: High and thin
  • Cumulus: Flat base that rises as lumpy domes or towers. Greater vertically than horizontally
  • Stratus: Sheets or layers of clouds. Greater horizontally than vertically.

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.

  • Cirrus: Thin and hooked tails that point in direction of air movement.
  • Cirrostratus: Sun halo, relatively transparent, and cover the entire sky. Form when a broad layer of air is lifted by convergence.
  • Cirrocumulus: Large formation of small puffs (mackerel sky).

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.

  • Altocumulus: Similar to cirrocumulus. Thicker and denser.
  • Altostratus: Fairly uniform greyish layer of clouds. No sun halo.
  • Altocumulus Castellanus (ACC): Tower-like projections billowing up from cloud bases.

Low Clouds – Below 2000m.

  • Stratus: Similar to altostratus, but lower. Associated with light precip.
  • Stratocumulus: Stratus clouds that form long rolls or globular patches.
  • Nimbostratus: Primary precip producer, and associated with stable conditions. Precip is light-moderate over a widespread area.

Other Clouds.

  • Clouds of vertical development (applicable to multiple height classes):
    • Towering Cumulus (TCU): Cumulus clouds that develop into towers. Forms vertically though instability and convection like CB clouds, but lack the anvil shape and darkness.
    • Cumulonimbus (CB): Greater vertical extent to TCU clouds, dark and dense, and produce heavy precip. The upper portion is spread by high winds near the tropopause.
  • Cirrus uncinus: High wispy clouds with a hook-like tail.
  • Fractus: Stratus or cumulus clouds broke into smaller pieces.
  • Mammatus: Clouds with strange rounded protuberance on the bottom surface.
  • Lenticular: Formed when a stable, moist air mass is topographically forced upward. Often lens-shaped, appearing on top of mountains.
  • Contrails: Formed by passing jets, as hot air mixes with cold air with a low vapor pressure. Indicator of humidity in the upper atmosphere.

Cloud Classification Chart

Fog. The composition of fog is the same as clouds, however, fog forms without the air mass rising. These processes are:

  • Cooling of the air near to the ground surface
  • Addition of water vapour to the air near the ground
  • Lateral movement of a parcel of air from a warmer area into a cooler area

There are several ways that fog can form:

  • Fog forming on clear nights with high humidity is a result of radiation cooling.
  • Advection fog forms when warm and moist air is blown over a cold surface.
  • Upslope fog forms when humid air moves up a slope.
  • Steam fog forms through the addition of water vapor caused by evaporation from a relatively warm surface condensation into a cooler air above.

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.

Precipitation – How It Occurs

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:

  • Bergeron process: Occurs in cold clouds, and involves ice crystal growth. Water droplets do not freeze unaided in temperatures below 0oC, and instead remain in a liquid form in a supercooled state. Initially, supercooled water requires a freezing nucleus to form into a small ice crystal. This results in the cloud comprising a mixture of ice crystals and supercooled water droplets. The water vapour pressure is higher over the water droplets compared with the ice crystals, so the vapour is supersaturated immediately around each ice crystal. Water vapour diffuses readily from the water droplets to the ice crystals, crystallising on the margins. Once the crystals reach a
    critical size, they begin to fall in response to gravity.

An illustration of the Bergeron process. Water molecules condense on an ice crystal due to a vapour pressure gradient between cloud droplets and ice crystals.

  • Collision-Coalescence process: Occurs in warm clouds located below the freezing level. Very large cloud droplets form due to the presence of giant condensation nuclei. These droplets fall and collide with other smaller droplets, forming into larger droplets. The droplets grow, and increase in speed, and eventually grow large enough to fall to the surface without evaporating. This may work in conjunction with the Bergeron process in cumulonimbus clouds.

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.

  • Plates and needles form at air temperatures warmer than -10oC
  • Dendrites (stellars) require -10oC to -20oC, as well as a high degree of supersaturation

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.

Diagram showing snow crystal growth as a function of temperature and supersaturation.

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.