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  1. Cloud/Fog Ingredients

    • Water vapor is evaporated from earth's surface into air.
    • A cooling mechanism to condense the water vapor into liquid or ice. Rising air is the cooling mechanism to create clouds. The rising air cools due to the adiabatic process. Fog is formed by five different cooling mechanisms. The name of the fog relates to that specific cooling mechanism.
    • Condensation nuclei such as smoke, pollen, dust, pollution, etc. that provides a surface for vapor to condense onto.

  2. Precipitation Processes

      There are two processes to create precipitation in a cloud:

      Collision-Coalescence occurs in clouds when larger liquid cloud droplets fall through the cloud and collide with smaller droplets that are either suspended in the cloud or falling more slowly than the larger droplets. Once the collision occurs, the large and small droplets coalesce (join) to create a new larger droplet. This larger droplet then repeats the process as it falls even faster through the cloud. The end result is an extremely large droplet called a raindrop leaving the cloud at the bottom.

      Bergeron Process occurs when a cloud is supercooled. A supercooled cloud is one that contains both liquid droplets and also ice crystals. The cloud temperature must be at or below freezing in order for this process to occur. Bergeron process occurs because water vapor molecules "prefer" to deposit onto an ice crystal (deposition) rather than to condense into a liquid droplet. Therefore, when water leaves a liquid droplet in the form of vapor, that vapor will deposit itself onto a nearby ice crystal. Once water vapor begins to deposit onto the ice crystal there are two results: the ice crystals grow and cloud droplets evaporate (thus getting smaller). When the ice crystal leaves the cloud it will be large enough to be called a snowflake.

  3. Precipitation Types

      Rain - must be liquid before hitting the surface and must not freeze onto surface. Therefore, the surface temperatures must be above freezing and the air above the ground must be above freezing.

      Snow - must begin as a snowflake (Bergeron) and remain a snowflake until reaching the ground. If flake melts into a drop it can never return to flake form again. Therefore, the cloud and all of the air beneath the cloud must be below freezing.

      Freezing Rain - rain that hits a below-freezing surface to form a glaze. The surface must be at or below freezing while the air above the surface must be warmer than freezing.

      Sleet - rain that freezes in the air and hits the ground as a small pellet. Typically, there is a warm layer of air aloft that keeps precipitation as rain but the layer of air above the surface is below freezing. When rain falls into that below-freezing layer of air, it turns to an ice pellet.

  4. Cloud Types

      Clouds are classified by altitude & appearance. There are four main groups of clouds: high, middle, low, and vertically-developed. Clouds can easily be identified by knowing these five cloud name fragments:

      cirr - high, ice, all white, very thin
      alto - middle, ice and/or liquid, white and grey, thin
      cumu - puffy appearance, all vertically-developed clouds begin with "cumu"
      nimb - cloud must be precipitating
      strat - cloud is spread out, most likely covers the sky

      Important: The terms above are NOT actual cloud types. One must combine these fragments to form an actual cloud type. For example, cirrostratus combines cirr- and strat-. The means a cirrostratus cloud is high, all white, made of ice, very thin, and is spread across the sky. Often the best way to identify a cloud is to use process of elimination. Start with color (is it all white [high], white and gray [middle], or gray [low]?) Then decide if it is vertically developed or not. Then consider if it has any "puffiness" (cumu) to the shape or if it is more flat and spread across the sky (strat).

  5. Fog Types

      Fog is denoted by the cooling mechanism involved. There are five main types of fog:

      Radiation Fog - occurs under clear, calm skies when infrared radiation (heat) escapes to upper atmosphere and outer space and the air is cooled to its dewpoint. Sometimes called valley fog or ground fog and is the most common fog over land in the world.

      Advection Fog - occurs when warm, humid air is cooled to its dewpoint by coming into contact with a cooler surface below. Snow, ice, and cold water are common surfaces that cause advection fog.

      Steam Fog - occurs when cooler air rests above warmer water and vapor that evaporates into the air cools to its dewpoint. Sometimes called Arctic sea smoke.

      Upslope Fog - occurs when air is forced to rise up a large slope and cools (adiabatically) to its dewpoint. Common in mountainous states.

      Frontal Fog - also knows as precipitation fog and occurs when rain drops fall into unsaturated, cooler air below. As the drops evaporate, water vapor is introduced into the cooler air. Very quickly, the vapor condenses into a small fog droplet.

Helpful Links:

Wiki: Precipitation
Wiki: Cloud Types
Wiki: Fog
Clouds & Precipitation - MUST SEE!
PSC Meteorology Cloud Boutique - MUST SEE!
Australian Severe Weather - MUST SEE!
Cloud Types
Snow Crystals Under A Microscope - Very cool!....pun intended.
Chasing the Storm: Weather Photography

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