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9.15 Clouds and Precipitation

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High level clouds

Ci: Cirrus clouds are curly, featherlike clouds and are often the first clouds to appear in a clear, blue sky. The shape and movement of cirrus clouds can often indicate the strength and direction of high altitude winds.

Cs: Cirrostratus are sheet-like, nearly transparent clouds. Cirrostratus clouds are so thin that the Sun and Moon can be clearly seen through them. When sunlight or moonlight passes through the ice crystals, it is bent in such a way that a halo may form. These clouds often indicate rain is on its way.

Cc: Cirrocumulus clouds look like small white puff balls high in the sky. The puff balls can occur individually or as long rows. When the puffs are in rows, they give the cloud a rippling appearance that resembles the scales of a fish.

Middle level clouds

As: Altostratus clouds are made up of both water droplets and ice crystals; they often indicate the increasing likelihood of rain. They cover huge areas of the sky, often over hundreds of square nautical miles. The Sun is visible through these clouds but it looks as if it is behind frosted glass. Altostratus cloud may produce rain or virga. Mainly small supercooled water droplets occur in this cloud producing light rime ice but if the cloud is thick enough clear ice is possible.

Ac: Altocumulus clouds are white or grey, or a mixture of both. They look puffy or like fuzzy bubbles in long rows. They generally have dark shadowed undersides. If this shading isn't visible, it's quite easy to mistake these clouds for high level cirrocumulus clouds. In case of doubt, hold your hand at arm’s length: if the puff is smaller than one finger width, you are looking at a cirrocumulus cloud. Altocumulus cloud is not usually associated with rain but may be quite turbulent. This cloud can contain super cooled water droplets to form rime ice.

Low level clouds

St: Stratus clouds form in a low layer and cover the sky like a blanket. They develop in horizontally stable layers rather than towering vertically columns. They can form within a few hundred feet above the ground while stratus cloud at ground level is known as fog. Stratus cloud may produce drizzle and possible rime ice may form but usually the temperature does not move below 0o C.

Ns:Nimbostratus clouds form a dark grey, wet looking, stable cloudy layer that totally mask the sun. The cloud produces heavy continuing rain or snow that prevails for hours or days. Clear ice will most certainly occur at and above the freezing level up to where the temperatures favour rime ice.

Sc: Stratocumulus clouds are grey with dark shading and spread in a puffy layer. The cloud may produce drizzle. Possible rime ice may form but the temperature usually does not drop below 0o C except over Tasmania and Victoria during winter.

Cu:Cumulus clouds look like white balls of cotton wool. They usually occur individually as small or large cells with blue sky between the clouds. The clouds can form from thermal convection producing flat bases and lumpy tops. Cumulus cloud may produce showers of rain or snow. In large cumulus cloud above the freezing level rime ice will occur and possibility clear ice.

Cb: Cumulonimbus clouds are thunderstorms. The top of these clouds can reach 60,000 ft in height around tropical areas (much higher than Mt Everest). There tops can penetrate into the stratosphere and are commonly characterised with an anvil-shaped ceiling. The bottom of a cumulonimbus cloud is made up mostly of water droplets whereas higher in the cloud, ice crystals dominate as the temperature is well below 0 °C.

Vertical winds speeds inside the clouds can be greater than 3000 ft/min. Cumulonimbus cloud produce turbulence, lightning, hail, rain, virga (rain that evaporates before it reaches the ground) and on occasion they create micro bursts, waterspouts and tornados. Clear ice will most certainly occur at and above the freezing level up to where the temperatures favour rime ice. Rime Ice will occur up to where ice crystals predominate. Risk of icing exists at all levels in the cloud.