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Page 2
Forming Clouds
Once the thermals rise above the surface, the potential for cloud development increases. Whether a cloud will arise from any particular thermal depends on the moisture content of that thermal when it forms, the height to which it rises, and the temperature and moisture content (or humidity) of the air through which it moves. At some altitude in the surrounding atmosphere, a thermal will reach its condensation level. At the condensation level, the relative humidity of the thermal air will be around 100 percent (saturated), and liquid droplets may arise from condensation of the thermal's water vapour. If the thermal is unable to reach its condensation level, no clouds form and skies remain clear. But if the thermal punches through this altitude, clouds begin to take shape. Since the surrounding air within the high pressure cell is rather uniform in its temperature and moisture properties, the condensation level (also called the lifting condensation level and the cloud base) occurs at a rather consistent altitude over the region (usually around 3200 to 5000 metres or 2000 to 3000 feet). Therefore, the bases of young cumulus clouds can all be found at a similar altitude. Above the cloud base, small visible clouds begin to appear as heaps atop the rising air column and, for this reason, are collectively called cumulus clouds. With conducive conditions for further growth, the initial cumulus can develop into larger and taller formations such as cumulus congestus, cumulus castellanus and the ultimate giant, cumulonimbus. But today, the sinking airmass and overlying elevated inversion prevent the cumuli from reaching great heights, and we have cumulus humilis. Humilis means humble in Latin, and accurately describes these smallest members of family Cumulus. Typically, cumuli humilis have cloud tops at 5000-8000 m (3000-5000 ft), making them shallow and very distinct in their outlines. As their family name denotes, they are small heaps of clouds with flat bottoms and rounded tops, and often look like they have been formed by a biscuit cutter out of cotton balls. Although it may appear that these clouds live a long life over the afternoon, each individual cloud actually lasts but five to forty minutes before slowly fading away as its air is mixed into the surroundings. But when one has faded, another will emerge, and the process continues on. Between each cumulus humilis, the air descends slowly to replace the rising thermals. As a result, the cloud field can look very regularly spaced with clouds aligning along "cloud streets." Later in the afternoon as the sun's heat wanes, the energy producing the cumulus field decreases, and thermals contain less excess heat and therefore rise less high. The resulting cloud depth decreases to give flat pancake-like clouds. Finally the production of thermals ceases altogether, and as each remaining cloud completes its lifespan, the sky clears up. By evening, the skies will likely again become completely clear. If the high pressure field continues to dominate the region, the process may reawaken with the next dawn.
The copyright of the article Cumulus Humilis: A Fair Weather Cloudscape - Page 2 in Meteorology is owned by . Permission to republish Cumulus Humilis: A Fair Weather Cloudscape - Page 2 in print or online must be granted by the author in writing.
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