On The Cloud

Bioprecipitation, the concept of rain-making bacteria, was proposed by David Sands from Montana State University. Such microbes – called ice nucleators – are found in rain, snow, HP Pavilion dm4-1008tu Battery and hail throughout the world. These bacteria may be part of a constant feedback between terrestrial ecosystems and clouds and may even have evolved the ability to promote rainstorms as a means of dispersal. They may rely on the rainfall to spread to new habitats, much as some plants rely on windblown pollen grainsHP Pavilion dm4-1008tx Battery.

[edit]Adding moisture to the airHP G62-112EE Battery

There are five main ways water vapor can be added to the air. Increased vapor content can result from wind convergence over water or moist ground into areas of upward motion.[8] Precipitation or virga falling from above also enhances moisture content.[9] Daytime heating causes water to evaporate from the surface of oceans, HP G62-112SO Batterywater bodies or wet land.[10] Transpiration from plants is another typical source of water vapor.[11] Lastly, cool or dry air moving over warmer water will become more humid. As with daytime heating, the addition of moisture to the air increases its heat content and instability and helps set into motion those processes that lead to the formation of cloud or fog. HP G62-113SO Battery [12]

[edit]Distribution in the troposphere: variable global prevalence

Main articles: Extratropical cyclone, Monsoon trough, and Subtropical ridge


An idealised view of three large circulation cells. HP G62-115SE Battery

Although the local distribution of clouds can be significantly influenced by topography, the global prevalence of cloud cover tends to vary more by latitude. This is the result of atmospheric motion driven by the uneven horizontal distribution of net incoming radiation from the sun. HP G62-115SO Battery Cloudiness reaches maxima close to the equator and near the 50th parallels of latitude in the northern and southern hemispheres.[13] These are zones of low pressure that encircle the Earth as part of a system of large latitudinal cells that influence atmospheric circulation. In both hemispheres working away from the equator, HP G62-117SO Battery they are the tropical Hadley cells, the mid-latitude Ferrel, and the polar cells. The 50th parallels coincide roughly with bands of low pressure situated just below the polar highs. These extratropical convergence zones are occupied by the polar fronts where air masses of polar origin meet and clash with those of tropical or subtropical originHP G62-118EO Battery.[14] This leads to the formation of weather-making extratropical cyclones.[15]

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