Per què el cel és blau

i els núvols són blancs?

Which clouds produce rain?

Posted by Costa M. a 4 Març 2013


First of all, we have to define a cloud, but what’s a cloud? Well, this is not an easy question. There is a huge debate among scientists about this question. According to the World Meteorological Organization (WMO) “a cloud is a hydrometeor consisting of minute particles of liquid water or ice, or of both, suspended in the free air and usually not touching the ground. It may also include larger particles of liquid water or ice as well as non-aqueous liquid or solid particles such as those present in fumes, smoke or dust.”

Then, what is a hydrometeor? A hydrometeor could be a cloud, as explained above, but also could be fog, mist or ice fog. More interesting, hydrometeors could be those consisting of a fall of an ensemble of particles (precipitation), which includes: rain, drizzle, snow, snow grains, snow pellets, diamond dust, hail and ice pellets.

Thus, in the context of this blog, more important than ask “which clouds produce rain?” is“which clouds produce water precipitation?” or “which kind of situations produce clouds that let water precipitates?” The word “water” here is remarked because what is important for us, the humanity, is that the precipitation gives water to the biosphere, not dust for example. And it is also important that this water could be in liquid form, snow or ice, because all of them are useful to retain fresh water provisions.

So, referring to clouds, there exist 10 genera and they are usually classified in three categories. By convention, the part of the atmosphere in which clouds are usually present has been vertically divided into three étages, depending on their base height:

  • HighCirrus (Ci), Cirrocumulus (Cc) and Cirrostratus (Cs)
  • MiddleAltocumulus (Ac), Altostratus (As) and Nimbostratus (Ns, which can extend into other étages)
  • LowStratus (St), Stratocumulus (Sc), Cumulus (Cu) and Cumulunimbus (Cb) (these two latter genera usually have their bases in the low étage, but their vertical extent is often so great that their tops may reach into the other étages).
Stratocumulus Photographer: Simon EugsterBy: Wikimedia commons

Stratocumulus Photographer: Simon Eugster
By: Wikimedia commons

All these clouds at the same time are subdivided in many species, varieties and supplementary features or accessory clouds.

Answering to the main question, clouds that produce rain and snow are mainly: As, Ns, Sc, Cu and Cb. Stratocumulus (St) can also produce snow, snow grains and drizzle. Snow pellets are produced by Sc, Cu and Cb. Hail is always caused by Cumulunimbus, but ice pellets come from Altostratus and Nimbostratus.

 

Cumulunimbus (Cb) producing rain in New Haven Harbor in New Haven. Author: Versageek on 2009/07/03.By: Wikimedia Commons

Cumulunimbus (Cb) producing rain in New Haven, CT. Author: Versageek on 2009/07/03.
By: Wikimedia Commons

 

Moreover, it is very interesting to distinguish between precipitation and shower. Showers are characterized by their abrupt beginning and ending and by the generally rapid and sometimes violent variations in the intensity. Drops and solid particles falling in a shower are generally larger than those falling in non-showery precipitation. Whether the hydrometeors occur as showers, or not, depends on the clouds in which they originate. Showers fall from dark convective clouds (mainly Cumulonimbus, rarely Cumulus); non-showery precipitation usually falls from stratiform clouds (mainly Altostratus and Nimbostratus).

The type of precipitation has many ecological implications. Showers are not desirable in terms of fresh water storage (due to the speed of the water falling, what make it not suitable to leak into the sub-soil aquifers). Moreover, they cause an important soil erosion and the water could be less absorbed by plants, because it’s rush or torrent behavior. Not forgetting that sometimes intense showers have caused important damages to society with flood episodes.

The behavior of the precipitation is clearly illustrated, for example, contrasting the Girona-Mediterranean climate and the London-Oceanic/Atlantic climate. The total year precipitation in London is 611 mm with 145 days/year of rainfall (data from MetOffice) and in Girona it raises to 737 mm with only 80 days/year of precipitation (data from Aemet). So, in London the precipitation is more uniform along the year and, without taking into account other differences like temperature, the vegetation differences are evident.

Finally, the type of hydrometeor is also ecologically important: snow provides a good storage of fresh water. For instance, in our country the snow accumulated in the Pyrenees and pre-Pyrenees Mountains is usually liberated over the land or into rivers, lakes… when temperature is rising (in spring) and provide fresh water when more scarce is this resource (in Summer time). On the contrary, hail and ice pellets could produce damage in crops (this and the hail control subject may be another entry to this blog).

References:

World Meteorological Organization (WMO), “International cloud atlas”, volume I (revised edition, 1975). Manual on the observation of clouds and other meteors.

Link relacionat: UdG Aigua

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