Clouds

Appeared in LE FIGARO
OCTOBER 30, 2007

by JEAN-LUC NOTHIAS

SO WHAT DO CLOUDS REVEAL TO US?

Cirrus, stratus, altocumulus, capillatus...Welcome to the major cloud families.

One sometimes sees clouds as nothing more than large pieces of cotton (more or less clean), floating languorously in the skies at the whim of the winds. In fact, they are short-lived but very dynamic entities, sometimes hyperactive, always moving and being transformed. Clouds paint for us, in a visible manner on the canvas of the sky, the state of the atmosphere. They inform us about temperature, humidity, winds, electricity, pollutants. Yet, not satisfied with being scientists, they are also poets and invite us to dream by the forms they display. Next to sheep, one finds the heads of dogs or wolves, herds of deer, ogres or giants, medieval castles...Everyone is here well-served.

For the kingdom of clouds is the atmosphere. It is 17 kilometers thick (17 at the equator, around 12 for France) and contains 5 billion million tons of air. If the earth is seen at the scale of a desk-top globe, the atmosphere is a sheet of paper.

The business of clouds, is, like for a refrigerator, all about transfering heat. Clouds make use of all the forms of heat transfer possible to form and transform themselves. Convection first of all is the manner in which hot air, lighter than cold, will rise creating ascending currents. Conduction is the transfer of heat through contact, the source of certain fogs at the end of cold nights. Refrigeration is also operative as for, when invisible water vapor condenses as a cloud, the passage from gas to liquid will create heat.

Also active is light from the sun, what are called radiation effects. The combination of all these elements as a function of circumstance constantly "clouds up" the atmosphere, from top to bottom around the planet. One could say there are as many clouds as places and circumstances. Which is not enough for man, who needs to classify and label. Clouds as much as anything, are taken to task. In 1802 a British pharmacist, Luke Howards, amateur meteorologist, proposed a classification system inspired by the classification of living things put forth by Carl von Linne. A clever enough system in that it allows one, starting from three (then four) basic categories, cumulus, stratus, cirrus and nimbus to mix these categories to define intermediate clouds. One thus has cirrostratus, altocumulus or cumulolimbus. And for specialists more subdivisions such as humilus, capillatus or mediocris...

THE FLEETING CUMULONIMBUS

The classification system is based on the altitude at which clouds form. The lowest are the stratus (less than 2000 meters), stack able clouds. Next come the cumulus and their cousins, those little round cottony ones (between 2 and 7 kilometers) and, at the very top, the cirrus (from the Latin "curly hair"), between 5 and 14 kilometers. And the first consequence of these different altitudes is that the forms and contours of these clouds are very different. Thus, a cumulus has very clear contours, because there is a distinct frontier between its humid interior and the drier air surrounding it. A cirrus, in turn, situated much higher, is composed of ice sequins and hence has more imprecise contours.

The least likable cloud is the one to which Baudelaire refers to in the verse "The low and heavy sky weighing like a pot-cover". This is the stratus, characteristic of grey and overcast skies. It won't bring heavy precipitation, but is generally opaque and obstinately hides the sun. The stratus nonetheless has an interesting side. It sometimes tires of flying and prefers to skim the ground. It is thus the only cloud in which we can actually walk. Even if we call it mist (less than one kilometer of visibility) or fog (from 1 to 5 kilometers of visibility).

The cumulus is also called the "nice weather cloud". It is incapable of crying, because it does not contain much water it is estimated that a great big summer cumulus several hundred meters in length contains a mere hundred litres of water. But there is a black sheep in the kingdom. It is the giant cumulonimbus bringer of heavy rains, lightning and thunder. Its base, somber and menacing, can be found at between 1 000 and 1500 meters and its immense body can range up to 12 000 - 13 000 meters. This "king of clouds" can move alone, but most often it is part of a group. Thus, the lifespan of a cumulonimbus might be approximately one hour, the storm, though, will last longer if there are many.

For more information: (reference is made to a French-language translation of a work by Gavin Pretor-Pinney of the U.K.)