|MadSci Network: Earth Sciences|
The problem with this question is that it is difficult to define things precisely. If we are talking about woolly clouds (cumulus) it is fairly clear how big "a cloud" is, although we should remember that sometimes there are little broken off bits. If we were to count them as separate clouds, it would make the average smaller! But what about storm clouds (nimbus) that might cover the whole of the sky for an area 100 kilometres in any direction? Or the high up wispy clouds, or mares' tails, or mottled clouds, or thin hazy cloud (cirrus, stratus, cirrostratus) -- do we count the separate strands or lumps as parts of a single cloud, or as separate clouds? Surprisingly, there is also a problem with the idea of an average when there is a large range of sizes involved. Let me give an example: What is the average size of the particles in concrete mix? You might think that that is fairly easy. You make up concrete by taking one part (by weight) of Portland cement, two parts of sand, and three parts of screenings. The particle size in the Portland cement is 0.01 mm, in the sand it is 1 mm, and in the screenings it is 10 mm. so the average is (0.01 x 1 + 1 x 2 + 10 x 3)/6, which comes to 5.3 mm. But if you were to take a sample, and actually count the particles, you would find, typically, that there were 1 billion cement particles for each 2 thousand sand particles for each 3 lumps of screenings. The average would be (0.01 x 1 000 000 000 + 1 X 2 000 + 10 x 3)/ 1 000 002 003 which comes to 0.01 mm ! Which is the 'true' average? Scientists actually talk about the 'number average' and the 'mass average' in these sorts of circumstances. Their values can be quite different. The number average mass of clouds would be quite different to the mass average. Finally, what do we mean by "the weight of a cloud". Are we referring to just the weight of the ice crystals in the cloud, or do we also include the weight of the air contained in the cloud? There is about 100 times as much air as ice in a typical cloud! So let us get back to clouds. I am going to talk about the "typical weight" of a cloud, rather than the average weight. I am going to talk just about the ice, not the air that is also present. And I am going to talk about a cumulus or cumulonimbus cloud -- the sort of woolly cloud that forms at a reasonably low height when humid air rises, and that might later turn darker coloured and lead to a thunderstorm type shower of rain. There are two ways that I can think of of estimating the weight of the cloud. The first and easiest is by thinking about the rainfall it might produce. An estimate of 5 mm of rain in an area 8 x 5 km seems not unreasonable for a single bank of showery cloud. (Translating for those who do not use international units, that means 20 points of rain over a 5 x 3 mile area). That amounts to a volume of 200 000 cubic metres of water, which would weigh 200 000 tonnes (I am much more comfortable with metric units. I will not stretch my translation skills any further, except to say that tonnes and tons are near enough to the same thing). The other way of estimating the weight of the cloud is a bit harder. You think about the water content of the parcel of air it comes from. Suppose that the air started off down at sea level at 25 deg C and 70% humidity. Then when that air rises it gets to a colder temperature, and soon it can no longer hold its water vapour content, and ice crystals start to form. We can calculate that the humid air contains about 16 g of water vapour per cubic metre, and somewhere between half and all of this will finish up as ice crystals in the cloud. If we work on this basis, then the ice in a lump of cloud that comes from an air parcel about 1 km long by 1 km wide by 200 m thick turns out to weigh 1 600 to 3 200 tonnes. If you have ever looked at satellite photographs of cloud, you will have seen tropical cyclones (hurricanes), where a bank of unbroken cloud stretches for perhaps 300 to 500 km in all directions from the centre of the storm. And you will have seen cloud banks associated with cold fronts, stretching perhaps 1000 km long and 50 km wide. If you regard these as single clouds, then they are huge. Wispy, high altitude clouds, on the other hand, can be really very thin and light.
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