Tue 11 Jun
Cloud Weight Watchers
How to weigh a cloud

Clouds are heavier than one might think. Here's why guessing a clouds weight can be important for meteorologists.

The weight of a cloud is the weight of its constituent water droplets or ice crystals (or snowflakes, sleet or hail ). The liquid water and the ice content are measured in grams per cubic meter of air.
Different types of cloud have different characteristic liquid water contents. For example, in a thunderstorm cloud (cumulonimbus) liquid water content can vary between 5 to 15 grams per cubic meter.
In non-rain clouds, the water droplets are numerous, but very tiny and thus such clouds seldom reach even 1 gram per cubic meter.
The weight of the ice content of cirrus clouds is even less, something between 0.05 and 0.5 grams per cubic meter.

Consider a humble cumulus cloud on a fine summer day. Maybe about one kilometre wide and long and also roughly as high. Our cloud is occupying a volume of one cubic kilometre, not especially large, but very typical for such clouds. A cloud like this has about 0.5 grams of liquid water in each cubic meter. If we multiply 1,000 x 1,000, x 1,000 x 0.5, we find that the cloud's liquid water weighs about 500 million grams, which is about 550 tons. The condensed water in this relatively small cloud is enough to fill a small public swimming pool 10m (33ft) wide, 25m (82ft) long and 2.2m (7ft) deep. Isn't it amazing that something this big and heavy can form from clear air within just a few minutes?

Impressed? Now imagine a solid afternoon thunderstorm cloud with the typical cauliflower texture. Even a very average one has a typical length and width in the order of about 10km. In the mid-latitudes such clouds tower up to a height of 5 to 7 km or so. If we multiply 10km x 10km x 6 km x 10 grams, we find that the cloud's liquid water weighs about 6000 billion grams, or 6 million tons. Enough water to fill a small reservoir lake. Can you believe how much a supercell thunderstorm does weigh? The typical size of such a mesoscale system is in the order of 100km by 100km, towering up to height of more than 10km - calculate yourself!

Why clouds do not fall from the sky? Read the second part of our Cloud Special!