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Heavy weather

Is it possible for a ground-based observer to calculate how much water is in a particular cloud? Do size and colour have any bearing? If this is possible, how is it done – I want to impress my friends. If it isn’t, can it be calculated by more scientific methods?

• As explained below, there is an approximate method to answer this question that will impress your friends, and a precise answer for which you’ll need an excellent Doppler radar system and a large grant from the government – Ed

• The amount of water inside a cloud is not different from the amount of water in the clear air around it. However, while the clear air contains water vapour, inside the cloud the air is saturated with water vapour and it has condensed out to produce the cloud. The difference between the two states is caused by temperature differences rather than the water content.

The colour of a cloud doesn’t make much difference either. In the higher part of a cloud, the water is in the form of ice crystals. Lower down it is a mixture of ice and liquid water. The colour of the cloud depends mainly on this ice/water mixture and the size of the water droplets, and less on the total amount of water.

An estimate of a cloud’s water content can be obtained from the amount of rain the cloud can produce (see Marcel Minnaert’s The Nature of Light and Colour in the Open Air published by Dover in the UK, Springer in the US).

If the entire atmosphere were saturated with water and it all fell in a steady stream, this could produce some 35 millimetres of rain, while the thickest actual clouds produce about 20 millimetres.

Cloudbursts can produce 50 millimetres or more, but this requires additional moisture from the surrounding atmosphere, which means such events are localised.

The heaviest cloudbursts roughly follow the following equation: the rainfall in millimetres equals 6.5 multiplied by the square root of the time in minutes that it has been raining. A more typical shower produces a few millimetres of rain, at a rate of perhaps 0.1 millimetre per minute. Normally 1 millimetre of rain corresponds to 1000 cubic metres of water — weighing a thousand tonnes — per cubic kilometre of cloud, though the thickest clouds can contain up to 20 times as much.

You can also estimate the amount of water from the volume of the cloud. By volume, the fraction of the cloud filled with water is about 1 millionth, or 0.0001 per cent. The cross-section area of a cloud can be measured from its shadow. A small cloud 500 metres by 500 metres and 100 metres high has a volume of 25 million cubic metres, of which roughly 25 cubic metres will be water, weighing 25 tonnes. Even if you can’t calculate the precise amount of water in a cloud, these numbers may still impress your friends.

Albert Zijlstra

Department of Physics

UMIST, Manchester, UK

• Sadly, just looking at a cloud does not give very precise information about how much water it contains. The colour of a cloud depends entirely on the relative position of the viewer and the physical structure of the cloud. Its apparent size is dependent on the altitude of the cloud, and this is generally very difficult to judge from a single observation point.

But knowing exactly how much water is contained in a cloud is important for producing accurate weather forecasts. In the UK, we are lucky to have very sophisticated tools like the Council for the Central Laboratory of the Research Councils’ Chilbolton Observatory (). This facility uses a Doppler radar.

The choice of frequency for the radar beam is very important. If the beam interacts too strongly with the water in a cloud, in terms of either reflecting or attenuating the signal, then the radar will have only a limited ability to penetrate cloud structure. If the interaction with water is too weak, then no useful information can be returned at all.

The Chilbolton facility can analyse and extract a huge variety of data and it has a maximum range of around 160 kilometres. It is able to provide information on droplet density, size, speed, and whether the droplets are water or ice.

Using this tool, your reader could work out fairly accurately the total water content contained within a cloud and, from the structure of the cloud, how likely it is to start raining – this technique has proved very useful at the Wimbledon tennis championships in past years, which are, of course, notorious for being interrupted by downpours.

Such radars help to produce detailed information on weather, from tracking hurricanes to helping to produce your daily weather forecast and predicting areas of turbulence on aircraft flights.

Dave Richards

Cambridge, UK

This week’s questions

Hidden gas

Where do neon light manufacturers get their neon from? I know from school that any neon present on Earth was originally made in supernovae. But where is it now? Being inert, it cannot form compounds from which we can extract it, so is it just floating about in the atmosphere and, if so, how do we isolate and concentrate it for use in light tubes?

Fabian Nesmith

Paihiatua, New Zealand

Soap on a hope

Is it possible to blow a toroidal soap bubble (one shaped like a ring doughnut)? And, if it is, would it collapse immediately to a sphere? Could its life be prolonged by spinning its surface, as with smoke rings?

Peter Gardner,

Blawith, Cumbria, UK

Natural effervescence

The labels on some bottles of sparkling mineral water inform us that its carbon dioxide content comes from the same source as the water. This is removed when the water is collected and added again to the packaged bottle when it is ready to be sold to the public. In what form is CO₂ found in natural mineral water springs, how does it get there and why is it removed and then added back before bottling?

Ray Train

Doncaster, South Yorkshire, UK