Reading clouds and understanding weather forecasts

When air rises, it reaches heights with lower air pressure. There it expands, and as it expands it cools – as long as no cloud forms, by about 1 °C per 100 metres. At some point it reaches its dew point, the water vapour condenses and a cloud forms. The flat base of many heap clouds shows exactly the height at which this happens.

How the air rises determines the shape of the cloud. If it rises strongly in individual places, for example over sunlit ground, heap clouds (cumulus) form. If air is lifted slowly over a large area, for example when warm air glides up over colder air, flat layer clouds (stratus and related forms) form. Very high up, where it is very cold, clouds consist of ice crystals and look fibrous (cirrus).

Condensation releases heat. This heat makes the air in the cloud rise further, sometimes to very great heights. In this way a harmless heap cloud can turn within a short time into a thundercloud (cumulonimbus) whose top spreads out into an anvil.

Small, flat heap clouds with clear edges on a sunny day are usually a sign of settled weather. If they are already growing noticeably upwards in the morning, towering up and darkening at the base, the air is unstable – showers and thunderstorms in the afternoon are then likely. A spreading anvil at the top and a dark, low base are a clear warning sign.

Ahead of a warm front, high feathery clouds (cirrus) often appear first. They thicken into a milky veil through which the sun sometimes shows a ring – a halo. Then the sun is only faintly visible as if through frosted glass, the clouds become lower and denser, and finally longer-lasting rain sets in. This sequence can stretch over many hours.

Lens- or UFO-shaped clouds over mountains that seem to stand still form in strong airflow over mountain ranges. They show strong wind aloft – on ridges and summits it can then be very stormy even if the valley is calm.

Modern weather forecasts are produced by computer models that divide the atmosphere into a huge grid and calculate how air, heat and moisture move. Because the starting state is never known perfectly and small differences grow over time, weather services often run many slightly different versions. The more of them show the same thing, the more certain the forecast.

This produces probabilities. A chance of rain says how likely it is that any noticeable precipitation falls in an area and period – not how long or how heavily it will rain. In thunderstorm situations, place and timing are especially uncertain: the model knows that storms are likely, but not exactly over which valley.

That is why the official forecast is the best basis, and looking at the sky is the check on site. If the weather arrives faster than forecast, what you see is what counts. Before the trip, also read the warnings from your national weather service, especially for thunderstorms and gales.

Towering clouds with dark bases, thunder or lightning mean: switch to thunderstorm safety immediately. Leave summits, ridges, open areas and the water, and avoid isolated trees and metal cables. According to a common safety rule, anyone who can hear thunder is already close enough to be struck by lightning.

You can roughly estimate the distance of a thunderstorm: count the seconds between lightning and thunder. Sound takes about three seconds to travel one kilometre. If the gaps get shorter, the storm is coming closer.

On mountain trips, set turnaround points and alternative routes in advance, and at signs of deterioration turn back earlier than planned rather than later. Weather rules from clouds never replace the warnings of weather services.

You learn to read clouds by observing. Look consciously at the sky once every day, note the clouds and compare them with the weather that comes a few hours later. A small notebook or dated photos help with this.

The World Meteorological Organization's International Cloud Atlas distinguishes ten cloud genera. You do not need to know them all reliably. If you can reliably recognise heap clouds, high feathery clouds, flat layer clouds and the thundercloud, you have already gained a lot for the trail.

And be careful with old folk weather rules. Some have a kernel of truth, but it only applies in certain regions and weather situations. In the mountains many local effects also arise that throw such rules off.