Layering by altitude…
Clouds: a mass of moist air that rises, progressively drops in temperature and therefore in its capacity to hold water vapour, until it reaches saturation point.
Once that limit is reached, the excess water is expelled in the form of fog, cloud, rain, hail, snow.
Clouds are classified by their height above the ground (high between 6 and 12 km; medium between 2 and 6 km; low below 2 km), by shape (stratus if they develop horizontally, cumulus if they develop vertically) and by consistency.
Low clouds (0-2000 m), not dangerous:
Stratus: a uniform blanket similar to fog, at most drizzle ¨ Stratocumulus: flat clouds with a dark base, between them you can glimpse the blue of the sky, light rain ¨ Cumulus: clouds with more or less vertical development, harmless, they are a sign of fine weather, but if rising air currents make them start to tower up, -watch out- they are turning into cumulonimbus.
Medium clouds (2000-6000 m), bringers of rain:
Cumulonimbus: the most feared by the summer mountaineer, these are clouds of massive vertical development, with intense swirling air currents inside them that “bode no good at all”. And indeed these clouds produce thundery downpours (of rain or hail) accompanied by lightning.
Altocumulus:
High clouds, regularly arranged, like little sheep; they are harmless.
Altostratus:
Compact clouds of an even grey colour; they make shadows disappear: they bring continuous rain and snow.
Nimbostratus:
Compact, black clouds, laden with a mixture of droplets, snowflakes and ice crystals in motion. They bring the classic heavy, long-lasting precipitation.
High clouds (above 6000 m), harmless:
- Cirrus
- Cirrocumulus
- Cirrostratus, arranged in filaments, they cast no shadows.
How to read the clouds during a hike
- Cumulus growing taller: if they start towering up during the day they are turning into cumulonimbus. That is the signal to bring your return forward and get off exposed ridges and summits before storms and lightning arrive.
- The sky veiling over and shadows disappearing: the typical sign of altostratus, which bring continuous precipitation; better to choose short routes with easy escape routes.
- Low, fog-like stratus: generally not very dangerous in terms of weather, but they cut visibility and make route-finding harder.
- Cirrus and altocumulus: harmless in themselves, but it is worth watching how the sky develops over the following hours.
To know how to behave in a thunderstorm, read the fact sheet on lightning; for other useful signs, see watching the sky to try to forecast the weather.