A storm out of nowhere? Nonsense! A storm cloud never just appears. It builds up gradually and sends out warning signals – and you can pick them up. But only if you know what to watch for. There are a few basic types of cloud – and each one tells you something about how the weather will develop over the next few hours.
They're clusters of tiny water droplets, ice crystals or both at once. They form when moist air rises, cools at altitude and the water vapour condenses on tiny particles of dust, pollen or sea-salt crystals.
Most clouds sit in the troposphere – the lowest layer of the atmosphere, where weather happens. The lowest ones have their base within two kilometres of the ground, while high clouds reach up to 13 kilometres. Low clouds are made of water droplets and bring fog, drizzle or poor visibility. The highest, the cirrus clouds, are made mainly of ice crystals.
It rains or snows when water droplets and ice crystals grow so big that the air currents can no longer hold them up – and they start falling to the ground. Low clouds are mostly water droplets, but they don't drop heavy rain. You get more of a mizzle, drizzle and lousy visibility. Heavy rain and storms come mainly from clouds that span several levels.
Which level a cloud sits at tells you how things are now. But what you care about is what's coming. That's why you need to watch how the clouds develop, too. Is a cloud growing upwards fast, thickening, darkening, dropping towards the ridge or spreading across the whole sky in a short time?
Small, separate heaped clouds (fair-weather puffs) can mean a stable day – but if they shoot up quickly it may signal an approaching storm. Or the other way round – high cloud (veils) gradually thickening and lowering can herald a front and rain within 12 to 48 hours.
When the wind drives moist air against a slope, the air is forced upwards, cools and the water vapour condenses. That way a cloud can form right on the windward side of the ridge. Or sit on top of it like a cap. It can then rain up there while the valley is still clear and relatively calm.
They form in the upper troposphere, where it's so cold that they're made mostly of ice crystals. They look harmless – thin wisps, a milky veil or small white scales. But they can also be a warning – if they thicken and keep multiplying, a front may be on its way within the next few hours to two days.
Delicate, fibrous clouds. They look like feathers or wisps. Made mostly of ice crystals. On their own they still mean calm weather – but if they multiply and thicken fast, they may be the first hint of a front arriving in the next 12 to 48 hours.
A high, milky veil. A thin, translucent layer covering much of the sky – often creating a halo (a bright ring) around the sun. It points to rising humidity and incoming frontal cloud – rain may follow within hours to a day.
Tiny cloudlets or fish scales – a mackerel sky. Small white puffs arranged in regular rows. They usually don't bring rain, but they can point to a change in the airflow and less stable air higher up. Watch whether they grow, merge and sink.
They cover a bigger part of the sky and you can't miss them. They can shroud the summits, create fog and gradually bring rain or snow. They're often part of a front moving in – the high veil thickens, the cloud base drops and the sky slowly "puts the lid on". So no short-lived change – we're talking about the weather turning bad for a while.
Mid-level cloudlets. Larger white to grey tufts arranged in wide fields. If they appear on a warm, humid morning and thicken during the day, they can foreshadow afternoon showers and storms. That's why in summer it pays to start your hike early.
A greyish veil. A uniform grey or blue-grey layer. It covers most of the sky and the sun shows only as a blurry bright patch. It's often an advanced stage of an approaching front – and rain may arrive within a few hours.
The rain cloud. A dark, thick, shapeless layer of cloud, usually covering the whole sky, with a ragged, blurry base. It brings steady rain or snow that can last for many hours. So not a quick shower – but a widespread turn for the worse.
A low cloud blanket. A uniform grey cloud that looks like fog that never lifted off the ground. It often hides slopes and ridges. It brings poor visibility, damp, drizzle and sometimes light snow. In the mountains, visibility can be much worse than in the valley.
Low, lumpy layers. Grey-white, bumpy clouds. They form a low blanket with patches of blue sometimes showing through the gaps. They mean fairly stable, damp weather with no showers or only light ones – often after a front has passed.
They form when the sun heats the ground and warm air rises. Small, flat-based cumulus clouds are a familiar sight on a stable summer day. But in the mountains the slopes heat up unevenly – and the terrain can boost the updrafts. Innocent little puffs can turn into massive towers that unleash storms.
White heaped clouds with flat bases. They form during the day as the ground warms up. They usually signal stable weather – but once they start shooting up like cauliflower towers and their bases darken, rain and storms may be on the way.
Towering giants. Cumulus clouds growing sharply upwards, looking like a huge cauliflower or castle walls. They indicate strong updrafts – and a high chance of heavy showers or further development into a storm cloud. Time to change plans and head down.
The thundercloud. A huge, tall tower with a dark base, its top often spread out into the classic anvil. It brings lightning, torrential rain, hail and strong wind. As soon as you spot a storm tower nearby, retreat – don't wait for the first flash.
The mountains themselves have a big say in how clouds form and move. And lens-shaped lenticular clouds, cloud caps or long banner clouds aren't just something to photograph. They can warn of strong wind and rapid changes in visibility – even when the valley stays calm and clear. That's why local differences in the mountains can be pretty big.
A lens above the ridge. It looks like a UFO or a stack of pancakes. It forms in strong airflow, so it's a warning of strong wind – and can come with a foehn too. So even if the valley is calm, an exposed ridge can be dangerous because of the strong wind.
It hugs the windward slope like a cap, or streams off it downwind like a long banner. On the windward side, fog and rain can build up quickly, while the leeward side stays drier for a while. Expect visibility to drop fast and the wind to pick up.
The foehn is a warm, dry and often very gusty downslope wind. It forms on the leeward side of mountains when air spills over the ridge and warms and dries out as it descends. On a hike it can be dangerous because of sudden gusts and sharp temperature swings – and in winter it also speeds up snowmelt and raises the avalanche danger. So when the foehn is blowing, calm weather in the valley doesn't necessarily mean safe conditions up high.
An experienced mountaineer doesn't just watch their feet. They keep an eye on what's going on above them, too. In the sky. Is it mostly high wisps, a milky veil, a low blanket, heaped clouds or massive towers? Are the clouds high up or hiding the surrounding peaks? Are they growing, thickening, darkening, lowering or breaking up? And which way are they moving?
In the mountains you need to heed the warning signs in time – and change your plan at the first sign of things getting worse. Not once it's already raining, blowing a gale and visibility is down to a few metres. Here are the main ones:
On its own, nothing. It's just a number that also changes with altitude – the higher you go, the lower the pressure, because there's less air above you. What matters is the trend. A slow, steady drop usually means a low-pressure system or a front is coming – the weather generally getting worse. A rapid drop, though, is a much louder warning – change is coming soon, for example as a storm.
This is the wrong question: "What should I do if a storm catches me up high?"
This is a better question: "What should I do so that a storm doesn't catch me up high?"
Because, as we said at the start, no storm comes out of nowhere. And if you have to face it somewhere on a rock face, on a via ferrata cable or on an open ridge, your life is already at risk. That's why you need to act in advance. Sometimes well in advance. The clouds will tell you when.
You check the forecast, look at the sky – and cut part of your hike. Or the whole thing. And then nothing happens. In the afternoon you'll be kicking yourself that the apocalypse never came. You could easily have gone. But you chose not to take the risk. Because then there are the stories of people who checked the forecast, looked at the sky – and set off anyway. And it ended badly. As the saying goes: the graveyards are full of heroes. But the mountains will still be here for you tomorrow.
