A summer morning, and the sky is dotted with white, fluffy clouds with flat bottoms, like a child's drawing. By lunchtime there are more and more of them, and by three o'clock one of the clouds has grown into a dark tower. We explain when cumulus clouds promise a fine day and when it's time to pack up the picnic.

Cumulus clouds are the most recognisable clouds in the sky. They're the ones children draw, the ones people gaze at lying on the grass, looking for familiar shapes. But for anyone spending the day outdoors, at the dacha, fishing or hiking, these clouds are also a clue. From how they look and behave, you can tell several hours in advance whether there'll be a thunderstorm.

How Cumulus Clouds Form

Cumulus clouds are born from warm air rising from the ground. The sun heats the soil, the soil warms the air near the surface, and "bubbles" of that air begin to float upwards.

At a certain height, the air cools to its dew point, the water vapour condenses into tiny droplets, and a cloud appears.

Hence their main feature: a flat base. It marks the height at which the rising air begins to condense. That's why all the cumulus clouds in the sky usually have their bases at the same level, while their tops are puffy and "billowing".

"Fair-Weather Clouds"

Small, flat cumulus clouds, which meteorologists call fair-weather cumulus, mean settled, clear weather.

"Cumulus clouds are harbingers of good weather," said Lidiya Iradionovna, an observer at the Uchebnaya weather station in Novosibirsk.

They usually appear in the morning, when the ground begins to warm up, and there are most of them around midday. In the evening, when the sun sinks and the flow of heat from the ground weakens, the clouds spread out into a flatter layer or gradually disappear. If scattered fluffy clouds hang in the sky all day and the sky clears by evening, that's a sign of settled weather.

When the Clouds Start to Grow

It's time to be on your guard if cumulus clouds stop being flat and start growing upwards. Hannah Christensen, an atmospheric physicist at the University of Oxford, explains that if small cumulus clouds stretch upwards, heavy rain will soon follow. In summer this happens often: morning cumulus clouds turn into thunderclouds by the middle of the day.

First the cloud grows into a towering cumulus: tall and dense, with a billowing top that looks like a cauliflower. This is the last stage before downpours and thunderstorms. The base of the cloud darkens, as less and less light gets through its thickness.

The Anvil: A Sure Sign of a Thunderstorm

The final stage is the cumulonimbus cloud. It rises so high that its top reaches the boundary above which the air stops rising. There, the cloud spreads out sideways, forming a flat "anvil". The upper part of such a cloud looks blurred and fibrous: it's already made of ice crystals.

If you see a cloud with an anvil, a thunderstorm is already under way or about to begin. Clouds like these bring downpours, hail, lightning and squalls.

Another warning sign is clouds with an "udder-like" underside that look like hanging sacks (known as mammatus). According to Lidiya Iradionovna, they "herald a thunderstorm or even hail".

Why Thunderstorms Are More Common in the Afternoon

Cumulus clouds grow as the ground heats up, so their development usually peaks in the second half of the day. On a hot, humid day, morning's fluffy clouds can grow into thunderclouds within a few hours: hence the typical summer picture of a clear morning and a thunderstorm at around 3–5pm.

That's why it's important not to look at the sky just once, but to watch how the clouds change. A cumulus cloud on its own doesn't tell you anything. What tells you something is how fast it's growing.

Signs the Weather Is Getting Worse

We've already mentioned the main sign: the clouds are growing upwards rather than outwards. American meteorologist Tim Vasquez, in an article for pilots, adds two more signs of deteriorating weather that are useful on the ground too:

  • the cloud bases are lower than usual;

  • the lower edge of the clouds stays ragged and doesn't even out over time.

And here are the signs that the weather will stay fine: the clouds are small and scattered, with an even, flat base, and they spread out or melt away by evening.

How to Use Clouds When You're Outdoors

  • Look at the sky at 10–11am

If by then the clouds have already grown noticeably and are piling up on top of each other, there's a high chance of a thunderstorm in the afternoon. If the sky is full of small, fluffy clouds, the day will most likely be clear.

  • Be back by 3pm

If the clouds were growing in the morning, it's best to plan hikes, fishing and work in the vegetable garden for the first half of the day, when a thunderstorm is less likely.

  • Seen an anvil? Take shelter

Don't wait for the first rumble of thunder: a storm can arrive within a few dozen minutes, and lightning can strike several kilometres from the cloud.

  • Check the forecast

Clouds are a good clue, but forecasters give the precise forecast of dangerous weather. If thunderstorms are forecast and towers are already growing in the sky in the morning, it's a reason to postpone your outing.