How to use clouds to predict the weather

So will it be sunny in Snowdonia today, or can we expect rain? Let’s look to the skies and find out. All you have to do is know your clouds!

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In his regular bog spot, writer and hiker Phil Thomas shares the trade secrets of his passion: the Great Outdoors. This week, he discusses everyone’s favourite topic – the weather!

When it comes to being in the mountains, it’s best to expect unpredictable weather. But, did you know? As well as checking the weather forecast before you set out, you can predict the weather during your adventures too.

So will it be sunny in Snowdonia today, or can we expect rain? Let’s look to the skies and find out. All you have to do is know your clouds!

Check the right forecast before you leave

Most of us are familiar with the BBC’s forecast, which for many years was provided by the Met Office (and now Meteo Group). But there’s also a specific forecasting service for the UK’s mountains. The Mountain Weather Information Service (MWIS) is the go-to place for more accurate weather expectations in Snowdonia.

Some accommodations regularly publish this information, and it is posted in popular jumping-off spots such as Ogwen Cottage. If you’ve got Internet access, go to before you set off and check the latest.

Blue sky in the morning…

Snowdonia’s location, standing over the west Wales coast, means you normally need to look westwards out to sea (or over the Llyn Peninsula or Anglesey) to see what’s heading our way.

Days that start bright and sunny can change quickly. But here’s a rule of thumb: If by mid-morning, heading towards lunchtime, your blue sky is only interrupted by cumulus clouds – fluffy, cotton-wool clouds – then rejoice, you can expect a rain-free day.

It’s also unlikely that the cloud level will drop to smother the peaks. In Snowdonia, this is a perfect summer’s day!

Wall of cloud in the west

Look westwards. If there’s a wall of cloud across the sea, Anglesey or the Llyn Peninsula, there’s a weather front coming. The weather is going to change.

You may have a couple of hours but probably less. If the weather forecast has predicted a deterioration, then that’s the start of it. It will cloud over first, the mountain tops will disappear, and shortly after there will likely be rain (or snow if it’s cold enough).

Snowdonia’s favourite – cumulonimbus clouds

If we did a cloud poll for Snowdonia, cumulonimbus clouds would easily come out tops! They’re bigger and darker then cumulus clouds, simply because they carry more moisture.

If the tops of the clouds are really high, and are forming an anvil shape, they could become torrential downpours, with hail and thunder. If they’ve formed before lunchtime, be careful. Late afternoon is the most common time for thunderstorms to strike (following the warmest part of the day). Rocky mountains are no place to be when there is lightning about.

From your mountain viewpoint it’s often easy to see if rain is already falling from a cumulonimbus cloud. You can often track the path of these showers too. They can be very localised in the mountains. It’s not uncommon for one valley to get a soaking and another to stay completely dry.

It’s not smoke – it’s a pannus cloud!

An overcast day means it could rain – or clear – at any time. But how do you know what the weather has in stall for you next? If you see low, fast-moving clouds – a layer of dark clouds under the overcast layer that can look like smoke – they’re pannus clouds.

Rain is imminent! You’ve got 10-15 minutes to take cover. If you’re already above the cloud base then of course you won’t see them. You’re probably already wet…

UFOs – or lenticular clouds?

These spectacular cloud formations are quote common in North Wales. They tend to form when the air is still and appear like saucers (often layered) above the hill and mountain peaks.

They’re more commonly seen in the afternoon as air starts to move in the upper atmosphere. They’re an indication that settled weather is going to change within the next 24 hours.

If the weather does take a turn for the worse, remember navigating the weather is just one way of staying safe. Why not read Phil’s blog on building a hiking GPS for less than £10 here?

So don’t forget, next time you’re walking the hills and mountains of Snowdonia. As well as looking down at where you’re putting your feet, stop long enough to look up at the sky too. It’s always giving you clues about what the weather is intending to do next.