Snowdon is the tallest mountain in Wales and has a more extreme and hostile climate than most places in the UK.
Weather is difficult to predict and nowhere is this more true than in mountains. You should always check the forecast before you set off but be properly prepared for all types of possible and unexpected weather on Snowdon/ Yr Wyddfa.
What is the difference between climate and weather?
Weather is the atmospheric conditions on a specific day or time. This could look like rain in the morning and sunshine in the afternoon, for example.
The climate is the average weather over a long period of time. The North Wales zone is a temperate maritime climate, whereas Snowdonia is a 'hyper-oceanic climate'. This means there is a lot of rain and sometimes it does feel like the ocean is falling from the sky.
Interestingly, all this rain makes Snowdonia's climate perfect for temperate rainforest. Extraordinarily, the National Park is home to the surviving fragments of an extremely rare temperate rainforest that once covered much of western Britain. So don't forget your waterproofs!
Is the climate on the summit different to the bottom of the mountain?
The climate at the top of a mountain is colder and windier than at its base. This is mainly due to the Lapse Rate and the Funnel Effect.
In addition, the summit of Snowdon sees more precipitation (usually rain) than the valleys below.
If hot air rises, why is it colder at the top of a mountain?
Lapse Rate
As you increase altitude, the air pressure decreases as the height of the air column pressing down on the Earth's surface decreases.
When air pressure decreases, there is less air filling up the same volume of space, so it undergoes expansion. As the air expands, it cools through the 'Joule–Thomson process'. This is why the compressed air in a tyre or ball escapes it feels colder. One reason for this cooling at lower air pressures is that there are fewer collisions between the air molecules. The collisions generate heat.
In addition, the atmosphere reflects solar energy in the form of heat back down to the earth's surface through 'the greenhouse effect'. The higher up you are, more heat has been reflected back to the earth's surface so, the cooler it is.
Why does it rain more on Snowdon?
Thinner air at higher altitudes is less able to retain heat. A cooler temperature means there is less evaporation, so there is more moisture in the air. This is why there is more precipitation at the top of a mountain. You will literally be walking in the clouds.
What is the Funnel Effect?
The wind is usually stronger the higher up the mountain you go. Windspeed increases as the moving air is squeezed between the wedge shape of the mountain and the upper atmosphere (tropopause).
The closer you are to the summit, the more the air is squeezed, increasing the force of the wind. This is known as the 'Funnel Effect'.
The air is also funnelled locally, increasing windspeeds in areas such as a saddle between two mountains.
What should you look for when checking the weather to climb Snowdon?
You should pay particular attention to the hazards which will be identified in most specialist mountain forecasts. The main weather considerations when climbing mountains are...
Temperature & feels like temperature
Ground conditions
Wind speed & direction
Precipitation (rain)
Lightning
UV
Weather changes
Cloud cover
Check the temperature of Snowdon summit
Because of the Lapse Rate, the temperature on the summit of Snowdon is usually around 10°C cooler than at sea level. Because of this average annual temperature, the top is just 5.3°C and even in the summer months the mercury struggles to top 13°C.
You should check the forecast, especially for the summit and pack appropriately.
Check the 'Feels like temperature'
The 'feels like temperature' is a more accurate indicator of how cold it will actually feel. This mainly takes into account the effect on the human body of the actual temperatures and the wind chill.
Check the ground conditions
Even when the air temperature is mild there can be treacherous snow and ice on the ground well into May and again in early October. This can pose a significant hazard to life, especially at accident blackspots on the main footpaths. For more information on this, check out our blog all about the Dangers of Climbing Snowdon.
Does wind speed affect the climb?
Because of the 'Funnel Effect,' the windspeed is usually faster the higher you ascend.
It is a major reason to change or adapt plans, especially on ridges such as Crib Goch.
Wind speed can make walking more difficult and slow you down. It can also make compass bearings difficult to follow.
Anything over 50mph is considered 'gale force' and can literally blow you off your feet or a cliff.
How will the wind direction affect the terrain?
Wind direction can have a big impact on your day, especially if windspeed is high.
Knowing where the wind is blowing from and planning your day accordingly can be the difference between a pleasant walk and an arduous battle.
The side of the mountain exposed to the wind is known as the 'windward' side. The side shielded from the wind is the 'leeward' side. Because of adiabatic warming, the leeward side is generally warmer and drier.
Try to use the terrain to your advantage and use 'leeward' slopes and the mountain itself as a wind barrier.
In the UK the wind usually blows from west to east. If this is the case then the PYG or Miners track or possibly Watkin Path could be a good choice of route up Snowdon as they would be on the 'leeward' side of the mountain.
Another factor to consider when planning your route is to choose a footpath that goes into the wind at the start of your walk when you are enthusiastic and fresh. Try to have the wind to your back later in the day when you are more tired.
To learn more about route planning, contour lines and more consider booking onto one of our Map Reading Courses or check out our blog to learn about the map reading basics.
Precipitation on Snowdon
If it is raining anywhere in Wales it is likely to be the top of Snowdon. Snow is also common. You should always take a waterproof jacket even if the forecast is hot and sunny.
Other Weather factors
The worst place you can be during a thunderstorm is somewhere high and exposed such as the top of a mountain. In July 2021 two people were struck by lightning on the summit of Snowdon.
UV is stronger the higher up you go as there is less atmosphere blocking the suns harmful rays so don't forget your sunglasses and sun cream.
Weather changes
It takes around 6-8 hours for most people to walk up and down Snowdon so make sure you check the forecast for the whole day and not just up to late morning.
Cloud cover
You want to have a good view from the summit of any mountain and it certainly makes the trek well worth it. The Met Office's Snowdonia forecast predicts the chances of cloud free summits. Super useful!
What weather sources can you trust?
Remember that the weather forecast is only a a prediction of the likely weather so the only weather forecast you can actually trust is what you can see now and on the horizon.
You can get a good picture of what to expect by cross referencing a few forecasts. If they all predict the same conditions they are likely to be accurate.
Use Met Office weather information for Snowdon
The Met Office do a really informative specialist Mountain Forecast for Snowdonia
The Snowdonia forecast for tomorrow and two day forecast are the most accurate. Importantly this provides information on weather hazards and gives really useful data on predicted temperature, feels like temperature, wind speed and wind direction at different altitudes.
Look at the wind conditions on Windy.com
If you know how to read contours then windy.com have interactive maps showing wind direction, speed and more.
This paints a really clear visual picture of where the windward and leeward slopes are allowing you to plan a sheltered route. The radar feature also shows where the bands of rain and showers are so you can see for yourself how likely it is to rain.
Download the MWIS (Mountain Weather Information Service) app
MWIS have an app providing detailed 1 and 2 day weather forecasts for Snowdonia and other mountain regions in the UK.
This app is a fantastic resource for checking the weather for Snowdonia tomorrow and includes really useful information for hillwalkers and climbers including the chance of cloud free summits, visibility and effect of the wind on you.
Read Ground conditions report
Check out the Ground Conditions Report from the Eryri/Snowdonia National Park Authority in spring, autumn and especially winter. You may need specialist equipment such as an ice axe and crampons on all footpaths such as the Llanberis Path.
Check the Snowdon Weather Station
The weather station on the summit of Snowdon measures the conditions at the time and keeps a record of previous weather conditions. Snowdon Live who operate the weather station provide information on the weather hazards on the day and sunset time.
When can you trust the weather forecast for Snowdon?
The Snowdonia forecast for today is the the most reliable. The forecast for tomorrow is also reasonably accurate with the margin for error decreasing quickly over time after this.
Changeable weather, such as showers, is more inaccurate whereas if the forecast is for one weather type all day (eg. rain) then this will probably happen.
Predicting the Snowdonia weather next month might give you an idea of the general conditions in Wales on an average day. This is why the Met Office use hedging words such as 'often', 'probably' and 'likely' to describe conditions this far out.
The weather in Snowdonia next week might give you a general idea of temperature and wind speed. As mountain professionals, we check the weather everyday and find frequent and sometimes major discrepancies, especially more than 4 days out.
The 3 day forecast sometimes gives you a reasonable picture of what to expect. If the weather the day before and the day after is the same then it is likely to be the middle of a weather front so you can have more confidence in this.
The Met Office and MWIS only give detailed forecasts two days ahead as mountain weather beyond this is very difficult to predict accurately.
"Believe only half of what you see and nothing that you hear." Edgar Allen Poe
Does the weather on Snowdon change quickly?
I remember a day in May when I took a client on the Snowdon Horseshoe. It was raining when we set off from the car park. A few minutes later the rain had stopped and we had warm sunshine. We stopped, packed our waterproofs away and started slapping on the sun cream. Two minutes later it was snowing!
The weather on Snowdon can change quickly. Make sure you are well prepared with warm clothes and waterproofs.
Plan your trip to Snowdon rain or shine with Walk Snowdonia
Knowing how much you can trust a weather forecast, what to wear, the best places to get a good view and which route is best or even safe based on the conditions comes with years of experience.
We climb Snowdon every day in all types of weather year round so we have built up an intimate knowledge of the mountain. This knowledge will keep you safe in all conditions possible. So, for a day to remember for all the right reasons, book your adventure with us today!
Frequently asked questions about the weather on Snowdon
Now you know it is important to check the weather before planning a trip to Snowdon to ensure safety and help you prepare accordingly. Check out our FAQs here:
How can you avoid wind chill?
Plan a route that is sheltered from the wind on the leeward aspect of a slope. Cover exposed skin with gloves etc. and have plenty of warm layers.
Waterproofs make an excellent windproof barrier so consider putting them on, even if it's not raining.
Should you expect freezing temperatures at the top of Mount Snowdon?
Yr Wyddfa's English name is Snowdon which literally means 'snow hill'. This is because snow can occur on the mountain as late as May and as early as October. Check the temperature at the summit and the ground conditions to get a good picture of what to expect.
Can you see through low cloud?
'Clag' (mountain fog) is common on mountains and can reduce visibility to little over 10 meters. This is often why there is no view from the summit of Snowdon even if it's a fine day in the rest of Snowdonia.
Conclusion
Although the weather forecast can be inaccurate, it is the best prediction available as to the likely conditions. You should check it before you set off but always be prepared for worse than expected conditions. Always take waterproofs and extra warm layers and be willing to turn around if necessary.
Grab your waterproofs and check out some other useful resources...
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