How to Catch a Cloud Inversion
January 21, 2025Of the many wonders found in the mountains, cloud inversions are one of the best. The sheer beauty of dark peaks rising like islands from an ocean of cloud is difficult to match. Cloud inversions are infrequent and elusive, which is part of their appeal. The specific meteorological conditions required can be challenging to predict, so there is always a bit of a gamble involved. With a bit of research, we can ascend into the clouds hoping to emerge above them into broad daylight, but we can never be certain until it happens.
Even on days where the conditions are favourable and an inversion seems likely, it is difficult to guess exactly which peaks will sit above the cloud, especially in Britain where our mountains are fairly small compared to other regions. This uncertainty can be frustrating and can lead to a day spent hiking in the cloud with little reward, but it is also part of the joy. One of the things that makes cloud inversions so enticing is the suspense of not knowing whether you will get above the cloud, and the sheer delight when you do.
This article aims to help improve your chances of catching a cloud inversion, though a degree of that uncertainty will always remain. There will be times when you succeed, and times when you don’t. Only getting out into the hills and observing the weather patterns, particularly those in a specific area, can provide the experience needed to become an expert cloud inversion hunter. I’m certainly still learning, and I expect to be for the rest of my life. But if you’re not sure where to start, then here are some useful tips.
What is a cloud inversion?
The scientific name for a cloud inversion is ‘temperature inversion’. Usually, air temperature will decrease as you ascend – although the rate of this decrease can vary. During a temperature inversion, this is reversed so that cooler air becomes trapped near the ground with a layer of warmer air above it. Clouds (or fog) can become trapped beneath an inversion, so they sit at a much lower height than usual – enabling people to hike or climb above the cloud and emerge into broad sunlight. This is what is known as a cloud inversion.
What to look for
- High pressure - High atmospheric pressure creates a more stable atmosphere and is more likely to provide clear skies overnight. This can allow air to cool rapidly and sink (to lower ground). This contributes to a layer of cooler air in places like valley bottoms being trapped by a layer of warmer air, resulting in an inversion. In contrast, low pressure brings more turbulence, mixing the air and preventing inversions.
- High moisture - Enables the formation of mist, fog and cloud which can be trapped beneath inversions, creating the ‘sea of cloud’ conditions that are associated with them. A combination of high pressure and high moisture makes cloud inversions especially likely.
- Little to no wind - Cloud inversions are typically associated with calm days and slow-moving weather patterns. Wind disturbs the air and blows the cloud across, preventing (or destroying) an inversion. However, as the inversion needs moisture to form cloud, a light breeze can be helpful in providing this moisture. I look for wind speeds of under 5mph as a rule of thumb, although I have experienced cloud inversions in higher wind speeds than this.
When to find cloud inversions
- Autumn and winter - Inversions are more common in autumn and winter, where the longer (and colder) nights allow for greater cooling of the air. Often in the mountains, the weak and low winter sun only reaches the valley bottom for a couple of hours, if at all, so the air near the grounds stays cooler for longer. As a result, inversions are more common and are likely to last longer in this time. However, like all of these recommendations there are always exceptions. For example, in Snowdonia big cloud inversions made of thick sea fog are common in Spring. One of the best cloud inversions I’ve ever seen was in on top of Ben More on the Isle of Mull in the middle of August. If other conditions are right, then you may find yourself above a cloud inversion regardless of the time of year!
- Early morning - Because the sun burns some inversions off as the day progresses, the chances of seeing one are also higher in the morning. This means either a very early start, enabling you to get above the cloud level before sunrise, or a high wild camp. However, some inversions last all day, or even for multiple days on end. These inversions are more likely to be made of thick cloud or fog, rather than morning mist that burns off as the day progresses. The most impressive inversions I’ve seen have been the prolonged ones, which provide spectacular sunsets above the clouds.
Where to go
Some locations are more suitable for the development of inversions than others. For example, inversions are more common on hills with large bodies of water because they contribute to greater cooling of the air at ground level and higher moisture levels. Peaks near large lakes, such as Helvellyn, with Ullswater and Thirlmere either side, are good locations for this reason. Tall peaks on islands, such as Ben More on the Isle of Mull or Mount Teide on Tenerife, get cloud inversions very frequently due to being surrounded by the sea. Eryri (Snowdonia) National Park has the combined benefit of proximity to the sea, and valleys with large lakes and rivers, making cloud inversions relatively common there.
In the absence of lakes or the sea, low-lying floodplains and large rivers can also contribute to a cloud inversion by providing moisture and aiding the cooling of the air. I have seen many cloud inversions on Pendle Hill, despite its modest height of 557 metres, because it benefits from being surrounded by rivers and broad, flat floodplains (which often have stagnant bodies of water in winter).
Interpreting weather apps
Weather apps provide a range of information, and it can be difficult to know how to interpret this information to predict cloud inversions. To my knowledge, no existing weather app explicitly predicts cloud inversions. I have occasionally seen cloud inversions predicted on the Mountain Weather Information Service (MWIS), but most cloud inversions that I have seen have not been predicted on the site. As a result, we are left largely dependent on our own calculations and guesswork. It is also worth noting that the information on weather apps can be unreliable. On many occasions, I have been on the summit of a hill above an inversion, despite the weather apps indicating otherwise by forecasting the summit to be in fog, or for the summit temperature to be lower than the temperature in the valleys. I look at weather apps to look for general patterns that may make inversions likely, such as high pressure, high moisture and low wind speeds, but I don’t always trust predictions of cloud base height. Ultimately, my primary judgment comes from walking outside and looking up at the mountains with my own eyes.
If you are looking at weather apps to inform your decisions, then you may want to consider:
- The presence or proximity of high-pressure systems, which appear on charts as widely spaced isobars.
- Wind speed, especially the forecast wind speed on summits.
- Temperature at valley level compared to temperature on the summits.
- Air moisture. This can be measured in several ways, including:
- Relative humidity – a percentage of how much moisture is in the air, compared to the maximum amount of moisture that the air can hold (so 100% is at capacity). The amount of moisture that the air can hold is influenced by current air temperature. The higher the relative humidity the better, and it is generally advisable to look for over 95% - but there can be exceptions to this.
- Dew point – this indicates the temperature at which dew (condensation) forms. A higher dew point indicates a larger amount of moisture in the air. If the dew point is about the same as the air temperature at valley level, then fog may form and if it does, you will likely find an inversion if you climb high enough.
I tend to use a combination of apps, because different apps provide different information and have different locations. Usually, I at least check: MWIS, Met Office mountain weather, Mountain Forecast and Yr (this is the forecast provided by the Norwegian Meteorological Institute, but with typical Scandinavian efficiency, it’s also very good at predicting UK weather). The website windy.com also has a variety of webcams in mountain areas to aid decision making.
Related phenomena
Before setting out in search of a cloud inversion, it can help to have an idea of some of the related phenomena that you may witness.
1. Brocken spectre - is when rainbow circles surround your shadow, in a halo-like shape. This occurs in the opposite direction of the sun, when the light casts your shadow onto a bank of cloud, so be sure to look in all directions when experiencing an inversion. Because the proximity of cloud is also important, brocken spectres usually occur when you’re emerging out of the cloud, rather than when you’re far above it.
2. Fog bow - looks like a rainbow of white light or extremely faded colour, and is much larger than a brocken spectre. It is caused by sunlight interacting with the water droplets in cloud or fog, similar to the way a rainbow is formed, but because the water droplets in fog are much smaller the colours are more muted. Fog bows need both sunlight and cloud to form, so they are often seen at the edge of the cloud layer during an inversion.
It should be apparent by now that inversions are dependent on a large number of variables and predicting them can be tricky. It does, however, get easier with time and experience. These days, although I briefly check the forecast in advance to see when an inversion is likely, most of the time I realise that there is an inversion when I leave my house in Yorkshire and see thick, freezing fog, or a low cloud base and unusually low temperatures. When alarm bells start ringing, I throw my pre-packed bag on my back and head up Pen-y-Ghent. Despite my best efforts in predicting inversions, I will never stop relishing the uncertainty of each experience, because it is that uncertainty that gets my adrenaline pumping and leads to overwhelming joy when I do break free of the cloud. Remember, inversions are a reward saved for those who dare to take a chance.