The term ‘inversion’ when talking about the weather refers to conditions when cold air is trapped below warmer air. The normal temperature profile of the atmosphere is for the air temperature to decrease with increasing height.
It isn’t the temperature itself that makes inversions in the mountains interesting, but the cloudscapes that can occur. The air holds a certain amount of water as a gas, and as the air cools some of this water condenses to form the tiny drops of water that we see as clouds. Most of the time you are more likely to encounter clouds as you ascend a mountain, as the air cools.
On day with an inversion, however, the coldest air is at ground level. Inversions can happen at any time of year, but they are most common in the autumn and winter on a still day when the skies have been clear the night before.
Early morning mist is due to a temperature inversion, like this faint mist on Ullswater in the morning. Such mists often ‘burn off’ during the day.
Cloudscapes formed by inversions are often more spectacular when you are up in the mountains, like this mist lurking below the peaks.
And sometimes proper clouds form, such as these seen from Helvellyn.
The best inversion I have ever encountered was on New Year’s Eve in the Lake District. The day did not start out promisingly, with very poor visibility, but we decided we’d get out anyway and tackle the walk to Grasmoor from Braithwaite via Grisedale Pike. This is the kind of view we had as we set off.
We’d reached something like 400 m altitude when the sky started to seem lighter. Brighter clouds are often just a symptom of wishful thinking, but not on this day. We could soon see sky, and then some hills above the layer of cloud.
Onwards and upwards, and we were rewarded by the sight of the various peaks around us poking out of a sea of cloud. That’s a bit of a cliche, but really does describes the views.
The conditions persisted all day, and it was only reluctantly that we set off back down into the gloom after one of the best winter walks I’ve had. The views were still spectacular in the fading light.
These two websites have more details on inversions. Interestingly, the video included in both of them was shot on the same day as my photos above.
And here’s a stitched together panorama of the view. Click here for a better view of it on Flickr (this blog site shrinks it too far!).
There’s no guarantee you will get such spectacular scenery if you come to the Lakes in the winter. But what is guaranteed is that you will never see it if you don’t come!