Mountain Weather
Information Service

What happens when inversions are higher up?

By MWIS forecaster Garry Nicholson:

Thursday's satellite picture had some rather interesting features to see. Look at the picture below, and notice the rippled cloud features. These are caused by stable weather conditions, and the fact that the incoming westerly airstream is forced up and over the hills both in western Britain and Ireland. This creates waveforms in the airflow. The rippled cloud structures are essentially the crest of a wave, where the air has risen slightly, cooling and condensing its moisture into a cloud as this occurs. The trough of the wave allows the air to warm ever so slightly as it descends, causing evaporation and a gap in the cloud.

Sometimes, such waveforms can be seen from underneath, but today's airstream was pretty saturated in the lower levels, and there was a lot of low cloud clagging in across western hills in particular - as this 'view' of Great Gable showed from the Wasdale Inn webcam!

(If you've spotted any wave clouds today, do please send us a picture via our social media channels!!)

So what was happening in today's atmosphere to create this intriguing satellite scene?

We had a frontal system clear through early in the day, but the cold front was fairly weak, and the air mass which followed it wasn't that cold, and therefore not particularly unstable and showery (as often is the case post-cold front). All quite normal, it's just how air masses interact sometimes. 

The waveforms were brought about by our old friend the inversion, but this time, occurring above the mountains, rather than lower down as we saw during the wonderful low level fog views in August.

The vertical profile of the atmosphere below is my favourite tool for seeing what is going on, and it tells the story perfectly. It's a complex chart, but look through the chaos and it's a fairly simple concept...

The red line is temperature, the blue line dew point, according to height. The nearer these lines are together, the 'wetter' the air. Today, an inversion existed at around 6000ft, so above all our hills. Above this, the air was drier and slightly warmer, capping the lower air and not allowing it to rise through convection.

The lower air was pretty saturated, with some moisture left from Wednesday's frontal system and high humidity air mass. This is why Great Gable and many other hills were very much in the fog! As the air was forced up over the hills, it eventually hit the 'lid' of the inversion, effectively bouncing the air back down, and once this occurs it induces a series of waves, which can continue a long way 'downstream' from the initial hills. How many waves can you count on the satellite image?! Some even travel out over the North Sea.

Beware of UFOs!

In similar conditions you'll sometimes see lenticular clouds, which tend to need a slightly higher level inversion, and less moisture lower down, so that they stand out as the eerie 'UFO' clouds sometimes seen over the hills. My image on the right isn't the best example, but you get the idea...

It's all just simply the atmosphere doing its day job and it's fascinating to observe the physical processes occurring as ever.