One of high pressure's most common phenomenon, is the development of a low-level inversion. This is a very narrow sliver of atmosphere, often only a few hundred metres above the ground, where temperature actually increases with height. The result of this inversion, is for cold, dense air to become trapped at the surface. When fog forms overnight, this inversion can sometimes prevent the fog from clearing all day. This was the case in mid-November 2005, when a cold and frosty high pressure morphed into one which created widespread fog, and this refused to clear all day in places.

However, the nature of low-level fog means that just a few hundred metres above the surface, it can be crystal clear with sunshine. On the 22nd November 2005, I used this notion and headed onto the Horseshoe Pass in Denbighshire, north-east Wales, which rises to around 500 metres above mean sea level. Sure enough, the steady climb to the summit soon revealed a sea of fog below me, and the result was like looking out of an aeroplane window!

The view from the Horseshoe pass, looking eastwards towards England. In fact, all that could be seen above the fog, were a few mountains which rose to over 450m asl.

Another view from the top of the world, this time looking
westwards towards Snowdonia.

Whilst climbing to around 500m asl, from the main carpark areas at 400m asl, I managed to get myself into position to witness some amazing fog-related phenomena. With the sun behind me, I was able to observe the equivalent of a rainbow cast in distant rain. This time however, the moisture was in the form of fog droplets, and this created a fog bow!

With the sun behind me, this fogbow was visible on the passing banks of fog.

Despite the fog becoming shallower, the lower portion of this fogbow was still visible!

Another classic optical affect to look for when viewing fog, is that of the 'Brocken Spectre'. This appears as a halo of light around the observer's head, provided that your shadow is being cast onto a passing fog bank. For maximum effect, you need to be in clear air, without any fog obscuring the sunshine behind you.

Here is the Brocken Spectre captured. Angelic!

This next diagram (above) shows the next step in this phenomenon, as the crest
of a wave moves up the valley, towards the mountain ridge. This was observed as
a rise in the height of the fog-top, or even as a large bulge spreading from valley
side to valley side.

When the wave reaches the ridge (above) it rears up in height, and
occasionally forms a breaking crest.
This image (below) shows this in reality, as the huge breaking crest of fog
reaches the ridge. If you look carefully on the right of this image, the same
wave can be seen in the valley in the background, also reaching the head of
that valley.

As the wave is now much higher than the ridge it is coming up against, it is able
to crash down onto the ridge itself, and then flow along its top (above).
As this image shows (below), the size of the cafe in comparison to the wave gives
some idea of scale. The following set of images shows the wave closing in on
the cafe, resembling an avalanche from this distance!

Now, the wave flows over the top of the ridge as a huge density current of
cold, moist air (above). The photographs below show two of these stages.
Firstly, the cafe buildings are caught up in this rapid flow of fog over the
top of the ridge, and the second image shows how some of the larger
waves even managed to flow down the other side of the mountain!

Finally, the wave disperses, with the rear of the wave sinking back into the valley from which it came. In this image (below) the forward edge of the wave is still clearly visible, whilst the fog following on disperses and returns to the valley.