I saw the propagation arrive from far away

I was out on my afternoon walk when I noticed a cloud formation which suggested enhanced propagation on VHF. It was a still sunny day on 31 July with some "nuisance" cloud towards the horizon. A careful look revealed a dark grey band below the distant cloud level. I recognised this as a band of still air above the North Sea, which indicates that a temperature inversion may be underway.

I put an inch to my step, hurrying down Ayton Law, past the alpacas sunning themselves in the still air, and home where I had a look at 2m and above.

Dark grey band just above the horizon, Ayton Law, 31 July 2024

Click to enlarge images if necessary, as per.

The dark grey band can form over the North Sea when pressure is high. When it comes ashore it produces the effect known in Scotland as the "haar". When warm air passes over the sea a foggy layer of condensation forms also known as a "sea fret". The large bank of air can be very still and stable, but the entire bank can sometimes extend ashore producing very strange conditions. When the haar arrives near GM4FVM it can be seen pouring over the headland to the south, spreading down the slopes and into the valley of the Eye Water in which this station is located.

On shore the haar produces a damp fog which feels wet and yet can be surprisingly cold for something produced below warm air. On 31 July when I approached home I also noticed a cold wind which often arrives at about the same time. This feels like the output from an air conditioner, and circulates around the stable air system. The Scots word haar is derived from a Dutch word for cold. Inside the fog it can be quite dark as if enveloped in cloud as in an aircraft, and with the Sun blocked out also strangely cold. A short distance away, outside the cloud, it will be sunny, still and warm. Overall these can be a remarkable effects.

A large bank of still air over the North Sea is just what is required for tropospheric propagation. The tropo predictions had not been too hopeful for that day, showing a detached patch of enhancement north of me and then some more over on the Denmark coast. Not promising as tropo requires a continuous path.

Hepburn Tropo Chart for 18:00 on 31 July 2024.

It seemed likely that a mass of air had formed over the North Sea and this was allowing propagation. The QSOs showed two distinct paths, one to the east to to SM and OZ, and one to the south east to DL and PA.

144MHz contacts at GM4FVM on 31 July 2024

Conditions on 144MHz have been pretty awful this year, reflecting the lack of these very conditions. So it was very agreeable to be able to work 15 stations across the North Sea, and the best DX to SM7KMJ in JO66 at 947km was pretty good.

On 70cm I worked across the North Sea to OZ2ND and PA3CWS both of whom I also worked on 2m. I saw nothing  on 23cm, despite trying hard.

Haar coming ashore is fairly rare round here. This time it did not arrive on shore, well not as far inland as GM4FVM anyway. However, seeing it in the distance and feeling that cold wind on an otherwise warm day put me on alert.

Now two days later the pressure has fallen and the usual strong winds associated with a weather system coming from the West have returned. There is no possibility of still enough air over the North Sea to hold a bank of air, and the warm weather has gone. VHF conditions have returned to their usual sleep. Even the alpacas are sheltering under the trees today.

Alpacas sheltering on Ayton Law, 2 August 2024.

Most propagation enhancements arrive silently and invisibly. Actually seeing one was a bit of a surprise.

73 Jim

GM4FVM