When winds blow over hills and mountains, they can produce some distinctive weather phenomena.
What is the Helm Wind?
The Helm Wind (the only named wind in the UK) is a strong north-easterly wind hitting the southwest slopes of Cross Fell in Cumbria.
When a wind blows at a constant speed and direction through a layer of stable air perpendicular to the ridge or peak of hills and mountains, the result is something called a lee wave. The air is squeezed as it passes over the high ground and descends briefly downwind (the lee side) of the hill. Because the air is stable, it tries to re-establish itself by rising again and this causes a waveform. Where the wave crests you can end up with clouds.
The Helm Wind is most common in late winter and spring, and when it blows, a heavy bank of cloud (the ‘helm’) rests along or just above the Cross Fell range. A slender, nearly stationary roll of whirling cloud (the ‘helm bar’), parallel with the ‘helm’, appears above a point 1 to 6 km (up to 3 miles) from the foot of the fell. The Helm Wind can be very gusty as it blows down the steep fell sides but ceases under the helm bar cloud. To the west of this point, a light westerly wind may blow over a short distance.
The term ‘Helm Wind’ is applied to similar winds with associated cloud elsewhere, such as in the Lake District or along the Pennines.