Location: Cumbria, United Kingdom
The helm wind is a strong, blustery easterly wind that descends the western slope of the Cross Fell Range in Cumbria, northern England. The Cross Fell area of the northern Pennines forms one of England's largest stretches of upland higher than 800m asl. To the west, there is a steep drop to the Eden Valley and beyond the valley lie the mountains of the Lake District.
The helm wind is basically a föhn-type lee wave caused by a prevailing north-easterly flow, blowing at a more or less right angle to the Cross Fell Range. The helm wind can last for days and is most strongly felt at Milburn, but is also noticeable all along the Fellside. An ideal setting for the phenomenon is a moderate, stable NNE to E wind over the Pennines. The wind speed is intensified as it drops the steep 600m escarpments to the Eden Valley. Near Penrith it ceases abruptly, though its roarings can still be heard, only to rise again a few kilometres away as the normal gusty north-easter.
The helm wind can be recognized by a bank of cloud appearing along the mountain tops or just above them, sometimes covering the summit of Cross Fell. This is the 'helm cloud'. The word 'helm' or means helmet or hilltop. The Cross Fell helm cloud is the little brother of the Alp's much larger föhn wall. However, at times, a further narrow stationary, but rotating roll cloud forms roughly parallel about 8 to 10 kilometers downwind (west) to the helm cloud. This is the helm bar. The helm bar is a rotor cloud and the surface winds die away beneath it. Farther away from the Fells there may be a light westerly wind towards the mountains.
The gap between the helm cloud and the helm bar represents the descending air and will vary according to temperature and pressure conditions. Within the helm bar, air is moving turbulently as convection currents operate. Warm air has risen from the ground, has cooled, become denser and sunk to the ground. Here it is warmed again, begins to rise and so the circular motion continues. The helm ends when the direction of the wind changes again.