This season brings the familiar but fascinating thrill we already knew from childhood: the coming summer holiday. And for a lot of us the mountains are the ultimate place to be.
For Europe the Alps are the most attractive; with the Mont Blanc a majestic massive; the highest of them all.

Mountains have their own special, unreliable climate. The weather one side of a mountain can be totally different from the conditions at the other side.
The föhn wind is famous in the Alps; they are so matched that some think the föhn wind is specific for the Alps, but that's not true. The term is used for all similar winds.
Föhn (its a German word) is a warm, strong and often very dry wind that often blows at the north slopes of the Alps, but can also be at other places.

The föhn often rises as the first front of a low; moving from the South of France to the Mediterranean. The wind will be south there, blowing warm and moistly air to the Alps. Along the south slopes the air is forced to ascend. While rising air cool down; the water vapour condenses and rain will ruin the day of many Italians and French.
On its way to the top of the mountains the air looses much of its humidity and becomes dryer. The temperature down the valleys is always higher than at the mountaintop. (Its about 1º C in 100 meter.) In the valleys there's a natural low pressure that draws the wind down the mountain. While descending the air will become warmer by compression and the wind speed will increase. A vigorous föhn storm can easily reach gale force.

The temperature in the föhn, at the north side of our mountain, can grow remarkable higher in a short time; sometimes about 10º C, and is much higher than at the other side. The humidity of the air can decline to 20%; these values are unknown in the UK.
A föhn can last from less than an hour to even several days.
The high crest of this wind wave leaves a specific formed cloud called a föhnwall (helm cloud).

Speaking about this helm cloud; the helm wind is famous in Cumbria. Its a strong blustery easterly wind that descends the western slope of the Cross Fell Range in the Northern Pennines.
But its a föhnwind, caused by an north-easterly flow; the ideal setting is a moderate, stable NNE to E wind. Near Penrith it ceases abruptly, but its roaring can still be heard.

The föhnwinds can cause trouble to many people. They get sick, have sleepless nights or a lack of concentration. Of course there's been a lot of research into these symptoms. It seems that there's a relation to vibrations caused by the waving airstreams. The vibrations prickle the auditory organs, irritating the nerve systems.
A nasty and problematic side effect of a wonderous phenomenon of nature.