18 Sep 2016

Explainer: What is the Mistral and what does it mean for pilots?

What is the Mistral wind and why does it matter to balloon pilots? With Mistral forecast we look at what it is and how it can help or hinder balloonists this week...

Picture southern France, the area around Provence, and you might think of buzzing bees and lavender fields in bloom. But when the Mistral blows hard, it can be a cold and unforgiving place.

This strong north wind has been blowing for millennia. Prehistoric excavations near Nice show that 400,000 years ago the region’s earliest inhabitants had built low rock walls on the north side of their fire places, to protect against the north wind.

And in more recent history, the traditional farmhouses that dot this ancient landscape typical face south, their rock walls facing the wind. Even the bell towers of village churches have open iron frameworks, to let the wind through.

The word Mistral comes from a Languedoc dialect and means ‘masterly’, and when it blows it certainly is that. It typically reaches speeds of 90km/h in the Rhone Valley, and has been recorded peaking at 185km/h. It has a major influence over the Mediterranean coast of France, bringing cold, dry air and sweeping the landscape dry.

It happens when there is an area of high pressure over the Bay of Biscay and an area of low pressure over the Gulf of Genoa (meteorologists talk of the ‘Genoa Low’).

These two areas of pressure rotate agains each other – one clockwise, the other anti-clockwise. Where they meet the air speeds quickly south. This brings in cold air from the north, which rushes down through the valleys of the southern Alps. The Rhone Valley works like a wind motorway, funnelling it all south, which further increases the speed of the wind.

You’d think a fast northerly airstream would be good for balloonists, but if low it means turbulence in the hills, and very difficult, if not dangerous landing conditions. The way to ride it out in a balloon is to be high – the Mistral reaches to 3,000m.

However, the effects of it are felt far offshore – many sunbathers on the beaches of the Cote d'Azur have been sent scurrying when the wind turns north, and indeed it can even turn the sea colder as it pushes warm water offshore.

For balloon pilots that means once they are in the Mistral flow they will find landing difficult and dangerous – they must ride it out until they find calmer air. And in this case that means going with the flow out towards the Mediterranean and open sea.

The Mistral typically lasts a few days, blowing out as the different areas of air pressure shift around Europe.

For more on the Mistral, try this Wikipedia page.

Illustration: A schematic diagram of the Mistral by Piotr Flatau / Public domain