22 January 2014

Seeing Italy

Lyon is some 160 kilometres from Chamonix, the ski resort on the Italian-French border. Mont Blanc, the mountain that separates the two countries at this point, is the highest not only in France, but in all the Alps, topping out at 4810 metres above mean sea level.

Although the Swiss border is much closer than the Italian one (Geneva is just 110km away), you can sometimes see Mont Blanc from Lyon's Fourvière hill, Croix-Rousse and other higher elevations, like the Mont d'Or just to the north-west.

On particularly clear days, you are treated to a whole mountain range stretching along the Italian border, and Mont Blanc itself appears to be just a few miles beyond the city limits. However, pretty as the sight is, it's not a good omen. 

To bowdlerise the immortal - and unfortunately apocryphal - words of former Alaskan Governor Sarah Palin, I can see Italy from my house. But it's anything other than a cause for celebration.

As veteran Lyonnais (or at least their mothers) will tell you, if the skies appear so unobscured that you can see Mont Blanc from Lyon, you had better start looking for your umbrella and Wellington boots.That's no old wives' tale, because I know from repeated experience that if this majestic mountain's snow-capped peak looms large on the horizon, you're guaranteed to get rain within the next 48 hours.

With the help of Lyon municipal library's superb Guichet du Savoir service and the book Téchnoguide de la Météo by Jean-Louis Vallée, I now believe that I understand the precise meteorological and geographical reasons behind this strange phenomenon:

Weathermen apparently consider visibility of more than 50km to be exceptional. However, that's barely a third of the distance between Lyon and Chamonix. So what we need is a bit of optical and meteorological trickery.

Apart from all that lovely air, our atmosphere is also filled with water vapour and, in part due to pollution, suspended dust particles. When humidity rises, these particles trap microscopic drops of water, refracting light and creating haze. By contrast, on clear days - say when rain has washed many of these particles out of the atmosphere - we can see much further. But even that's not enough. The object we are viewing from afar (in this case a mountain) has to be brightly illuminated, either because the sun is shining on it and reflected off the snow or because the object is back-lit. 

Even better, just like at a cinema, it's best if the area between the viewer and the viewed object is in shade or unlit. This is where clouds help us Alp-spotting Lyonnais.

Two more factors come into play here in Lyon: southerly winds funnelled up the Rhône Valley dry the atmosphere and blow dust particles away, making the air clearer and thus less hazy. Finally what you need is a warm front, which tends to move from the south-west to the north-east. This warmth will not only dry out the atmosphere, temporarily increasing visibility further, but it draws rain behind it.

Thus when you're marvelling at the sight of Mont Blanc, you should really turn and look in the other direction. Chances are, you'll see the clouds pushed ahead of the warm front, which are indirectly helping you see the mountains. And maybe the rain clouds that will inevitably follow.

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