Where weather systems clash
Travellers on the Bernina Line of the RhB are able to witness a meteorological phenomenon: the dividing line between two weather systems.The line separates the weather of the northern and southern sides of the Alps. In an interview, weather expert Thomas Bucheli (50) of Swiss Television explains the significance that the Bernina massif has for the weather in Switzerland, and about why he always reaches for his camera whenever he is in such a place.
Mr Bucheli, what exactly is a meteorological divide?
It is defined as „a term used to describe a section of landscape that has an influence over various weather conditions in its surrounding area“. This influence must be effective enough to control, in a long-term manner, the temperature and humidity of the air, as it moves from one side of thelandscape to the other. Mountain ridges are the most well-known type of meteorological divide, but parts of valleys with abrupt slopes and / orchanges of direction or narrow passages, and even larger lakes, can play the same role.
How does such a phenomenon arise, and which factors play a part in it?
The most effective divides are high mountain ranges standing crosswise to the prevailing wind. As the wind hits this obstacle, it has to flow either sideways or upwards. The associated changes in temperature and humidity on the windward side of the mountain range often lead to fog, clouds and precipitation, while the air on the leeward side is much drier and warmer, with generally better weather (the socalled „foehn effect“). Many of the world’s deserts lie right behind a meteorological divide. The dry Tibetan highlands, for example, are in the rain shadow of the Himalayas, as does the Atacama Desert of northern Chile is in the lee of the Andes.
How significant is the Bernina meteorological divide to Switzerland?
The Bernina group, the highest range in the eastern Alps, is a major meteorological divide for southerly winds, affecting the weather of Upper Engadin in particular. It can also influence the weather further north. Typical results of this phenomenon can be heavy precipitation on the southern Alpine slopes, with barely any rain or snow in Upper Engadin, and a foehn wind in central Graubünden. The effects of the Bernina meteorological divide are at their best when the upper layers of air do not contain too much humidity. If this happens, Puschlav and the valleys to its south often lie under high-altitude mist, while the sun shines all day in Engadin.
How does a meteorological divide affect the landscape?
Meteorological divides have a long-term effect on humidity and precipitation. The weather on the leeward, or downwind, side of a mountain is significantly drier than on the windward side. This affects local vegetation, soil-quality and the area’s agriculture. The Engadin is bounded by high mountain ranges on both sides, making it – along with Wallis – one of Switzerland’s driest regions.
What can the meteorological divide mean for outdoor pursuits like hiking, mountain-biking or ski touring?
You shouldn’t put blind faith in the benign effects of the meteorological divide. If the wind changes direction, or if humidity suddenly rises, a thick mist can soon come down. So it’s always a good idea to keep track of the situation. In fact, it’s always advisable to check the weather forecast before setting out!
To what extent does the meteorological divide influence you, as a weatherman, when making calculations and drawing up forecasts?
I always take meteorological divides into account. Our country of Switzerland basically consists entirely of a series of meteorological divides, such as the Jura, the central ranges of hills, the foothills of the Alps and all the other mountain ranges of the Alps themselves. Whenever I’m drawing up a forecast for a particular place, the first thing I want to know is: which way is the wind blowing? The second question is always: which mountain stands in its way?
What fascinates you most about the meteorological divide?
Whenever a „foehn wall“ forms at one, I am seized by a desire to go and photograph it. The picture you get is of a striking wall of cloud in front of an almost totally blue sky. I have managed to collect a number of fantastic photos of these foehn walls from all over Switzerland.
Finally: do you dare to give us a forecast for 2012?
Given the changing positions of the sun, we inevitably look set to go through various seasons in 2012. But the answer to the question on how this is going to express itself in actual weather conditions, and on whether we are all going to be satisfied with it, is largely written in the stars...