The Bise is a northeasterly wind that blows mainly over the Swiss plateau. In summer, the Bise brings dry air and temperatures that generally reflect the time of year. In winter, the Bise produces cold and relatively moist air that favours cloud formation in lower-lying areas.
Bise occurs when there is an area of high pressure over the northern part of central or northern Europe in addition to an area of low pressure over the Mediterranean. The area of high pressure prevents low-pressure areas from the Atlantic from circulating over Europe, and gives rise to an easterly to northeasterly flow over southern Europe and thus over Switzerland. This flow is called the Bise.
The Bise is mainly experienced in the Swiss plateau and especially in the area of Lake Geneva. The easterly wind is channelled between the Jura and the Alps and picks up speed towards the west as the distance between the two mountain ranges decreases. When the Bise is strong and temperatures are below zero, freezing spray can form on the shores of lakes. The Geneva region is particularly susceptible to such conditions.
In summer, continental air flowing in from the east is relatively dry. Sunny weather prevails throughout the country, and temperatures are normal for the time of year. In winter, the temperature of the incoming air is often lower, and relative humidity much higher, when Bise is present. The vertical width of this layer of cool or cold air with its high moisture content is between 500 and 2,000 m. Above this layer, warm, dry air can be found, due to the subsidence of the cold air (sinking under the influence of a high-pressure area). These two air masses are separated by an inversion layer, which is not especially thick, but is nevertheless pronounced. In an inversion layer, temperatures increase with altitude, rather than decreasing as would normally be expected. Inversions occur in high-pressure areas where the air sinks and warms up over a wide area. This causes the air to dry out and the clouds to dissipate. Underneath, the cold air condenses to fog. The horizontal inversion layer prevents the vertical exchange of air.
In the layer of moist air close to the ground, strong winds cause pronounced turbulence.
If the air is sufficiently humid, a low layer of cloud forms (stratus/low stratus). The top of this cloud is the height of the bottom of the inversion layer. The height of the base of the cloud depends on the moisture content of the air.
Even though the Bise is often associated with dry, stable weather conditions, it can sometimes occur with clouds and rain (or snow). In this case, it is known as the “black Bise”. This occurs when a low near the Alps brings moisture along with an easterly to southeasterly flow.