April 2012

Smoke on the water


  Icy car Versoix 2012

Most of Europe will remember the first two weeks of February 2012 as the coldest in a long time – as a matter of fact since 1985. At its origin was a dense Siberian high-pressure system with abnormally high surface pressures, on occasion reaching 1060 hPa (MSL). With the aid of an upper level low-pressure system to the east, a continental Arctic air mass pushed its way toward the western portion of the continent on brisk and cold northeasterly winds, known as the bise (pronounced “beeze”) in western Switzerland. It is well-known locally that these conditions, when cold enough, blow waters into freezing spray along the immediate windward shorelines of the lake, creating dramatic ice sculptures as objects and structures are sheathed in layer upon layer of airborne ice.

Another less dramatic phenomenon is steam fog. Under the same general bise conditions, frigid air is transported over bodies of water allowing the formation of steam fog over lakes or sea smoke over oceans and seas. This phenomenon was observed several times on Lake Geneva during the two-week cold snap (see photo). In general, in order for steam fog to form, the overlying air has to be between 5° to 40° Celsius colder than surface water temperatures. The actual temperature difference required depends on the humidity of the air and, in the case of oceans and seas, the salinity of the water. The greater the difference, the more spectacular the steam fog or sea smoke tends to be.

Steam fog over Lake Geneva

If the wind is not too strong, the air in close proximity to the water surface tends to be even colder than when no wind is blowing (wind-chill effect) so the temperature difference would be even greater, further favouring the appearance of the steam fog or sea smoke. If the winds are too strong, however, the steam tends to evaporate quite quickly and disappear altogether. The narrow conditions suitable for the formation of steam fog over Lake Geneva occur quite infrequently, making their appearance in February a scientifically interesting and visually delightful event.

When surface water temperatures fall to 4°C, another interesting environmental process also comes into play and impacts on the temperature difference between the surface water and the overlying air. This happens because fresh water becomes densest when its temperature is near 4°C. In the mild weather conditions generally seen around Lake Geneva, when a northeasterly wind blows across the lake, the warm surface water tends to be pushed towards the southwest (Geneva), which in turn triggers the upwelling of deeper cooler water to the surface at the eastern end of the lake (Montreux). During February, the exact opposite happened. Average surface temperatures across the lake fell to 4° C and lower. The relatively warmer and denser 4°C water sank and colder surface waters remained on top (the reason why ice forms at the surface of the water). This colder surface was then blown towards the southwest (Geneva), and the deeper denser (but warmer) waters were upwelled at the eastern end of the lake (Montreux). This phenomenon was confirmed by measurements.

by Lionel Peyraud, (Météo-Suisse, Geneva)

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