Winds of Change

Winds of Change

Wed, 19/02/2014 - 07:35
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Image: Some of the local wind names and their location. 

In many areas of the world, regional conditions give rise to winds that have been identified by the locals as having a special effect or occurring during a particular season. Quite often these winds are given a name by local inhabitants.

The effect of these local winds depends on the temperature and humidity changes they bring and the speed of wind itself. Warm winds, such as the Santa Ana in California and the Sukhavey in Russia, are extremely dry and cause widespread desiccation of plants. Such warm winds can also be highly beneficial – the Chinook of Canada, for example, often hastens the melting of snow in spring to permit early planting activities.

Here are just a few examples of local winds:

Katabatic winds (from Greek – katabatikos meaning ‘going down’) form when cold air above a plateau, mountain, glacier, or even a hill flows down a slope due to gravity. Katabatic winds are cool, dry and can be strong. They are very common in the Antarctic and can achieve wind speeds of 190 mph. The 'Bora' is a katabatic wind occurring in the Adriatic Sea, Black Sea, Southeastern Europe and Turkey. ‘Bora’ winds arises most often in the winter when high density cold air flows towards the sea. The ‘Mistral’ is another example of a katabatic wind which blows in the Rhone valley towards the Mediterranean. In Alaska, a katabatic wind known as a 'Williwaw' can pose a danger to ships in harbour.

The Föhn (German name) or Foehn is a dry, warm and strong, gusty wind, which develops on the leeward side of the mountains. In various countries Föhn winds have been given local names e.g. ‘Chinook’ on the eastern side of the Rocky Mountains and ‘Zonda’ on the eastern slope of the Andes. A prerequisite for the occurrence of a Föhn wind is when the large scale flow is perpendicular to the mountain range. The air is forced across the mountain and as it rises it cools and condenses to form clouds, producing rain or snow on the windward side of the mountain. The drier air crosses the mountains and descends on the other side and it warms as it does so. The sinking air has lost most of its moisture and therefore heats up more than it cooled on its ascent (this is because adiabatic lapse rate of dry air is greater than that of moist air).The resulting Föhn winds are drier and much warmer than the air at the same height on the opposite side of the mountains. The higher the mountain range, the greater the differences in temperature on either side. In the winter Föhn winds cause rapid melting, and even sublimation (direct evaporation) of snow. Summer Föhn winds sometimes cause leaves to dry quickly and drop.

Mountain and valley breezes are local winds caused by the diurnal cycle, due to different rate of heating of the air in the bottom of the valleys and slopes, which causes high-contrasts in temperature and pressure. In the day time wind blows from the valleys along their axis and the adjacent slopes towards higher mountains (anabatic wind). On clear nights the mountain winds often blow from the mountains slope toward the valley.