It is the global air patterns that give rise to the principal wind systems on earth, such as the westerlies, the trade winds and the polar easterlies. These global patterns also influence some of the smaller scale winds such as the monsoon. But at a much smaller and local scale there are a large number of strong and often violent winds caused by local, and sometimes unique, topography and climatic conditions or by the interaction of large scale weather systems with local landmasses.
In 1981, after extensive research on damaging thunderstorm winds, Dr. Fujita (who also developed the Fujita Tornado Damage Scale) coined the term 'Downburst' to describe a type of destructive wind associated with a thunderstorm.
The microburst is a downburst which is confined to a small area, less than 4 km (2.5 miles) in diameter from the initial point of downdraft impact. An intense microburst can result in damaging winds near 270 km/hr (170 mph).
Downbursts occur in a severe thunderstorm where the air is accelerated downward by exceptionally strong evaporative cooling or by very heavy rain which drags dry air down with it. When the rapidly descending air strikes the ground, it spreads outward in all directions.
Dr. Fujita has defined downbursts as “a surface wind in excess of 39 mph caused by a small-scale downdraft from the base of a convective cloud.”
The damage caused by a downburst is frequently mistaken for a tornado, particularly directly beneath the downburst. However, damage patterns of a downburst are distinguished away from the impact area by characteristic straight-line wind damage rather than the twisted pattern of tornado damage.
The Chinook wind provides a welcome respite from the long winter along the eastern slopes of the Rockies. The change in temperature that the Chinook brings can be a dramatic one. On Jan. 11, 1983, the temperature in Calgary rose 30°C (from –17°C to 13°C) in just 4 hours, and on February 7, 1964, the temperature rose 28°C (51°F°).
Moist weather patterns originate off the Pacific coast and cool as they climb up the western slopes of the Rockies. As they drop down the eastern side of the mountains they rapidly warm and increase in speed, bringing a pleasant warm and breezy flow to the cold residents of the eastern Rockies. The Chinook will begin with a sudden change in wind direction towards the west or southwest and a rapid increase in speed.
Peter Mayle, the author of 'A Year in Provence' described the Mistral as a "brutal, exhausting wind that can blow the ears off a donkey". The Mistral blows as often as 100 days of the year, gusting to force 10 and above.
A complex set of conditions gives rise to a Mistral. When a high-pressure settles on the plateau of the Massif Central, and a low-pressure system lies over the Mediterranean, the cold mountain air can begin to flow downhill, accelerating as it thunders through the gap of the Rhône valley, before it turns and heads eastward up the coast. Most tourists miss the winter worst of the Mistral, when the wind pours down the Rhône valley during the winter months, a relentless icy flow blows that can last a fortnight.The Mistral is also common in spring, and can affect even a Summer break.
The Föhn is a dry, warm lee wind, it is the name of the wind that blows in the European Alps, but now is used generically for any similar lee wind. Much like the Chinook, the Fohn gains its warmth from the air being compressed as it descends down the lee side of a mountain range or slope. It is also often blamed for symptoms such as headaches and depression among people living in its path.
The Sirocco blows hot and dry out of North Africa and is caused by a strong southerly wind forming ahead of a depression moving into the central Mediterranean. The Sirocco can be very hot and dry and often carries Saharan dust. If the conditions are right these winds blow across the Mediterranean picking up moisture from the sea and by the time they reach Europe they bring warm, humid air and low cloud.
These are some of the more famous local winds, but examples can be found throughout the world. For instance a sea breeze may occur on a clear sunny day. As the sun heats the land, the land heats up much more quickly than the sea causing the air to rise over land and the cooler sea air to rush in and fill the gap. At night the reverse may happen and is known as a land breeze.
