Atmosphere Local Winds
My Blogs (olelog) are mainly based on my daily reading of earth science news. Here on whatonearth.olehnielsen.dk I try to weave some of the pieces together to a greater whole with added background info.

  Local Winds

Santa Ana (California)

Posted in OleLog as: San Diego, Santa Ana and Wildfires on Friday 26 October 2007

She starts when masses of cold air and accompanying high pressure form over the Great Basin in Utah and Nevada East of the Sierra Nevada Mountains. The air circulates clockwise around the high pressure area bringing winds from the east and Northeast to Southern California (the reverse of the westerly winds characteristic of the latitude). Some call her the Santa Ana. Others call her the Devil's Breath.

The Santa Ana winds usually form during autumn and early spring when the desert is relatively cold, although they may form at virtually any time of year. When the winds are moderate, they blow air pollution out to sea and make life in Southern California more pleasant. This week they have been blowing at near record speeds. They grow warmer, dryer and stronger as they spill south and west, falling several kilometres down through mountain canyons towards the ocean. The southern California coastal region gets some of its hottest weather of the year during autumn while Santa Ana winds are blowing. During Santa Ana conditions it is typically hotter along the coast than in the deserts and the humidity plummets to less than 15%.

On the satellite image there are no clouds. The weather is clearly dry. Large smoke plumes are driven from the large wildfires (red spots) towards the west far out over the Pacific Ocean by strong winds. Wind speeds as high as around 160 km/hr were measured in a couple of areas. That is the speed of winds in a Category 2 hurricane.

In Europe we know this type of winds from the Alps (the Föhn), but it is also found in other parts of the world (under different names):

* Zonda winds in Argentina
* Chinook winds in the Rocky Mountains, United States/Canada and the Chugach Mountains of Alaska, United States
* Diablo winds in the San Francisco Bay Area, United States
* The Nor'wester in Canterbury and Otago, New Zealand
* Halny in the Carpathian Mountains, Eastern Europe
* Fogony in the Catalan Pyrenees
* Bergwind in South Africa
* Viento Sur in the Cantabrian region (northern Spain)
* Terral in Málaga (southern Spain)
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* http://www.npr.org/templates/story/story.php?storyId=15584420
* http://en.wikipedia.org/wiki/Santa_Ana_wind

Find below a diagram showing the principles of the Föhn winds, with description underneath.

Moist air is forced to rise over the mountains. Condensation produces thick clouds and the air cools slowly at 0.5°C/100m. Heavy rain falls over the mountains and the air loses most of its moisture content. On the other side of the mountains the now dry air descends and warms up at 1°C/100m. The wind speed increases.

Bora, Northern Adriatic

Posted in OleLog as: Bora on Saturday 1 November 2008

Today I am slowly moving from linguistics back to earth sciences - or at least to meteorology. Winds. Winds can have all sorts of weird names, but also more straight ones based on their direction, like the (mid-latitude) westerlies in my part of the world - although many of the more straight ones are rooted in local languages, or dialects. So let’s move from Santa Ana or the Devil's Breath to Northerners and Southerners.

Luckily I didn’t experience the Bora, when I was in Slovenia and Croatia. The Bora is known throughout the northern Adriatic. The name derives from the Ancient Greek word for north, “borea”, which we also find in boreal - boreal forest and so on. Or maybe rather from the Greek mythological figure of Boreas, the North Wind. In Croatian it is called bura and in Slovenian burja.

The bora is a strong, cold and gusty north-easterly wind which descends to the Adriatic Sea from the Dinaric Alps, the mountains behind the Dalmatian coast (the coast of Croatia). It is mainly a winter phenomenon that develops when a slow-moving depression is centred over the Plain of Hungary and western Balkans so that winds are blowing from the east towards the Dinaric Alps. These mountains form a barrier which trap the cold air to the east of them whilst the Adriatic coast remains comparatively mild. Gradually, though, the depth of the cold air increases until the air flows over passes and through valleys to reach the Adriatic Sea.

The bora begins suddenly and without warning and the cold air typically descends to the coast so rapidly that it has little time to warm up. The bora can reach speeds of more than 100 km/h and has been known to overturn vehicles and blow people off their feet. In the Trieste area the wind was even more powerful in the past, since the slopes of the Karst were barer than they are today, as a result of the great number of trees that were needed to build Venice.

Down-slope winds flowing from high elevations of mountains, plateaus, and hills down their slopes to the valleys, planes or sea below are called katabatic winds. Katabatic is another word derived from the Greek, namely from katabaino - to go down. An upslope wind is called anabatic. Kata means down and ana means up.

When a katabatic wind is warmed by compression during its descent into denser air, it is called a föhn.

Katabatic flows slumping down from uplands or mountains may be funneled and strengthened by the landscape and are then known as mountain gap wind such as the Santa Ana (the one that is causing wildfires in California).

Cold and usually dry katabatic winds, like the Bora, result from the downslope gravity flow of cold, dense air. A large-scale katabatic wind that descends too rapidly to warm up is called a fall wind. The Bora is such a fall wind.

Slovenia and Croatia also have their own name for a wind coming from the south, namely the jugo. Some of you may remember that Jugoslavia actually meant South Slavia - in fact called Südslavien in German. Jugo is the local name for the Sirocco. The Sirocco is a Mediterranean wind that comes from the Sahara and reaches hurricane speeds in North Africa and Southern Europe, but that is yet another story.

• http://www.istrianet.org/istria/geosciences/meteorology/winds-bora-adr.htm
• http://en.wikipedia.org/wiki/Burja
• http://www.weatheronline.net.nz/reports/wxfacts/Katabatic-winds.htm
• http://www.rmets.org/activities/schools/local_winds.php

Norwesters, Bangladesh

See Norwesters in Bangladesh posted on Thursday 8 May 2008

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