Pre-frontal Winds

Pre-frontal Winds Can Be Pleasant Or Almost Unendurable

If They're Unpleasant Enough They'll Get Their Own Name

Pre-frontal Winds

While often being honoured with local names, Pre-frontal Windsare a different group to the other groups of local winds,Foehn Winds and Outflow Winds. (General information on winds can be found in the Introduction To Winds pages.

Here's a general rule of thumb for weather systems.

Firstly they tend to move from east to west, and the junctionbetween them is called a front - a discontinuity in the airpressure gradient. Fronts are usually associated with lowpressure systems, and behind the front the winds generallyblow from the poles.

In advance of a cold front the air pressure is higher, but theair is still rotating, and blowing from warmer areas - more or less opposite to the winds behind the front. So as the front passes through we can expect a change in both wind direction and temperature, if not storms, rain or snow.

If the air pressure gradient is high (the isobars ona synoptic weather map are close together) the winds will be strong. So that sets the scene for the hot Pre-frontal Winds in areas poleward from hot arid desert areas. When strongly developed, we can expect hot blustery winds blowing straight from the overheated desert, and occasionally laden with dust or sand. They may last for days, but eventually the winds will change as a trough or front passes through, bringing cooler air and maybe the relief of a rainstorm.

The most famous of these winds are the winds blowing off the Sahara desert - depending on the geography they may be known as the Sirocco, Ghibli, Khamsin or Leveche - there are many other local names, and the same conditions occur in Asia and Australia.

The brief strong bursts of cool air, sometimes associated with a thunderstorm, rain or dust, also have their names. A great favorite of old time movie and radio serials was the Haboob, a Saharan wind often accompanied by sandstorms. The name has also been used in the southern USA for a similar wind.

And closer to my home, Sydney, Australia had it's sequence of frontal winds. The Brickfielder, so named because it picked up dust from the brick making areas of the early city, was a tiring hot northwesterly which could raise the temperature well above the Fahrenheit century. If the conditions were right, relief would come in a boisterous wind change known as the Southerly Buster - sometimes accompanied by a storm, sometimes by a linear roll cloud, and sometimes by nothing at all apart from the colder wind. No matter how it arrived, it came suddenly and 30°F+ drops in temperature were not uncommon. Both names are now out of use as our perceptions of weather changes are blunted by air conditioning and inside workplaces.

Names for the post frontal winds are less common, but a famous one is the southern Saharan Harmattan - a cooler wind which may follow a Haboob (love that word!)

Variants of the front related winds are those associated with monsoonal conditions - seasonal changes in wind direction in areas near hot, often high deserts. The main difference is that the Siroccos and their counterparts generally only last for days, but the monsoon winds last for months. Other Local Winds

There are many other local winds, some of which are quite complex but share some of the characteristics of the three types described earlier. In older settled areas, almost every wind has its own name, but elsewhere only the more spectacular examples attract a local name - a bit like naming hurricanes but not subtropical storms.

An interesting variant is the Barat of parts of Indonesia. This is actually a monsoonal wind which becomes significant in its strength when it blows along the seaways between islands. Its strength is magnified by the gaps between high island and the winds and rough seas present problems to sailors.

Like all aspects of weather, winds are complex phenomena caused by different combinations of circumstances.

So although it is fair to divide the winds with local names into three broad types, the real situation is not quite as simple. Foehn winds locally become gap winds, outflow winds have some foehn characteristics, and pre-frontal winds are also modified by gaps, outflows and downslope warming.

And all winds can be magnified by the effects of the larger picture of air mass movements and the effects of high altitude jetstreams. And if two weather systems combine and reinforce each other, the resultant weather bomb can provide more than enough excitement for the average onlooker.

For a short discussion of general characteristics of winds, visit the Introduction To Winds pages, while more specific information is available elsewhere on the other two main types of local winds, Foehn Winds and Outflow Winds.

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