Pre-frontal Winds Can Be Pleasant Or Almost Unendurable
If They're Unpleasant Enough They'll Get Their Own Name
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
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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