Local winds occur on a small spatial scale, their horizontal dimensions typically
several tens to a few hundreds of kilometres. They also tend to be short-lived
lasting typically several hours to a day. There are many such winds around
the world, some of them cold, some warm, some wet, some dry. There are many
hazards associated with the wind.
The main local winds are:
Sea breezes and land breezes, Anabatic and katabatic winds,The bora and mistral, The Főhn.
wind is caused by thermal (heat) processes. Anabatic (upslope) winds
occur over slopes which are heated by the sun. Air which is in contact
with slopes that are warmed expands upward and cool and sinks over neighbouring
valleys (see diagram). Anabatic winds are usually slow, at only 1-2m/s
and are rarely importance expect near coasts where they can increase
the strength of sea breezes.
Katabatic (downslope) winds occur over slopes which are cooled. Katabatic
winds occur where air in contact with sloping ground is colder than
air at the same level away from the hillside over the valley (see diagram
below). Katabatic winds are nocturnal phenomena in most parts of the
world (i.e. they tend to happen at night) as there is surface cooling,
especially when there is little cloud
and due to lack of heating by the sun.
Katabatic wind speeds do not typically not exceed 3 or 4 m/s. However,
where the ground is covered with snow or ice, katabatic winds can occur
at any time of day or night with speeds often reaching 10 m/s, or even
more if funnelling through narrow valleys occurs. Katabatic winds may
lead to the formation of frost, mist and fog in valleys.
breezes are the result of differential heating of the land and the sea.
Sea breezes occur by day, when the land becomes warmer than the sea.
Warm air from the land cannot expand into the sea as the air is cooler
and more dense, so it expands up into the atmosphere. Cumulus clouds
tend to form as the warm air rises over the land to about 500-1500m.
The diagram below shows the whole sea breeze process.
Air in sea breezes is cool and moist compared to the air over the land.
Sea breezes can move 70km inland in temperate climates by 9pm in the
evening. Sea breezes can be noticed several kilometres out to sea. In
the tropics they can be felt 20km from the land. Wind speeds from sea
breezes can be about 4-8m/s but can be even stronger.
Land breezes occur at night and in the early morning, when the land
is cooler than the sea. This is because as the air cools in the night
time (as there is less heating from the sun) it contracts. Pressure
is higher over the land than the sea. This causes the air to flow from
the land to the sea which is known as a land breeze. The circulation
is completed by air rising and moving towards the land at 100-300m.
This is shown on the diagram below. Cumulus
clouds from where there is rising air. Land sea breeze fronts tend to
only affect a small area of 10-15km out to sea, in comparison to the
much larger effect of sea breezes. Wind speeds are also lower at 2-3m/s.
The Föhn (or foehn)
Föhn is a warm, dry, gusty wind which occurs over the lower slopes on
the lee side (the side which is not directly exposed to wind and weather)
of a mountain barrier. It is a result of forcing stable air over a mountain
barrier. The onset of a Föhn is generally sudden. For example, the temperature
may rise more than 10°C in five minutes and the wind strength increase
from almost calm to gale force just as quickly. Föhn winds occur quite
often in the Alps (where the name föhn originated) and in the Rockies
(where the name chinook is used). They also occur in the Moray Firth
and over eastern parts of New Zealand’s South Island. In addition, they
occur over eastern Sri Lanka during the south-west monsoon.
danger of a Föhn where there are steep snow-covered slopes is that avalanches
may result from the sudden warming and blustery conditions. In Föhn
conditions, relative humidity may fall to less than 30%, causing vegetation
and wooden buildings to dry out. This is a long-standing problem in
Switzerland, where so many fire disasters have occurred during Föhn
conditions that fire-watching is obligatory when a Föhn is blowing.
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 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.
mistral is also a strong and often violent wind. It blows from the north
or north-west down the Rhône Valley of southern France and across the
Rhône Delta to the Golfe du Lion and sometimes beyond. Though strongest
and most frequent in winter, it may blow at any time of year and develops
when stable air is forced through the Rhône Valley. It occurs when a
depression is centred over north-west Italy and the Ligurian Sea and
a ridge of high pressure extends north-eastward across the Bay of Biscay.
It may blow continuously for a day or two and attain speeds of 100 km/h,
causing considerable damage to crops and making driving conditions difficult
in the Rhône Valley.