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Where does the wind come from?
The definition of wind that we experience at ground
level is the sum of complex movements of the atmosphere due to a number of
different factors;
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The revolution of the planet. The earth is a solid
spinning ball surrounded by gas. Due to the effects of friction the rough
ground surface of the planet, the ball, has a tendency to drag the
atmosphere, the gas, along with it and cause it to rotate in the same
general direction. However the rotational speed at the equator is faster
than that nearer to the poles therefore this has the effect of creating an
offset so winds in the northern hemisphere tend to move from the
south-west towards the north-east and from the north-west towards the
south-east in the southern hemisphere.
-
Solar Radiation or heat. Everyone knows that hot
air rises and cold air sinks. This has three major effects on the
atmosphere;
a) As the air at a given location is heated it
rises causing the column of air to rise to a greater height than cooler
air. Because a taller column weighs more than a shorter column this has
the effect of creating a high pressure zone under warm air and a low
pressure column under cold air. As nature wishes everything to be in
equilibrium at ground level air moves from the high pressure zone to the
low pressure zone creating another cause of atmospheric movement.
b) The heating effect on the air, or the cooling
for that matter, need not come directly from the sun but from contact with
or radiation from other surfaces like the Sahara Dessert, the Arctic Ice
Sheets or the rocks of the Himalayan mountains. The effect can also happen
in coastal regions on the boundary between the land and the sea. The
ability for land and sea to absorb, store and radiate of heat from the sun
is different consequently during the day the air over the land tends to be
hotter, whilst at night it is the other way around. This difference tends
to cause on-shore breezes during the day and off-shore breezes at night.
These movements are called Anabatic and Catabatic respectively and also
relate to up slope and down slope movements of air in mountainous regions.
c) The Squall. These are sudden increases in air
movement, sometime but not always associated with a change of the
direction of movement. They are caused by sudden cooling of the air as a
result of precipitation, i.e.. rain, sleet or snow. Squalls can last for
some period of time along a storm front, but as these fronts are
themselves moving quite rapidly the squall to the stationery observer
appears to last only a few minutes.
-
The shape of the land or obstacles. In relation to
the atmosphere as a whole we consider here only major obstacles such as
mountain regions. Ignoring the local effects, described in 2b above, air
movements created on a larger scale by 1 or 2a simply have to flow around
or over them creating different effects known as Orographic wind
movements.
It is the resultant sum of all these global
movements of the atmosphere that we experience as wind. We must however be
aware that there are local influences that will cause variation, sometime
dramatic, to these more general movements. In the city high building,
sky-scrapers, can create updrafts and downdrafts of air so severe that they
can blow you right off your feet. Similarly if you are within a deep forest
you may be completely oblivious to the fact that outside the trees there is
a gale blowing.
The terminology used when describing the wind
Wind Speed and the Beaufort Scale
How far does the air move in a period of time the
expression of which can thus take different forms;
-
k.p.h. - Kilometres per hour.
-
m.p.h. - Miles per hour.
-
kts. - Knots or Nautical Miles per hour.
Before we had accurate instruments to measure
velocity in 1805 Admiral Sir Francis Beaufort invented a method of
categorising wind speed in a manner that could easily be understood by
others
The Beaufort Scale for describing wind force
| Force |
m.p.h. |
kts. |
Description |
Terrestrial Specification |
Nautical Specification |
| 0 |
0-1 |
0-1 |
Calm |
Calm, smoke rises vertical. |
Sea surface like a mirror. |
|
1 |
1-3 |
1-3 |
Light Air |
Direction of wind shown by smoke drift, but not by wind vanes. |
Ripples with the appearance of scales are formed, but without foam crests. |
| 2 |
4-7 |
4-6 |
Light Breeze |
Wind felt on face; leaves rustle; ordinary vanes moved by wind. |
Small wavelets, still short, but more pronounced. Crests have a glassy
appearance and do not break. |
| 3 |
8-12 |
7-10 |
Gentle Breeze |
Leaves and small twigs in constant motion; wind extends light flag. |
Large wavelets. Crests begin to break. Foam of glassy appearance. Perhaps
scattered white horses. |
| 4 |
13-18 |
11-16 |
Moderate Breeze |
Raises dust and loose paper; small branches are moved. |
Small waves, becoming larger; fairly frequent white horses. |
| 5 |
19-24 |
17-21 |
Fresh Breeze |
Small trees in leaf begin to sway; crested wavelets form on inland waters. |
Moderate waves, taking a more pronounced long form; many white horses are
formed. Chance of some spray. |
| 6 |
25-31 |
22-27 |
Strong Breeze |
Large branches in motion; whistling heard in telegraph wires; umbrellas used
with difficulty. |
Large waves begin to form; the white foam crests are more extensive
everywhere. Probably some spray. |
| 7 |
32-38 |
28-33 |
Near Gale |
Whole trees in motion; inconvenience felt when walking against the wind. |
Sea heaps up and white foam from breaking waves begins to be blown in
streaks along the direction of the wind. |
| 8 |
39-46 |
34-40 |
Gale |
Breaks twigs off trees; generally impedes progress. |
Moderately high waves of greater length; edges of crests begin to break into
spindrift. The foam is blown in well-marked streaks along the direction of the wind. |
| 9 |
47-54 |
41-47 |
Severe Gale |
Slight structural damage occurs (chimney-pots and slates removed). |
High waves. Dense streaks of foam along the direction of the wind. Crests of
waves begin to topple, tumble and roll over. Spray may affect visibility. |
| 10 |
55-63 |
48-55 |
Storm |
Seldom experienced inland; trees uprooted; considerable structural damage
occurs. |
Very high waves with long over-hanging crests. The resulting foam, in great
patches, is blown in dense white streaks along the direction of the wind.
On the whole the surface of the sea takes on a white appearance. The
'tumbling' of the sea becomes heavy and shock-like. Visibility affected. |
| 11 |
64-72 |
56-63 |
Violent Storm |
Very rarely experienced inland; accompanied by wide-spread damage. |
Exceptionally high waves (small and medium-size ships might be for a time
lost to view behind the waves). The sea is completely covered with long
white patches of foam lying along the direction of the wind. Everywhere the
edges of the wave crests are blown into froth. Visibility affected. |
| 12 |
73-83 |
64-71 |
Hurricane |
Considerable damage and possible loss of life |
The air is filled with foam and spray. Sea completely white with driving
spray; visibility very seriously affected. |
The air usually does not move with a constant
velocity but its motion usually gently fluctuates however there are times
when the velocity suddenly increases for while and then falls back to
previous levels. Where the wind speed increases by at least 10 m.p.h. for a
period exceeding 20 seconds this is described as a "gust"
Cyclone and Anti-cyclone
Thee two terms are used to describe the motion of
the wind. A cyclone is an area of low pressure around which winds blow
counter-clockwise in the Northern Hemisphere and clockwise in the Southern
Hemisphere. Also the term used for a hurricane in the Indian Ocean and in
the Western Pacific Ocean. Where as an anti-cyclone is a large area of high
pressure around which the winds blow clockwise in the Northern Hemisphere
and counter-clockwise in the Southern Hemisphere.
Hurricane and Typhoon
A tropical storm in which the maximum sustained
surface winds equal or exceed 64 knots (74 mph). These tropical cyclones are
called hurricanes when located in the Northern Hemisphere. Everywhere else
they are generally called typhoons or cyclones. The intensity of a hurricane
is measured on the Saffir-Simpson Scale, which ranks hurricanes on a scale
of 1 to 5 according to wind speed, barometric pressure, storm surge, and
damage potential. Winds blow in a large spiral around a relatively calm
centre of extremely low pressure known as the eye. Around the rim of the
eye, winds may gust to more than 200 mph.
Tornado
A twisting, spinning funnel of low pressure air. The
most unpredictable weather event, tornadoes are created during powerful
thunderstorms. As a column of warm air rises, air rushes in at ground level
and begins to spin. If the storm gathers energy, a twisting, spinning funnel
develops. Because of the funnel's cloud and rain composition and the dust,
soil, and debris it draws up, the funnel appears blackish in colour. The
most energetic storms result in the funnel touching the ground. In these
tornadoes, the roaring winds in the funnel can reach 300 mph, the strongest
winds on Earth.
The Wind Chill Temperature (Factor) is a calculation
performed to estimate what the temperature really feels like. As humans and
animals perspire the effect of moving air over their bodies evaporates the
resultant perspiration making them feel cooler. Obviously the stronger the
wind the greater the effect of wind chill.
Wind Run
In simple terms this is explained the distance the
wind travels over a period of time or better still how far, horizontally, a
balloon would travel if you let it go. Wind Run is calculated to provide the
Daily, Monthly and Annual distances.
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