Wind is the
flow of
gases on a large scale. On
Earth,
wind consists of the bulk movement of air. In
outer space,
solar
wind is the movement of gases or charged particles from the
sun through space, while planetary wind is the
outgassing of light
chemical elements from a planet's
atmosphere into space. Winds are commonly classified by their
spatial scale, their
speed, the types of forces that cause them, the
regions in which they occur, and their effect. The strongest
observed winds on a planet in our
solar
system occur on
Neptune and
Saturn.
In
meteorology, winds are measured
according to the direction from which the wind is blowing as well
as their strength. Shorter duration winds, such as wind gusts, can
cause substantial damage to power lines and suspension bridges.
Strong winds of intermediate duration (around one minute) are
termed
squalls. Long-duration winds have
various names associated with their average strength, such as
breeze,
gale,
storm,
hurricane, and typhoon. Wind occurs on a
range of scales, from thunderstorm flows lasting tens of minutes,
to local breezes generated by heating of land surfaces and lasting
a few hours, to
global winds resulting from
the difference in absorption of
solar
energy between the
climate zones on
Earth. The two main causes of large scale
atmospheric circulation are the
differential heating between the equator and the poles, and the
rotation of the planet (
Coriolis
effect). Within the tropics,
thermal
low circulations over terrain and high plateaus can drive
monsoon circulations. In coastal areas the
sea breeze/land breeze cycle can define
local winds; in areas that have variable terrain, mountain and
valley breezes can dominate local winds.
In
human civilization, wind has inspired
mythology, influenced the events of
history, expanded the range of
transport and
warfare, and
provided a
power source for
mechanical work,
electricity, and
recreation. Wind has been used to power the
voyages of
sailing ships across vast
oceans.
Hot air balloons use the
wind to take short trips, and powered flight uses it to increase
lift and reduce fuel consumption. Areas of
wind shear caused by various
weather phenomena can lead to dangerous situations
for
aircraft. When winds become strong,
trees and man-made structures are damaged or destroyed.
Winds can shape landforms, via a variety of
aeolian processes such as the formation of
fertile soils, such as
loess, and by
erosion.
Dust from large
deserts can be moved great distances from its
source region by the
prevailing
winds; winds that are accelerated by rough topography and
associated with dust outbreaks have been assigned regional names in
various parts of the world because of their significant effects on
those regions. Wind helps to spread
wildfires. Winds disperse seeds from various
plants, enabling the survival and dispersal of those plant species,
as well as flying insect populations. When combined with cold
temperatures, wind has a negative impact on
livestock. Wind affects animals' food stores, as
well as their hunting and defensive strategies.
Cause
Wind is caused by differences in pressure. When a
difference in pressure exists, the
air is accelerated from higher to lower pressure. On a rotating
planet the air will be deflected by the
Coriolis effect, except exactly on the
equator. Globally, the two major driving factors of large scale
winds (the
atmospheric
circulation) are the differential heating between the equator
and the poles (difference in absorption of
solar energy leading to
buoyancy forces) and the
rotation of the planet. Outside the tropics
and aloft from frictional effects of the surface, the large-scale
winds tend to approach
geostrophic
balance. Near the Earth's surface,
friction causes the wind to be slower than it would
be otherwise. Surface friction also causes winds to blow more
inward into low pressure areas.
Winds defined by an equilibrium of physical forces are used in the
decomposition and analysis of wind profiles. They are useful for
simplifying the atmospheric
equations of motion and for making
qualitative arguments about the horizontal and vertical
distribution of winds. The
geostrophic
wind component is the result of the balance between Coriolis
force and pressure gradient force. It flows parallel to
isobar and approximates the
flow above the
atmospheric
boundary layer in the midlatitudes. The
thermal wind is the
difference in the
geostrophic wind between two levels in the atmosphere. It exists
only in an atmosphere with horizontal
temperature gradients. The
ageostrophic wind component is the difference
between actual and geostrophic wind, which is responsible for air
"filling up" cyclones over time. The
gradient wind is similar to the geostrophic
wind but also includes
centrifugal
force (or
centripetal
acceleration).
Measurement
Wind direction is reported by the
direction from which it originates. For example, a
northerly wind blows from the north to the south.
Weather vanes pivot to indicate the direction
of the wind. At airports,
windsocks are
primarily used to indicate wind direction, but can also be used to
estimate wind speed by its angle of hang. Wind speed is measured by
anemometers, most commonly using rotating
cups or propellers. When a high measurement frequency is needed
(such as in research applications), wind can be measured by the
propagation speed of
ultrasound signals
or by the effect of ventilation on the resistance of a heated wire.
Another type of anemometer uses
pitot
tubes that take advantage of the pressure differential between
an inner tube and an outer tube that is exposed to the wind to
determine the dynamic pressure, which is then used to compute the
wind speed.
Sustained wind speeds are reported globally at a height and are
averaged over a 10 minute time frame.
The United States
reports winds over a 2 minute average, while
India
typically reports winds over a 3 minute
average. Knowing the wind sampling average is important, as
the value of a one-minute sustained wind is typically
14 percent greater than a ten-minute sustained wind. Shorter
bursts of higher winds, known as wind gusts, are defined as maxima
that exceed the lowest wind speed measured during a ten minute time
interval by . A
squall is a doubling of the
wind speed above a certain threshold, which lasts for a minute or
more.
To determine winds aloft,
rawinsondes
determine wind speed by
GPS,
radio navigation, or
radar
tracking of the probe. Alternatively, movement of the parent
weather balloon position can be
tracked from the ground visually using
theodolites.
Remote
sensing techniques for wind include
SODAR,
Doppler LIDARs
and
RADARs, which can measure the
doppler shift of
electromagnetic radiation
scattered or reflected off suspended
aerosols or
molecules, and
radiometers and radars can be used to
measure the surface roughness of the ocean from space or airplanes.
Ocean roughness can be used to estimate wind velocity close to the
sea surface over oceans. Geostationary satellite imagery can be
used to estimate the winds throughout the atmosphere based upon how
far clouds move from one image to the next.
Wind Engineering describes the study of the
effects of the wind on the built environment, including buildings,
bridges and other man-made objects.
Wind force scale
Historically, the
Beaufort wind force
scale provides an empirical description of wind speed based on
observed sea conditions. Originally it was a 13-level scale, but
during the 1940s, the scale was expanded to 17 levels. There
are general terms that differentiate winds of different average
speeds such as a breeze, a gale, a storm, tornado, or a hurricane.
Within the Beaufort scale, gale-force winds lie between and with
preceding adjectives such as moderate, fresh, strong, and whole
used to differentiate the wind's strength within the gale category.
A storm has winds of to . The terminology for tropical cyclones
differs from one region to another globally. Most ocean basins use
the average wind speed to determine the tropical cyclone's
category. Below is a summary of the classifications used by
Regional
Specialized Meteorological Centers worldwide:
! Beaufort scale
! 10-minute sustained winds (knots)
! General term
! N Indian Ocean
IMD
! SW Indian Ocean
MF
! Australia
BOM
! SW Pacific
FMS
! NW Pacific
JMA
! NW Pacific
JTWC
! NE Pacific &
N Atlantic
NHC & CPHC
General Wind
Classifications |
Tropical Cyclone
Classifications (all winds are 10-minute averages) |
|
0 |
<1></1> |
Calm |
Depression |
Tropical Disturbance |
Tropical Low |
Tropical Depression |
Tropical Depression |
Tropical Depression |
Tropical Depression |
1 |
1–3 |
Light air |
2 |
4–6 |
Light breeze |
3 |
7–10 |
Gentle breeze |
4 |
11–16 |
Moderate breeze |
5 |
17–21 |
Fresh breeze |
6 |
22–27 |
Strong breeze |
7 |
28–29 |
Moderate gale |
Deep Depression |
Tropical Depression |
30–33 |
8 |
34–40 |
Fresh gale |
Cyclonic Storm |
Moderate Tropical Storm |
Tropical Cyclone (1) |
Tropical Cyclone (1) |
Tropical Storm |
Tropical Storm |
Tropical Storm |
9 |
41–47 |
Strong gale |
10 |
48–55 |
Whole gale |
Severe Cyclonic Storm |
Severe Tropical Storm |
Tropical Cyclone (2) |
Tropical Cyclone (2) |
Severe Tropical Storm |
11 |
56–63 |
Storm |
12 |
64–72 |
Hurricane |
Very Severe Cyclonic Storm |
Tropical Cyclone |
Severe Tropical Cyclone (3) |
Severe Tropical Cyclone (3) |
Typhoon |
Typhoon |
Hurricane (1) |
13 |
73–85 |
Hurricane (2) |
14 |
86–89 |
Severe Tropical Cyclone (4) |
Severe Tropical Cyclone (4) |
Major Hurricane (3) |
15 |
90–99 |
Intense Tropical Cyclone |
16 |
100–106 |
Major Hurricane (4) |
17 |
107–114 |
Severe Tropical Cyclone (5) |
Severe Tropical Cyclone (5) |
115–119 |
Very Intense Tropical Cyclone |
Super Typhoon |
>120 |
Super Cyclonic Storm |
Major Hurricane (5) |
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Wind plotting within a station
model
The
station model plotted on surface
weather maps uses a wind barb to show
both wind direction and speed. The wind barb shows the speed using
"flags" on the end.
- Each half of a flag depicts of wind.
- Each full flag depicts of wind.
- Each pennant (filled
triangle) depicts of wind.
Winds are depicted as blowing from the direction the barb is
facing. Therefore, a northeast wind will be depicted with a line
extending from the cloud circle to the northeast, with flags
indicating wind speed on the northeast end of this line. Once
plotted on a map, an analysis of
isotachs
(lines of equal wind speeds) can be accomplished. Isotachs are
particularly useful in diagnosing the location of the jet stream on
upper level constant pressure charts, and are usually located at or
above the 300 hPa level.
Global climatology
The westerlies and trade winds
Winds are part of Earth's atmospheric circulation.
Easterly winds, on average, dominate the flow pattern across the
poles, westerly winds blow across the
mid-latitudes of the earth, to the north of
the
subtropical ridge, while
easterlies again dominate the
tropics.
Directly under the
subtropical
ridge are the doldrums, or horse latitudes, where winds are
lighter. Many of the Earth's deserts lie near the average latitude
of the subtropical ridge, where descent reduces the
relative humidity of the airmass. The
strongest winds are in the mid-latitudes where cold Arctic air
meets warm air from the tropics.
Tropics
The trade winds (also called trades) are the prevailing pattern of
easterly surface winds found in the
tropics towards the Earth's
equator.
The trade winds blow predominantly from the
northeast in the Northern Hemisphere
and from the southeast in the Southern
Hemisphere
. The trade winds act as the
steering flow for
tropical cyclones that form over world's
oceans.
Trade winds also steer African dust westward
across the Atlantic
Ocean
into the Caribbean Sea
, as well as portions of southeast North America.
A
monsoon is a seasonal prevailing wind that
lasts for several months within tropical regions.
The term was first
used in English in India
, Bangladesh
, Pakistan
, and
neighboring countries to refer to the big seasonal winds blowing
from the Indian
Ocean
and Arabian
Sea
in the southwest bringing heavy rainfall to the area. Its poleward
progression is accelerated by the development off a heat low over
the Asian, African, and North American continents during May
through July, and over Australia in December.
Westerlies and their impact
The Westerlies or the Prevailing Westerlies are the
prevailing winds in the
middle latitudes between 35 and
65 degrees
latitude. These prevailing
winds blow from the
west to the
east to the north of the subtropical ridge, and steer
extratropical cyclones in this general manner.
The winds are
predominantly from the southwest in the Northern
Hemisphere
and from the northwest in the Southern
Hemisphere
. They are strongest in the winter when the
pressure is lower over the poles, and weakest during the summer and
when pressures are higher over the poles.
Together with the
trade winds, the
westerlies enabled a round-trip trade route for sailing ships
crossing the Atlantic and Pacific Oceans, as the westerlies lead to
the development of strong ocean currents on the western sides of
oceans in both hemispheres through the process of western
intensification. These western ocean currents transport warm,
tropical water polewards toward the
polar
regions. The westerlies can be particularly strong, especially
in the southern hemisphere, where there is less land in the middle
latitudes to cause the flow pattern to amplify, which slows the
winds down. The strongest westerly winds in the middle latitudes
are within a band known as the
Roaring
Forties, between 40 and 50 degrees latitude south of the
equator. The Westerlies play an important role in carrying the
warm, equatorial waters and winds to the western coasts of
continents, especially in the southern hemisphere because of its
vast oceanic expanse.
Polar easterlies
The polar
easterlies, also known as Polar Hadley cells, are dry, cold
prevailing winds that blow from the high-pressure areas of the
polar highs at the north
and south poles
towards the low-pressure areas within the
Westerlies at high latitudes. Unlike the Westerlies, these
prevailing winds blow from the
east to the
west, and are often weak and irregular. Because
of the low sun angle,
cold air builds up and
subsides at the pole creating
surface high-pressure areas, forcing an equatorward outflow of air;
that outflow is deflected eastward by the Coriolis effect.
Local considerations
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Local winds around the world.
These winds are formed trough the heating of land (from
mountains or flat terrain)
Sea and land breezes
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A: Sea breeze (occurs at daytime), B:
Land breeze (occurs at night)
In coastal regions, sea breezes and land breezes can be important
factors in a location's prevailing winds. The
sea is warmed by the sun more slowly because of
water's greater
specific heat compared
to land. As the temperature of the surface of the
land rises, the land heats the air above it by
conduction. The warm air is less dense than the surrounding
environment and so it rises. This causes a pressure gradient of
about 2 millibars from the ocean to the land. The cooler air above
the sea, now with higher
sea level
pressure, flows inland into the lower pressure, creating a
cooler breeze near the coast. When large-scale winds are calm, the
strength of the sea breeze is directly proportional to the
temperature difference between the land mass and the sea. If an
offshore wind of exists, the sea breeze is not likely to
develop.
At night, the land cools off more quickly than the ocean because of
differences in their
specific heat
values. This temperature change causes the daytime sea breeze to
dissipate. When the temperature onshore cools below the temperature
offshore, the pressure over the water will be lower than that of
the land, establishing a land breeze, as long as an onshore wind is
not strong enough to oppose it.
Near mountains
The wind flows towards a mountain and produces a first
oscillation (A).
A second wave occurs further away and higher.
The lenticular clouds form at the peak of the waves (B).
Over elevated surfaces, heating of the ground exceeds the heating
of the surrounding air at the same altitude above
sea level, creating an associated thermal low over
the terrain and enhancing any thermal lows that would have
otherwise existed, and changing the wind circulation of the region.
In areas where there is rugged
topography
that significantly interrupts the environmental wind flow, the wind
circulation between mountains and valleys is the most important
contributor to the prevailing winds. Hills and valleys
substantially distort the airflow by increasing friction between
the atmosphere and landmass by acting as a physical block to the
flow, deflecting the wind parallel to the range just upstream of
the topography, which is known as a
barrier
jet. This barrier jet can increase the low level wind by
45 percent. Wind direction also changes because of the contour
of the land.
If there is a
pass in the mountain
range, winds will rush through the pass with considerable speed
because of the
Bernoulli
principle that describes an inverse relationship between speed
and pressure. The airflow can remain turbulent and erratic for some
distance downwind into the flatter countryside. These conditions
are dangerous to ascending and descending
airplanes. Cool winds accelerating through mountain
gaps have been given regional names.
In Central America, examples include the
Papagayo wind, the Panama
wind, and
the Tehuano wind. In
Europe, similar winds are known as the
Bora,
Tramontane, and
Mistral. When these winds blow over
open waters, they increase mixing of the upper layers of the ocean
that elevates cool, nutrient rich waters to the surface, which
leads to increased marine life.
In mountainous areas, local distortion of the airflow becomes
severe. Jagged terrain combines to produce unpredictable flow
patterns and turbulence, such as
rotor,
which can be topped by
lenticular
clouds. Strong
updrafts, downdrafts and
eddies develop as the air flows over hills
and down valleys. Orographic
precipitation occurs on the
windward side of mountains and is caused by
the rising air motion of a large-scale flow of moist air across the
mountain ridge, also known as upslope flow, resulting in
adiabatic cooling and condensation. In
mountainous parts of the world subjected to relatively consistent
winds (for example, the trade winds), a more moist climate usually
prevails on the windward side of a mountain than on the
leeward or downwind side. Moisture is removed by
orographic lift, leaving drier air on the descending and generally
warming, leeward side where a
rain
shadow is observed. Winds that flow over mountains down into
lower elevations are known as downslope winds. These winds are warm
and dry. In Europe downwind of the
Alps, they
are known as
foehn.
In Poland
, an example
is the halny wiatr. In Argentina, the local name for
downsloped winds is
zonda. In Java, the
local name for such winds is koembang.
In New Zealand
, they are known as the Nor'west arch, and are accompanied by the
cloud formation they are named after that has inspired artwork over
the years. In the Great Plains of the United States, the
winds are known as a
chinook. In
California, downsloped winds are funneled through mountain passes,
which intensify their effect, and examples into
Santa Ana and
Sundowner winds. Wind speeds during
downslope wind effect can exceed .
Average wind speeds
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The average windspeeds at 10 m
altitude
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The average windspeeds at 80 m
altitude
As described earlier, prevailing and local winds are not spread
evenly across the earth, which means that wind speeds also differ
by region. In addition, the wind speed also increases with the
altitude.
Wind Power Density
Nowadays, a yardstick used to determine the best locations for wind
energy development is referred to as Wind Power Density (WPD). It
is a calculation relating to the effective force of the wind at a
particular location, frequently expressed in terms of the elevation
above ground level over a period of time. It takes into account
wind velocity and mass. Color coded maps are prepared for a
particular area are described as, for example, "Mean Annual Power
Density at 50 Meters." The results of the above calculation
are included in an index developed by the National Renewable Energy
Lab and referred to as "NREL CLASS." The larger the WPD
calculation, the higher it is rated by class. At the end of 2008,
worldwide
nameplate capacity of
wind-powered generators was 120.8
gigawatts. Although wind produces only about
1.5 percent of worldwide electricity use, it is growing
rapidly, having doubled in the three years between 2005 and 2008.
In several countries it has achieved relatively high levels of
penetration, accounting for approximately 19 percent of
electricity production in
Denmark, 10 percent in
Spain and
Portugal, and 7 percent in
Germany and the
Republic of Ireland in 2008. One study
indicates that an entirely renewable energy supply based on
70 percent wind is attainable at today's power prices by
linking
wind farms with an
HVDC supergrid.
Shear
Wind shear, sometimes referred to as windshear or
wind gradient, is a difference in wind speed
and direction over a relatively short distance in the Earth's
atmosphere. Wind shear can be broken down into vertical and
horizontal components, with horizontal wind shear seen across
weather fronts and near the coast,
and vertical shear typically near the surface, though also at
higher levels in the atmosphere near upper level jets and frontal
zones aloft.
Wind shear itself is a
microscale
meteorological phenomenon occurring over a very small distance,
but it can be associated with
mesoscale or
synoptic scale weather features such as
squall lines and
cold fronts. It is commonly observed near
microbursts and
downbursts caused by
thunderstorms, weather fronts, areas of locally
higher low level winds referred to as low level jets, near
mountains, radiation inversions that occur because
of clear skies and calm winds, buildings,
wind turbines, and
sailboats.
Wind shear has a significant effect during
take-off and landing of aircraft because of their effects on
control of the aircraft, and was a significant cause of aircraft
accidents involving large loss of life within the United States
.
Sound movement through the atmosphere is affected by wind shear,
which can bend the wave front, causing sounds to be heard where
they normally would not, or vice versa. Strong vertical wind shear
within the troposphere also inhibits
tropical cyclone development, but helps to
organize individual thunderstorms into living longer life cycles
that can then produce
severe weather.
The
thermal wind concept explains how
differences in wind speed with height are dependent on horizontal
temperature differences, and explains the existence of the jet
stream.
Usage of wind
History
As a natural force, the wind was often personified as one or more
wind gods or as an expression of the
supernatural in many cultures.
Vayu is the Hindu God of Wind. The Greek wind gods
include
Boreas,
Notus,
Eurus, and
Zephyrus.
Aeolus, in varying interpretations the ruler
or keeper of the four winds, has also been described as
Astraeus, the god of dusk who fathered the four
winds with
Eos, goddess of dawn.
The Ancient Greeks also observed the seasonal
change of the winds, as evidenced by the Tower of the
Winds
in Athens
.
Venti are the Roman gods of the winds.
Fūjin, the Japanese wind god and is one of the
eldest
Shinto gods. According to legend, he
was present at the creation of the world and first let the winds
out of his bag to clear the world of mist. In
Norse mythology,
Njord
is the god of the wind. There are also four dvärgar (
Norse dwarves), named
Norðri, Suðri, Austri
and Vestri, and probably the
four stags of Yggdrasil, personify
the four winds, and parallel the four Greek wind gods.
Stribog is the name of the
Slavic god of winds, sky and air. He is
said to be the ancestor (grandfather) of the winds of the eight
directions.
Kamikaze (神風) is a Japanese word,
usually translated as divine wind, believed to be a gift from the
gods. The term is first known to have been used as the name of a
pair or series of typhoons that are said to have saved Japan from
two Mongol fleets under Kublai Khan that attacked Japan in 1274 and
again in 1281.
Protestant
Wind is a name for the storm that deterred the Spanish Armada from an invasion of England
in 1588 where the wind played a pivotal role, or
the favorable winds that enabled William of Orange to invade England
in 1688. During
Napoleon's
Egyptian Campaign, the
French soldiers had a hard time with the
khamsin wind: when the storm appeared "as a
blood-stint in the distant sky", the natives went to take cover,
while the French "did not react until it was too late, then choked
and fainted in the blinding, suffocating walls of dust." During the
North African Campaign of the
World War II, "allied and German troops
were several times forced to halt in mid-battle because of
sandstorms caused by khamsin ... Grains of sand whirled by the wind
blinded the soldiers and created electrical disturbances that
rendered compasses useless."
Transportation
There are many different forms of sailing ships, but they all have
certain basic things in common. Except for
rotor ships using the
Magnus effect, every sailing ship has a
hull,
rigging and
at least one
mast to hold up the
sails that use the wind to power the ship.
Ocean journeys by sailing ship can take many months, and a common
hazard is becoming becalmed because of lack of wind, or being blown
off course by severe
storms or winds that do
not allow progress in the desired direction. A severe storm could
lead to
shipwreck, and the loss of all
hands. Sailing ships can only carry a certain quantity of supplies
in their
hold, so they have to plan long
voyages carefully to include
appropriate
provisions, including
fresh
water.
While
aircraft usually travel under an
internal power source,
tail winds affect
groundspeed, and in the case of
hot air
balloons and other lighter-than-air vehicles, wind may play a
significant role in their movement and ground track. In addition,
the direction of wind plays a role in the takeoff and landing of
fixed-wing aircraft and
airfield runways are usually aligned to take the
direction of wind into account. Of all factors affecting the
direction of flight operations at an airport, wind direction is
considered the primary governing factor. While taking off with a
tailwind may be permissible under certain circumstances, it is
generally considered the least desirable choice because of
performance and safety considerations, with a headwind the
desirable choice. A tailwind will increase takeoff distance and
decrease climb gradient such that runway length and obstacle
clearance may become limiting factors. An airship, or dirigible, is
a
lighter-than-air aircraft that can be
steered and propelled through the air using
rudders and
propellers or
other
thrust. Unlike other
aerodynamic aircraft such as
fixed-wing aircraft and
helicopters, which produce
lift by moving a wing, or
airfoil, through the air, aerostatic aircraft, such
as airships and hot air balloons, stay aloft by filling a large
cavity, such as a
balloon, with a
lifting gas. The main types of airship are
non-rigid ,
semi-rigid and
rigid. Blimps are small airships without
internal skeletons. Semi-rigid airships are slightly larger and
have some form of internal support such as a fixed keel. Rigid
airships with full skeletons, such as the massive
Zeppelin transoceanic models, all but disappeared
after several high-profile catastrophic accidents during the
mid-20th century.
Power source
Historically, the ancient Sinhalese of Anuradhapura
and in other cities around Sri Lanka
used the monsoon winds to power furnaces as early
as 300 BCE. The furnaces were
constructed on the path of the monsoon winds to exploit the wind
power, to bring the temperatures inside up to . An early historical
reference to a rudimentary
windmill was
used to power an
organ in the first
century CE.
The first practical windmills were later built in Sistan, Afghanistan
, from the 7th century CE. These were
vertical-axle windmills, which had long vertical
driveshafts with rectangle shaped blades. Made of
six to twelve
sail covered in
reed mat or cloth material, these windmills
were used to grind corn and draw up water, and were used in the
gristmilling and sugarcane industries.
Horizontal-axle windmills were later used extensively in
Northwestern Europe to grind flour beginning in the 1180s, and many
Dutch windmills still exist.
High altitude
wind power is the focus of over 30 companies worldwide using
tethered technology rather than ground-hugging
compressive-towers.
Recreation
Wind figures prominently in several popular sports, including
recreational
hang gliding,
hot air ballooning,
kite
flying,
kite landboarding,
kite surfing,
paragliding,
sailing, and
windsurfing. In gliding, wind gradients
just above the surface affect the takeoff and landing phases of
flight of a
glider. Wind gradient can have a
noticeable effect on
ground launches,
also known as winch launches or wire launches. If the wind gradient
is significant or sudden, or both, and the pilot maintains the same
pitch attitude, the indicated airspeed will increase, possibly
exceeding the maximum ground launch tow speed. The pilot must
adjust the airspeed to deal with the effect of the gradient. When
landing, wind shear is also a hazard, particularly when the winds
are strong. As the glider descends through the wind gradient on
final approach to landing, airspeed decreases while sink rate
increases, and there is insufficient time to accelerate prior to
ground contact. The pilot must anticipate the wind gradient and use
a higher approach speed to compensate for it.
Role in the natural world
In arid climates, the main source of erosion is wind. The general
wind circulation moves small particulates such as dust across wide
oceans thousands of kilometers downwind of their point of origin,
which is known as deflation. Westerly winds in the mid-latitudes of
the planet drive the movement of ocean currents from west to east
across the world's oceans. Wind has a very important role in aiding
plants and other immobile organisms in dispersal of seeds, spores,
pollen, etc. Although wind is not the primary form of seed
dispersal in plants, it provides dispersal for a large percentage
of the biomass of land plants.
Erosion
Erosion can be the result of material movement by the wind. There
are two main effects. First, wind causes small particles to be
lifted and therefore moved to another region. This is called
deflation. Second, these suspended particles may impact on solid
objects causing erosion by abrasion (ecological succession). Wind
erosion generally occurs in areas with little or no vegetation,
often in areas where there is insufficient rainfall to support
vegetation. An example is the formation of sand
dunes, on a beach or in a desert. Loess is a
homogeneous, typically nonstratified, porous,
friable, slightly coherent, often calcareous,
fine-grained,
silty, pale yellow or buff,
windblown (aeolian)
sediment. It generally
occurs as a widespread blanket deposit that covers areas of
hundreds of square kilometers and tens of meters thick. Loess often
stands in either steep or vertical faces. Loess tends to develop
into highly rich soils. Under appropriate climatic conditions,
areas with loess are among the most agriculturally productive in
the world. Loess deposits are geologically unstable by nature, and
will erode very readily. Therefore, windbreaks (such as big trees
and bushes) are often planted by farmers to reduce the wind erosion
of loess.
Desert dust migration
During
mid-summer (July), the westward-moving trade winds south of the
northward-moving subtropical ridge expand northwestward from the
Caribbean
Sea
into southeastern North America. When dust
from the
Sahara moving around the southern
periphery of the ridge within the belt of trade winds moves over
land, rainfall is suppressed and the sky changes from a blue to a
white appearance, which leads to an increase in red sunsets. Its
presence negatively impacts
air quality
by adding to the count of airborne particulates. Over
50 percent of the African dust that reaches the United States
affects Florida. Since 1970, dust outbreaks have worsened because
of periods of drought in Africa. There is a large variability in
the dust transport to the Caribbean and Florida from year to year.
Dust events have been linked to a decline in the health of
coral reefs across the Caribbean and Florida,
primarily since the 1970s. Similar dust plumes originate in the
Gobi desert, which combined with
pollutants, spread large distances downwind, or eastward, into
North America.
There are local names for winds associated with sand and dust
storms.
The Calima carries dust on southeast winds
into the Canary
islands
. The Harmattan
carries dust during the winter into the Gulf of Guinea
. The Sirocco brings
dust from north Africa into southern Europe because of the movement
of extratropical cyclones through the Mediterranean Sea
. Spring storm systems moving across the
eastern Mediterranean Sea cause dust to carry across Egypt
and the
Arabian peninsula, which are
locally known as Khamsin.
The
Shamal is caused by cold fronts
lifting dust into the atmosphere for days at a time across the
Persian
Gulf
states.
Effect on plants
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Tumbleweed blown against a fence
Wind dispersal of seeds, or
anemochory,
is one of the more primitive means of dispersal. Wind dispersal can
take on one of two primary forms: seeds can float on the breeze or
alternatively, they can flutter to the ground. The classic examples
of these dispersal mechanisms include
dandelions (
Taraxacum spp.,
Asteraceae), which have a feathery
pappus attached to their seeds and
can be dispersed long distances, and
maples
(
Acer spp.,
Sapindaceae), which have winged seeds and
flutter to the ground. An important constraint on wind dispersal is
the need for abundant seed production to maximize the likelihood of
a seed landing in a site suitable for
germination. There are also strong evolutionary
constraints on this dispersal mechanism. For instance, species in
the Asteraceae on
islands tended to have
reduced dispersal capabilities (i.e., larger seed mass and smaller
pappus) relative to the same species on the mainland. Reliance upon
wind dispersal is common among many
weedy or
ruderal species. Unusual mechanisms of wind
dispersal include
tumbleweeds. A related
process to anemochory is
anemophily,
which is the process where pollen is distributed by wind. Large
families of plants are pollinated in this manner, which is favored
when individuals of the dominant plant species are spaced closely
together.
Wind also limits tree growth. On
coasts and
isolated mountains, the tree line is often much lower than in
corresponding altitudes inland and in larger, more complex mountain
systems, because strong winds reduce tree growth. High winds scour
away thin
soils through erosion, as well as
damage limbs and twigs. When high winds knock down or uproot trees,
the process is known as
windthrow. This is
most likely on
windward slopes of
mountains, with severe cases generally occurring to
tree stands that are 75 years or older.
Plant varieties near the coast, such as the
Sitka spruce and
sea grape, are
prune back by wind and salt spray near the
coastline.
Effect on animals
Cattle and
sheep are
prone to
wind chill caused by a
combination of wind and cold temperatures, when winds exceed that
renders their hair and wool coverings ineffective. Although
penguins use both a layer of
fat and
feathers to help guard
against coldness in both water and air, their
flipper and feet are less immune to the
cold.
In
the coldest climates such as Antarctica
, emperor penguins
use huddling behavior to survive the wind and
cold, continuously alternating the members on the outside of the
assembled group, which reduces heat loss by 50%. Flying
insects, a subset of
arthropods, are swept along by the prevailing
winds, while birds follow their own course taking advantage of wind
conditions, in order to either fly or glide. As such, fine line
patterns within
weather radar imagery,
associated with converging winds, are dominated by insect returns.
Bird migration, which tends to occur overnight within the lowest of
the
Earth's atmosphere,
contaminates wind profiles gathered by weather radar, particularly
the
WSR-88D, by increasing the environmental
wind returns by to .
Pikas use a wall of pebbles to store dry plants
and grasses for the winter in order to protect the food from being
blown away.
Cockroaches use slight winds
that precede the attacks of potential
predators, such as
toads, to
survive their encounters. Their
cerci are very
sensitive to the wind, and help them survive half of their attacks.
Elk has a keen sense of smell that can detect
potential upwind predators at a distance of . Increases in wind
above signals
glaucous gulls to
increase their foraging and aerial attacks on thick-billed
murres.
Related damage
High winds are known to cause damage, depending upon their
strength. Infrequent wind gusts can cause poorly designed
suspension bridges to sway.
When wind gusts are
at a similar frequency to the swaying of the bridge, the bridge can
be destroyed more easily, such as what occurred with the Tacoma
Narrows Bridge
in 1940. Wind speeds as low as can lead to
power outages due to tree branches disrupting the flow of energy
through power lines. While no species of tree is guaranteed to
stand up to hurricane-force winds, those with shallow roots are
more prone to uproot, and brittle trees such as
eucalyptus, sea
hibiscus,
and
avocado are more prone to damage.
Hurricane-force winds cause substantial damage to mobile homes, and
begin to structurally damage homes with foundations. Winds of this
strength due to downsloped winds off terrain have been known to
shatter windows and sandblast paint from cars. Once winds exceed ,
homes completely collapse, and significant damage is done to larger
buildings. Total destruction to man-made structures occurs when
winds reach . The
Saffir-Simpson
scale and
Enhanced Fujita
scale were designed to help estimate wind speed from the damage
caused by high winds related to tropical cyclones and
tornadoes, and vice versa.
Mount
Washington
is famous for its dangerously erratic weather,
holding the record for the highest wind gust directly measured at
the Earth's surface, at 231 mph (372 km/h) on the
afternoon of April 12, 1934.
Wildfire intensity increases during daytime hours. For example,
burn rates of
smoldering logs are up to
five times greater during the day because of lower humidity,
increased temperatures, and increased wind speeds. Sunlight warms
the ground during the day and causes air currents to travel uphill,
and downhill during the night as the land cools. Wildfires are
fanned by these winds and often follow the air currents over hills
and through valleys. United States wildfire operations revolve
around a 24-hour
fire day that begins at 10:00 a.m.
because of the predictable increase in intensity resulting from the
daytime warmth.
In space
The solar wind is quite different from a terrestrial wind, in that
its origin is the sun, and it is composed of charged particles that
have escaped the sun's atmosphere. Similar to the solar wind, the
planetary wind is composed of light
gases that escape planetary atmospheres. Over long periods of time,
the planetary wind can radically change the composition of
planetary atmospheres.
Planetary
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Possible future for Earth due to the
planetary wind: Venus
The hydrodynamic wind within the upper portion of a planet's
atmosphere allows light chemical elements such as
Hydrogen to move up to the
exobase, the lower limit of the
exosphere, where the gases can then reach
escape velocity, entering
outer space without impacting other particles of
gas. This type of gas loss from a planet into space is known as
planetary wind. Such a process over
geologic time causes water-rich planets such
as the Earth to evolve into planets such as
Venus over billions of years. Planets with hot lower
atmospheres could result in humid upper atmospheres that accelerate
the loss of hydrogen.
Solar
Rather than air, the solar wind is a
stream of charged particles—a
plasma—ejected from the
upper atmosphere of the sun at a rate of
. It consists mostly of
electrons and
protons with energies of about 1
keV. The stream of particles varies in
temperature and speed with the passage of time. These particles are
able to escape the sun's
gravity, in part
because of the high
temperature of the
corona, but also because of high
kinetic energy that particles gain through a
process that is not well-understood. The solar wind creates the
Heliosphere, a vast bubble in the
interstellar medium surrounding
the solar system. Planets require large magnetic fields in order to
reduce the ionization of their upper atmosphere by the solar wind.
Other phenomena include
geomagnetic
storms that can knock out power grids on Earth, the
aurora such as the
Northern Lights, and the plasma tails of
comets that always point away from the
sun.
On other planets
Strong winds at Venus's cloud tops circle the planet every four to
five earth days. When the poles of
Mars are
exposed to sunlight after their
winter, the
frozen CO
2 sublimes, creating significant winds
that sweep off the poles as fast as , which subsequently transports
large amounts of dust and water vapor over its
landscape. On
Jupiter, wind
speeds of are common in zonal jet streams. Saturn's winds are among
the solar system's fastest.
Cassini–Huygens data indicated peak
easterly winds of . On
Uranus, northern
hemisphere wind speeds reach as high as near 50 degrees north
latitude. At the cloud tops of
Neptune,
prevailing winds range in speed from along the equator to at the
poles. At 70° S latitude on Neptune, a high-speed jet stream
travels at a speed of .
See also
References
- Kansas Wind Energy Project, Affiliated Atlantic & Western
Group Inc, 5250 W 94th Terrace, Prairie Village, Kansas 66207
External links