Meteorology and oceanography
Winds on the Earth, local winds.
Factors affecting wind
Geostrophic wind
Gradient wind
Surface wind
Vertical airflow
General winds
Local winds
Wind.
factors affecting wind
Wind is the movement of atmospheric air relative to the Earth's surface. It is created
due to uneven atmospheric pressure distribution in the atmosphere.
Air flows from areas of higher pressure to areas of lower pressure.
The wind compensates inequalities in the atmospheric pressure field.
Unequal heating of Earth’s surface continually generates pressure differences. Solar
radiation is the primary energy source for most wind.
Wind.
factors affecting wind
If Earth didn’t rotate and if there were no friction, air would flow directly from areas of
higher pressure to areas of lower pressure.
Because both factors exist, wind is controlled by a combination of forces:
✔
pressure gradient force
✔
Coriolis force
✔
friction
Wind.
pressure gradient force
In order for any particle to accelerate, it needs unbalanced force from one direction. The
force that generates winds results from horizontal pressure differences.
Pressure differences cause the wind to blow, and the greater these differences,
the greater the wind speed.
The value of the pressure gradient on the synoptic charts illustrates the density of the
isobars - the smaller the distance between them, the higher the gradient and thus
the stronger the wind blows.
Wind.
pressure gradient force
H
1020
1016
1012
1008
1004
1000
L
The pressure gradient is perpendicular to the isobar at every point.
Wind.
pressure gradient force
A pressure gradient can be analyzed in the vertical dimension, with the prevailing trend
being a downward gradient (toward the Earth's surface). Vertical pressure gradients are
considered more substantially in terms of atmospheric lifting and instability.
Surface-level winds and large, regional scale, airflow are more often analyzed in a
horizontal sense. In areas underneath a gentle horizontal pressure gradient, where
there is little change in the air pressure, the weather is generally calm and stable, with
little wind or very light winds.
In areas with a steep pressure gradient, the weather is often unstable and generally
changeable. You can have moderate to severe winds, and moderate to severe weather
(rain, snow, or sleet depending on temperature conditions and other factors).
Wind.
pressure gradient force
The magnitude of the pressure gradient force is a function of the pressure difference
between two points and air density.
1 Δp
F FG = ∗
d Δn
FFG – pressure gradient force
d – density of air
Δp – pressure difference between two points
Δn – distance between two points
, where
Wind.
pressure gradient force
Pressure gradient degree is a change in the atmospheric pressure value per
1° distance on the meridian, or 60 NM (111 km).
In meteorology, the distance = 100km was conventionally taken.
In practice, we draw a perpendicular line between isobars, calculate its length in reality
and calculate the pressure gradient (Pg) by reading value pressure differences between
the isobars.
For example: Pg = (4hPa * Δk [km]) / k [km] ,where
- Δk :length of the segment corresponding to the assumed length of 100 km on the scale of a
given map (k = 100km)
- k :distance between two isobars (km) with a cut every 4hPa
The air velocity is assumed to be approximately equal to three times the degree of
pressure gradient expressed in hPa.
Wind.
Coriolis force
The wind doesn’t cross the isobars at right angles, as the pressure gradient force
directs. The deviation is the result of Earth's rotation and has been named the
Coriolis force after the French scientist Gaspard Gustave Coriolis, who first expressed
its magnitude numerically.
All free-moving objects (including wind) are deflected to the right of their path in the
Northern Hemisphere and the left in the Southern Hemisphere.
© EarthHow.com
Wind.
Coriolis force
The Coriolis force is the
result of the difference in
angular velocity and the
linear velocity of the rotating
Earth.
© CC 3.0
Wind.
Coriolis force
Wind.
Coriolis force
After a few hours the winds along the 20th, 40th, and 60th parallels appear to be veering off course.
Nevertheless, the Coriolis force is imperceptible to an observer unrelated to the reference system.
When viewed from space, it is apparent that these winds have maintained their original direction.
In fact, points on the surface of the Earth have changed their position during the rotation of the
Earth around its axis.
© Lutgens, Tarbuck
Wind.
Coriolis force
Coriolis force acts to change the direction of a moving body to the right in the
Northern Hemisphere, and to the left in the Southern Hemisphere.
Coriolis force:
- is always directed at right angles to the direction of airflow
- affects only wind direction, not wind speed
- is affected by wind speed (the stronger the wind, the greater the deflecting force)
- is strongest at the poles and weakens equatorward, becoming nonexistent at the
equator
Wind.
Coriolis force
where:
m - body weight,
ω - vector of angular velocity of the rotating system,
v - vector of the linear velocity of the body measured in a rotating
reference system
The Coriolis force does not affect the bodies that are at rest.
The magnitude of the effect is given by:
2νω sin ϕ
in which ν is the velocity of the object, ω is the angular velocity of the Earth, and ϕ is the latitude.
Wind.
Coriolis force
Suppose that from the top of the high
tower (from point A) we want to vertically
drop some body.
west
At point A, it has a higher linear velocity
than on the earth's surface (although the
angular velocity is the same).
During the flight from the tower, this body
will move down due to the force of gravity
and to the east - due to the inertia force, it
will not drop to point B' but to point B'’.
Free-falling bodies deflected to the east
everywhere beyond the Earth's poles.
east
The body dropped from
the top of the Eiffel
Tower (height 273 m
from the highest
terrace) will fall shifted
by 6.50103512 cm to
the east.
Wind.
Coriolis force
Is it a true that water goes down a sink in one direction in the Northern Hemisphere and in
the opposite direction in the Southern Hemisphere?
NO !
Typical sink is less then a meter in diameter and drains in a matter of seconds. On this scale the
Coriolis force is minuscule. Therefore the shape of the sink and how level it is has more to do with
the direction of the water flow than the Coriolis force. The difference between the angular and
linear velocity of the Earth in the center and at the edge of the sink is almost imperceptible.
Sneezing of a fly flying nearby may have a greater impact on the direction of water flow even :-)
In case of an atmosphere situation, when we observe the contribution of Coriolis force, a typical
cyclone is more than 1000 kilometers in diameter and may exist for several days. Then the
Coriolis force has a significant value.
You can read more about ‘fake Coriolis’ here: https://personal.ems.psu.edu/~fraser/Bad/BadCoriolis.html
Wind.
friction
The pressure gradient force is the primary driving force of the wind. As an unbalanced
force, it causes air to accelerate from regions of higher pressure to regions of lower
pressure. Thus the wind speed should continually accelerate (increase) for as long as
this imbalance exists.
Some other force (or forces) must oppose the pressure gradient force to moderate
airflow.
As we know, friction acts to slow a moving object.
The meaning of friction depends on the height above the surface of the Earth.
Wind.
geostrophic wind
This type of airflow applies to the atmosphere at the height of a few kilometers, where
the effects of friction are small enough to disregard.
Initially, the parcel of air does not move, the Coriolis force does not affect it, therefore,
only the force of the pressure gradient acts.
As a result of the force of the pressure gradient, the particle begins to move
perpendicular to the isobar towards the low pressure region.
As soon as the flow begins, the
Coriolis force comes into play and
causes a deflection to the right for
wind in the Northern Hemisphere
(left in the Southern Hemisphere).
© Lutgens, Tarbuck
Wind.
geostrophic wind
As the parcel continues to accelerate, the Coriolis force intensifies, because the
magnitude of the Coriolis force is proportional to wind speed.
Then, the increased speed results in further deflection.
Finally the wind turns so that it is flowing parallel to the isobars.
The pressure gradient force is balanced by the opposing Coriolis force.
As long as these forces remain balanced, the resulting wind will continue to flow
parallel to the isobars at a constant speed.
Wind.
geostrophic wind
Under these idealized conditions the Coriolis force is exactly equal and opposite to the
pressure gradient force, the airflow is in so called geostrophic balance (geostrophic
means: turned by Earth).
Geostrophic winds flow in a straight path, parallel to the isobars, with velocities
proportional to the pressure gradient force.
A high pressure gradient creates strong winds, a weak pressure gradient creates
light winds.
In a real atmosphere, such a movement can occur only in a free atmosphere (>500m).
Such conditions exist above the friction layer, where the isobars are close to straight
lines.
Wind.
gradient wind
Geostrophic winds exist in locations where there are no frictional forces and the isobars
are straight.
However, such locations are quite rare. Isobars are almost always curved and are very
rarely evenly spaced.
This changes the geostrophic winds so that they are no longer geostrophic but are
instead in gradient wind balance.
They still blow parallel to the isobars, but are no longer balanced by only the pressure
gradient and Coriolis forces, and do not have the same velocity as geostrophic winds.
Wind.
gradient wind
The gradient wind wind blows in a free atmosphere, above the friction layer, around a
circular isobar. The forces here are Coriolis force, centrifugal force and pressure
gradient force. The gradient wind, similar to the geostrophic one, blows parallel to the
isobar, leaving a lower pressure on the northern hemisphere on the left side, and on the
southern hemisphere on the right side.
the initial route of the air
© Lutgens, Tarbuck
Wind.
gradient wind
In this case, the centrifugal force acts in the same direction as the Coriolis force.
As the parcel moves north, it moves slightly away from the center - decreases the
centrifugal force.
The pressure gradient force becomes slightly more dominant and the parcel moves
back to the original radius.
This allows the gradient wind to blow parallel to the isobars.
© ww2010.atmos.uiuc.edu
Wind.
gradient wind
Since the pressure gradient force
doesn't change, and all the forces must
balance, the Coriolis force becomes
weaker.
This in turn decreases the overall wind
speed.
This is where the gradient wind differs
from the geostrophic winds.
In this case of a low pressure system
or trough, the gradient wind blows
parallel to the isobars at a less than
geostrophic speed.
Wind.
gradient wind
In this case, again starting from point A, the geostrophic wind will blow straight south.
This time the centrifugal force is pushing in the same direction as the pressure gradient
force, and when it gets slightly further away from the center, the centrifugal force again
reduces, but this time that makes the Coriolis Force more dominant and the air parcel
will move back to its original radius - again with the end result being wind blowing
parallel to the isobars.
© ww2010.atmos.uiuc.edu
Wind.
gradient wind
Since the pressure gradient force still
doesn't change, the Coriolis force must
again adjust to balance the forces.
However now it becomes stronger,
which in turn increases the overall
wind speed.
This means that in a high pressure
system or ridge, the gradient wind
blows parallel to the isobars faster than
geostrophic speed.
Wind.
surface wind
Friction as a factor affecting wind is important only within the first few kilometers of
Earth’s surface.
Friction acts to slow the movement of air. Friction also reduces the Coriolis force, which
is proportional to wind speed.
Air flow at an angle across the isobars, toward the
area of lower pressure.
In this way, there is a gradual equalization of
pressure between the areas of higher and lower
pressure.
The deflection and slowing effect of friction
decreases with the height.
A significant increase in speed is already recorded
30-50m above the ground.
© Lutgens, Tarbuck
Wind.
surface wind
In the Northern Hemisphere winds blow counterclockwise around a cyclone and
clockwise around an anticyclone, with winds nearly parallel to the isobars.
When we add the effect of friction, we notice that the
airflow crosses the isobars at varying angles,
depending on the roughness of the terrain, but
always from higher to lower pressure.
In whatever hemisphere, friction causes a net inflow
(convergence) around a cyclone and a net outflow
(divergence) around an anticyclone.
Wind.
surface wind
Environment Canada – national marine weather guide
Wind.
surface wind
The roughness of the terrain determines the angle at which the air flow across the
isobars as well as influence the speed at which it will move.
Over smooth ocean surface, where the friction is low, air moves at an angle of
10° to 20° to the isobars and at speed roughly two-thirds of geostrophic flow.
Over rugged terrain, where fiction is high, the angle can be as great as 45° from the
isobars, with wind speed reduced by as much as 50 percent.
© ww2010.atmos.uiuc.edu
Wind.
vertical airflow
Cyclonic circulation has converging
surface winds and rising air causing
cloudy conditions.
Anticyclonic circulation has diverging
surface winds and descending air,
which provides clear skies and fair
weather.
© Lutgens, Tarbuck
Surface convergence about a cyclone causes a net upward movement. The rate of this
vertical movement is slow, generally less than 1 kilometers per day. However it’s
enough to create a cloud cover and precipitations.
Wind.
vertical airflow
Friction can cause convergence and divergence in several ways. When air flows from
the relatively smooth ocean surface onto land, the increased friction causes an abrupt
drop in wind speed. This reduction of wind speed results a pile-up of an air stream.
Thus, converging winds and ascending air accompany flow off the ocean. This effect
contributes to the cloudy conditions over land often associated with a sea breeze in a
humid regions.
Conversely, when air flows from land onto the ocean, general divergence and
subsidence accompany the seaward flow of air because of lower friction which
increases wind speed over the water.
Wind.
winds distribution
Zonal wind distribution according to the
three-cell global circulation model.
© Lutgens, Tarbuck
Wind.
winds distribution
Wind.
winds distribution
Wind.
local winds
Local winds appear in a given area at specific times of a day or a year and show a significant
independence from the general atmosphere circulating in a given place.
These local circulations overlap with the general circulation and can sometimes significantly or
even completely change its most important features.
►Local winds may be the result of air circulation having a local range, independent of the general
atmospheric circulation, but overlapping with it. An example of such winds are sea breezes and
mountain breezes.
►Another type of local winds are currents of the general atmosphere circulation, which under the
influence of local factors (eg, orography) are deformed. An example of this type of wind is fen or
bora.
►Sometimes, as local winds, characterized by a characteristic specificity, we treat small air
currents that are actually currents of the general atmosphere circulation. Such winds include, for
example, sirocco, hamsin, samum.
Wind.
local winds – sea breeze
© Lutgens, Tarbuck
Wind.
local winds – valley/mountain breeze (anabatic/katabatic wind)
© Lutgens, Tarbuck
Wind.
local winds – foehn type winds (foehn, halny, chinook, fén-fēng, etc.)
Wind.
local winds – most known local winds on the Earth
Wind.
local winds – most known local winds of North America
The Norther
This name for a wind is used in more than one
place. In Chile, a Norther is a northerly gale with
rain. It usually occurs in winter but occasionally
occurs at other times of year. Typically, it can be
identified by falling air pressure, a cloudy or overcast
sky, good visibility and water levels below normal
along the coast.
Over the Gulf of Mexico and western parts of the
Caribbean Sea, Northers are strong, cool, northerly
winds which blow mainly in winter. Over the Gulf of
Mexico, they are sometimes humid and
accompanied by precipitation, but over the Gulf of
Tehuantepec they are dry winds.
Wind.
local winds – most known local winds of North America
Chinook (warm dry westerly off the Rocky Mountains)
Chinook winds (Chinooks), are foehn winds in the interior West of North
America, where the Canadian Prairies and Great Plains meet various
mountain ranges, although the original usage is in reference to wet, warm
coastal winds in the Pacific Northwest.
A strong wind can make snow 30 cm deep almost vanish in one day. The
snow partly melts and partly sublimates in the dry wind. Chinook winds
have been observed to raise winter temperature, often from below −20°C
to as high as 10–20°C for a few hours or days, then temperatures
plummet to their base levels. The greatest recorded temperature change
in 24 hours was caused by Chinook winds on 15 January 1972, in Loma,
Montana; the temperature rose from −48°C to +9°C.
Where Chinooks occur most frequently.
Wind.
local winds – most known local winds of North America
Blizzards
A blizzard is a severe snowstorm characterized by strong
sustained winds of at least (56 km/h) and lasting for a prolonged
period of time — typically three hours or more.
A ground blizzard is a weather condition where snow is not falling but loose
snow on the ground is lifted and blown by strong winds. Blizzards can have an
immense size, which can usually be larger than a few states in the United
States.
A nor'easter is a macro-scale storm that occurs off the New England and
Atlantic Canada coastlines. It gets its name from the direction the wind is
coming from. The usage of the term in North America comes from the
wind associated with many different types of storms some of which can
form in the North Atlantic Ocean and some of which form as far south as
the Gulf of Mexico. The term is most often used in the coastal areas of
New England and Atlantic Canada. This type of storm has characteristics
similar to a hurricane.
Wind.
local winds – most known local winds of South America
Pampero (Argentina), very strong wind which blows
in the Pampa.
The pampero is a burst of cold polar air from the
west, southwest or south on the pampas in the
south of Brazil, Argentina, Uruguay, Paraguay and
Bolivia.
This wind (often violently) picks up during the
passage of a cold front of an active low passing by.
It takes the form of a squall line. There is a marked
drop in temperature after its passing. The Pampero
is most common at winter in the southern
hemisphere (May to August). During the summers in
the region around Buenos Aires, the pampero
storms are a welcome sing marking the end of long
periods of high humidity and extreme heat.
Wind.
local winds – most known local winds of Australia
Southerly (Southerly Buster) - rapidly arriving low
pressure cell that dramatically cools Sydney,
Australia during summer.
These storms can be cold and have bad weather.
In Wellington, New Zealand these storms are
normally short and frequently have winds gusting
between 120 km/h and 160 km/h though higher
speeds are known.
Wind.
local winds – most known local winds of Australia
Brickfielder
A hot, dry, dusty wind of southern or central
Australia.
Name of wind from the location name Brickfield Hill,
after the hill in Surry Hills (now in inner Sydney) from
the direction of which a hot wind blew into Sydney in
its early days.
Black nor'easter
Persistent and potentially violent Sydney northeasterly storm is known as a "black nor'easter". This
is not a convection wind, but a storm system that
develops offshore which can last several days. This
is heralded by the rapid build-up of dense black
cloud that can convert to a gale in well under one
hour.
Wind.
local winds – most known local winds of Africa
Sirocco
It arises from a warm, dry, tropical air-mass that is
pulled northward by low-pressure cells moving
eastward across the Mediterranean Sea, with the
wind originating in the Arabian or Sahara deserts.
The hotter, drier continental air mixes with the
cooler, wetter air of the maritime cyclone, and the
counter-clockwise circulation of the low propels the
mixed air across the southern coasts of Europe.
The sirocco's duration may be as short as half a day
or may last several days. While passing over the
Mediterranean Sea, the sirocco picks up moisture;
this results in rainfall in the southern part of Italy,
known locally as "blood rain" due to the red sand
mixed with the falling rain.
Wind.
local winds – most known local winds of Africa
Khamsin
Dry, hot, sandy local wind, blowing from the south, in
North Africa and the Arabian Peninsula.
From the Arabic word for "fifty", these windstorms
often blow sporadically over a fifty-day period in
spring, hence the name.
When the storm passes over an area, lasting for
several hours, it carries great quantities of sand and
dust from the deserts, with a speed up to 140 km/h,
and the humidity in that area drops below 5%. Even
in winter, the temperatures rise above 45°C due to
the storm.
Wind.
local winds – most known local winds of Africa
Harmattan
Dry and dusty northeasterly trade wind, which blows
from the Sahara Desert over West Africa into the
Gulf of Guinea.
The Harmattan blows during the dry season, which
occurs during the lowest-sun months. In this season
the subtropical ridge of high pressure stays over the
central Sahara Desert and the low-pressure
Intertropical Convergence Zone (ITCZ) stays over
the Gulf of Guinea. On its passage over the Sahara,
the harmattan picks up fine dust and sand particles
(between 0.5 and 10 microns).
Wind.
local winds – most known local winds of Africa
Haboob
A haboob is a type of intense dust storm carried on an
atmospheric gravity current, also known as a weather
front. Haboobs occur regularly in dry land area regions
throughout the world.
African haboobs result from the northward summer shift
of the inter-tropical front into North Africa, bringing
moisture from the Gulf of Guinea.
Haboobs have been observed in the Sahara desert
(typically Sudan, where they were named and described),
as well as across the Arabian Peninsula, throughout
Kuwait, and in the most arid regions of Iraq. Haboob
winds in the Arabian Peninsula, Iraq, Kuwait are
frequently created by the collapse of a thunderstorm.
Wind.
local winds – most known local winds of Africa
Berg Wind
When the air that has been heated on the extensive
central plateau flows down the escarpment to the coast it
undergoes further warming by adiabatic processes. This
accounts for the hot and dry properties of these off-shore
winds, wherever they occur along South Africa's
coastline.
Berg winds are usually accompanied by coastal lows.
These coastal lows owe their existence to the
configuration of the plateau, escarpment and coastal
plain, in that they are confined to the coastal areas,
always below the escarpment. Though they can arise
almost anywhere along the coast, they often first appear
on the west coast, or even on the Namibian coast.
Wind.
local winds – most known local winds of Asia
Buran
Wind which blows across Iran, eastern Asia,
specifically Xinjiang, Siberia, and Kazakhstan. Over
the tundra, it is also known as purga. It is a wind of
cold air, sometimes very strong, characteristic of the
steppes of the East European Plain, to the west of
the Urals.
The buran takes two forms: in summer, it is a hot, dry
wind, whipping up sandstorms; in winter, it is bitterly
cold and often accompanied by blizzards. Winter
buran winds are strong and full of ice and snow. The
sky is often laden with snow, which swirls about and
reduces the visibility to near zero at times. In Alaska
this severe northeasterly wind is known as burga and
brings snow and ice pellets.
Wind.
local winds – most known local winds of Asia
Karaburan
Also called black storm, black buran.
A violent northeast wind of Central Asia occurring
during spring and summer.
It resembles the white buran of winter but, instead of
snow, it carries clouds of dust that darken the sky.
Wind.
local winds – most known local winds of Europe
Helm
The Helm Wind is a named wind in Cumbria,
England, a strong north-easterly wind which blows
down the south-west slope of the Cross Fell
escarpment.
It is the only named wind in the British Isles, although
many other mountain regions in Britain exhibit the
same phenomenon when the weather conditions are
favourable.
Wind.
local winds – most known local winds of Europe
Bora
The bora is a northern to north-eastern katabatic wind
in the Adriatic Sea. Similar nomenclature is used for
north-eastern winds in other littoral areas of eastern
Mediterranean and Black Sea basins.
The bora is most common during the winter.
It blows hardest when a polar high-pressure area sits
over the snow-covered mountains of the interior
plateau behind the Dinaric coastal mountain range
and a calm low-pressure area lies further south over
the warmer Adriatic. As the air grows even colder and
thus denser at night, the bora increases. Its initial
temperature is so low that even with the warming
occasioned by its descent it reaches the lowlands as
a cold wind.
Wind.
local winds – most known local winds of Europe
Mistral
Strong, cold, northwesterly wind that blows from
southern France into the Gulf of Lion in the northern
Mediterranean, with sustained winds often exceeding
66 km/h, sometimes reaching 185 km/h. It is most
common in the winter and spring, and strongest in the
transition between the two seasons. Periods of the
wind exceeding 30 km/h for more than sixty-five hours
have been reported.
The mistral is usually accompanied by clear, fresh
weather.
The mistral usually blows in winter or spring, though it
occurs in all seasons. It sometimes lasts only one or
two days, frequently lasts several days, and
sometimes lasts more than a week.
Wind.
local winds – most known local winds of Europe
Foehn
Foehn 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.
Wind.
local winds – most known local winds of Europe
Levant
A moist wind which blows from the east over the Strait
of Gibraltar. It is frequently accompanied by haze or
fog and may occur at any time of year, though it is
most common in the period June to October. A feature
is the occurrence of a ‘banner cloud’ extending a
kilometer or more downwind from the summit of the
Rock of Gibraltar.
The strength of the Levant does not normally exceed
Beaufort Force 5. When it is strong, however,
complex and vigorous atmospheric eddies form in the
lee of the Rock, causing difficult conditions for
yachtsmen and the pilots of aircraft.
The Levant can also cause persistently foggy weather
on the coast of Spain.
Wind.
local winds – most known local winds of Europe
Etesians (Meltemi)
The strong northerly winds which blow at times over
the Aegean Sea and eastern parts of the
Mediterranean Sea during the period May to October.
The winds are known as Meltemi in Turkey.
commons.wikimedia.org
Wind.
local winds – most known local winds of the Mediterranean
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