Wind

What is Wind?
By: Mhykaela T. Bautista
BSED II- T

WIND
Wind is moving air and is caused by
differences in air pressure within our
atmosphere. Air under high pressure
moves toward areas of low pressure.
The greater the difference in pressure,
the faster the air flows.

Wind is the movement of air across the Earth’s surface and is
produced by differences in air pressure between one places to
another. Wind strength can vary from a light breeze to
hurricane force. Winds are named from the direction from
which they originate. Since wind is produced by differences
in air pressure, it is important to understand that concept when
studying wind as well. Air pressure is created by the motion,
size, and number of gas molecules present in the air. This
varies based on the temperature and density of the air mass.

Within the atmosphere, there are several forces that impact the speed and
direction of winds. The most important though is the Earth’s
gravitational force. As gravity compresses the Earth’s atmosphere, it
creates air pressure- the driving force of wind. Without gravity, there
would be no atmosphere or air pressure and thus, no wind.
The force actually responsible for causing the movement of air though is
the pressure gradient force.

Pressure Gradient Force and Wind

Differences in air pressure and the pressure gradient force are caused by the unequal
heating of the Earth’s surface when incoming solar radiation concentrates at the
equator. Because of the energy surplus at low latitudes for example, the air there is
warmer than that at the poles. Warm air is less dense and has a lower barometric
pressure than the cold air at high latitudes. These differences in barometric pressure
are what create the pressure gradient force and wind as air constantly moves between
areas of high and low pressure.
Finally, the Coriolis force and friction both significantly affect wind across the globe.
The Coriolis force makes wind deflect from its straight path between high and low
pressure areas and the friction force slows wind down as it travels over the Earth’s
surface.

Within the atmosphere, there are different levels of air
circulation. However those in the middle and
upper troposphere are an important part of the entire
atmosphere's air circulation. To map these circulation
patterns upper air pressure maps use 500 millibars (mb)
as a reference point. This means that the height above sea
level is only plotted in areas with an air pressure level of
500 mb.

The jet stream is a fast flowing, river of air found in the atmosphere at around 12 km above the
surface of the Earth just under the tropopause. They form at the boundaries of adjacent air masses
with significant differences in temperature, such as of the polar region and the warmer air to the
south. Because of the effect of the Earth's rotation the streams flow west to east, propagating in a
serpentine or wave-like manner at lower speeds than that of the actual wind within the flow.
Jet Stream

Rossby Waves
Rossby Waves are like rivers of air in the upper troposphere and they gradually
meander. The meander loops get bigger and bigger until their wavelength from
trough to trough could be as much as 8000 kms. When the Waves are well
developed and cover a wide range of latitude they are said to have a low zonal
index - which leads to the formation of ridges of blocking, high pressure
systems and dry stable conditions. When they are almost straight and cover a
narrow zone of latitude they are said to have a high zonal index - which leads to
a succession of low pressure systems and unsettled weather. The waves evolve
then they straighten up and then meanders form again in an endless cycle. The
wave evolution cycle lasts about 6 weeks.

Sta Ana Winds
The Santa Ana winds are strong, extremely dry down-slope winds that
originate inland and affect coastal Southern California and northern
Baja California. Santa Ana winds blow mostly in autumn and winter,
but can arise at other times of the year. They can range from hot to
cold, depending on the prevailing temperatures in the source regions,
the Great Basin and upper Mojave Desert. The winds are known
especially for the hot dry weather (often the hottest of the year) that
they bring in the fall, and are infamous for fanning regional wildfires.
For these reasons, they are sometimes known as the "devil winds"
across Southern California.

On a warm summer day along the coast, this differential heating of land
and sea leads to the development of local winds called sea breezes. As air
above the land surface is heated by radiation from the Sun, it expands
and begins to rise, being lighter than the surrounding air. To replace the
rising air, cooler air is drawn in from above the surface of the sea. This is
the sea breeze, and can offer a pleasant cooling influence on hot summer
afternoons.

A land breeze occurs at night when the land cools faster than the
sea. In this case, it is air above the warmer surface water that is
heated and rises, pulling in air from the cooler land surface.

A valley breeze develops during the day as the Sun heats the land surface and air
at the valley bottom and sides . As the air heats it becomes less dense and buoyant
and begins to flow gently up the valley sides. Vertical ascent of the air rising
along the sides of the mountain is usually limited by the presence of
a temperature inversion layer. When the ascending air currents encounter the
inversion they are forced to move horizontally and then back down to the valley
floor. This creates a self-contained circulation system. If conditions are right, the
rising air can condense and form into cumuliform clouds.

During the night, the air along the mountain slopes begins to cool quickly because
of longwave radiation loss. As the air cools, it becomes more dense and begins to flow
downslope causing a mountain breeze. Convergence of the draining air occurs at the
valley floor and forces the air to move vertically upward. The upward movement is
usually limited by the presence of a temperature inversion which forces the air to begin
moving horizontally. This horizontal movement completes the circulation cell system. In
narrowing terrain, mountain winds can accelerate in speed because of the venturi effect.
Such winds can attain speeds as high has 150 kilometers per hour.

A monsoon is a seasonal wind, found especially in Asia that reverses
direction between summer and winter and often brings heavy rains. In
the summer, a high pressure area lies over the Indian Ocean while a
low exists over the Asian continent. The air masses move from the
high pressure over the ocean to the low over the continent, bringing
moisture-laden air to south Asia. During winter, the process is reversed
and a low sits over the Indian Ocean while a high lies over the Tibetan
plateau so air flows down the Himalaya and south to the ocean. The
migration of trade winds and westerlies also contributes to the
monsoons. Smaller monsoons take place in equatorial Africa, northern
Australia, and, to a lesser extent, in the southwestern United States.

The cause of this wind shift, or monsoon.

The monsoon, which is essentially the seasonal reversal in wind direction, causes
most of the rainfall received in India and some other parts of the world. The primary
cause of monsoons is the difference between annual temperature trends over land and
sea. The apparent position of the Sun with reference to the Earth oscillates from the
Tropic of Cancer to the Tropic of Capricorn. Thus the low pressure region created by
solar heating also changes latitude. The northeast and southeast trade winds converge
in this low pressure zone, which is also known as the Intertropical Convergence Zone
or ITCZ. This low pressure regions sees continuous rise of moist wind from the sea
surface to the upper layers of the atmosphere, where the cooling means the air can no
longer hold so much moisture resulting in precipitation. The rainy seasons of East
Asia, sub-Saharan Africa, Australia and the southern parts of North America coincide
with the shift of ITCZ towards these regions.

Though monsoons are often associated with devastating effects, including famine, the overall effects of
the season are positive.
Drought: The countries affected by monsoon season welcome the rain as a point at which drought ends.
However, if the rainfall in this season is below average then further drought can be devastating. This leads
to food supplies dwindling and crops unable to grow, and many of the area's inhabitants may die of
famine.
Floods: Those who make their living from the land welcome the season, where the rains will make a large
contribution to the crops they produce. However, when too much rain comes in a short period of time this
can cause flash floods that not only damage crops and drown livestock, but also harm inhabitants of the
area. However, in less extreme forms these floods help to fertilise the soil.
Agriculture and Economy: The patterns of the monsoon have a huge impact on farmers and their crops
and livestock. If the rains don't come as expected then farming patterns can be disrupted. Not to mention
the effects of drought and flood, as covered above. The economy of many Indian countries greatly
depends on agriculture, meaning that the disruption not only harms health and availability of food, but the
economy as a whole and food prices rise dramatically.

Prevailing wind patterns in the Philippines

Amihan refers to the season dominated by the trade winds, which are
experienced in the Philippines as a cool northeast wind. It is
characterized by moderate temperatures, little or no rainfall, and a
prevailing wind from the east.
As a general rule of thumb, the Philippines' Amihan weather pattern
begins sometime in September or October and ends sometime in May
or June. There may, however, be wide variations from year to year.

The trade winds are just air movements toward the equator. They are
warm, steady breezes that blow almost continuously. The Coriolis Effect
makes the trade winds appear to be curving to the west, whether they are
traveling to the equator from the south or north.
What are the trade winds?

The Philippines experiences the west or southwest wind; south-
west monsoon, which in turn is referred to as the Habagat.
The Habagat season is characterized by hot and humid weather,
frequent heavy rainfall, and a prevailing wind from the west.
Habagat is also known as monsoon or southwest monsoon in the
Philippines.

Different Wind Patterns
found across the Earth

The equator receives the Sun's direct rays. Here, air is heated and rises,
leaving low pressure areas behind. Moving to about thirty degrees north
and south of the equator, the warm air from the equator begins to cool and
sink. Between thirty degrees latitude and the equator, most of the cooling
sinking air moves back to the equator. The rest of the air flows toward the
poles.

At about sixty degrees latitude in both hemispheres, the prevailing
westerlies join with the polar easterlies to reduce upward motion.
The polar easterlies form when the atmosphere over the poles
cools. This cool air then sinks and spreads over the surface. As the
air flows away from the poles, it is turned to the west by the
Coriolis effect. Again, because these winds begin in the east, they
are called easterlies.

Between thirty and sixty degrees latitude, the winds that
move toward the poles appear to curve to the east. Because
winds are named from the direction in which they originate,
these winds are called prevailing westerlies. Prevailing
westerlies in the Northern Hemisphere are responsible for
many of the weather movements across the United States
and Canada.

The trade winds are just air movements toward
the equator. They are warm, steady breezes that
blow almost continuously. The Coriolis Effect
makes the trade winds appear to be curving to the
west, whether they are traveling to the equator
from the south or north.

What are the doldrums?
The doldrums is an area of calm weather. The trade winds
coming from the south and the north meet near the equator.
These converging trade winds produce general upward winds
as they are heated, so there are no steady surface winds.

In the southern hemisphere the belts are reversed. The
southeast trade winds blow from the southeast toward the
equator. The southern equivalent of the horse latitudes (or
Variables of Cancer) is called the Variables of Capricorn.
The southern westerlies start somewhat south of South Africa.
They tend to be stronger than the northern westerlies because
they are mostly over water (roaring forties). The southern polar
easterlies are mostly over Antarctica.

All of the belts move north during the northern summer and south
during the northern winter. Because global heating and cooling
lags behind the position of the sun, they reach their northernmost
latitude at or after the end of the northern summer. This brought
the trade winds within reach of the Spain and Portugal and
determined the sailing time of the Spanish treasure fleet. The
northernmost position of the wind belts corresponds to the
Atlantic hurricane season.

Land gains and loses heat more rapidly than water.
During the day, the land warms more rapidly than the
water. The air above land warms, becomes thinner, and
rises, drawing cooler air landward from the sea. At
night, the process reverses, and cool heavy air from the
land flows out to sea. These land and sea breezes are
important along the coast.

The annual equivalent of the daily land and sea breezes is the
yearly monsoon. During summer, the continents heat more rapidly than the
oceans. Air over the continents warms, thins and rises drawing cooler
moist ocean air landward, producing a wet season. During winter, the
process reverses and cold, dry heavy air flows outward from the
continents, producing a dry season. The monsoon is most striking in south
Asia because of the size of the Eurasian landmass and because the
Himalayas tend to bottle up the air above the continent. Approximations of
the Indian monsoon exist in other places, but they are poorly developed.

The fact that the westerlies and trade winds blow in opposite
directions and that the continents prevent water from circling
the globe contributes to the formation of circular ocean currents,
clockwise in the northern hemisphere and counterclockwise in
the southern hemisphere. The Coriolis force also plays a part.
The trade winds push water west. At the doldrums it flows back
east producing the equatorial counter current.

Coriolis force,
Hadley cell and
other things

In the northern hemisphere, the Coriolis effect causes wind and water
currents to bend to the right (clockwise). Cold heavy air flows south from
the north pole and is bent west, forming the polar easterlies. Warm air rises
at the equator drawing air from the north which bends to the west,
contributing to the trade winds. The Coriolis effect bends the westerlies and
trade winds slightly clockwise in the northern hemisphere.
Hot air rises at the doldrums. As it rises, it cools producing thunderstorms.
The dry air flows north at a high altitude and descends at the horse latitudes
and flows back to the equator with the trade winds. This is called the Hadley
cell. There is also a Ferrel cell over the westerlies and a polar cell over the
pole. There are other complexities, not all of which are properly understood.

Wind

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