Global circulation patterns of atmosphere

1. Global Circulation Patterns Of Atmosphere
Definition:
Over the major parts of the Earth's surface there are large-scale wind circulations present.
The global circulation can be described as the world-wide system of winds by which the
necessary transport of heat from tropical to polar latitudes is accomplished.
Explanation:
At any one time there are multiple weather systems active around the globe with variable
winds. When these winds are averaged over many years a well-defined global circulation
pattern appears. The circulation of wind in the atmosphere is driven by the rotation of the earth
and the incoming energy from the sun. Wind circulates in each hemisphere in three distinct cells
which help transport energy and heat from the equator to the poles. The winds are driven by the
energy from the sun at the surface as warm air rises and colder air sinks. Atmospheric
circulation is the large-scale movement of air, and the means by which thermal energy is
distributed on the surface of the Earth. The large-scale structure of the atmospheric
circulation varies from year to year, but the basic climatological structure remains fairly
constant. Individual weather systems mid-latitude depressions, or tropical convective cells
occur "randomly", and it is accepted that weather cannot be predicted beyond a fairly short
limit: perhaps a month in theory, or about ten days in practice. Nonetheless, as the climate
is the average of these systems and patterns where and when they tend to occur again and
again it is stable over longer periods of time. As a rule, the "cells" of Earth's atmosphere
shift pole wards in warmer climates e.g. interglacials compared to glacials, but remain
largely constant even due to continental drift; they are, fundamentally, a property of the
Earth's size, rotation rate, heating and atmospheric depth, all of which change little.
However, a tectonic uplift can significantly alter their major elements, for example, the jet
stream, and plate tectonics may shift ocean currents. In the extremely hot climates of the
Mesozoic, indications of a third desert belt at the Equator has been found; it was perhaps
caused by convection.
Troposphere:
The lowest region of the atmosphere between the earth's surface and the tropopause,
characterized by decreasing temperature with increasing altitude. It is the the lowest region
of the atmosphere, extending from the earth's surface to a height of about 6–10 km. It is the
part of the atmosphere where most of the weather takes place. n each hemisphere there
are three cells in which air circulates through the entire depth of the troposphere. These
are:
1. Hadley Cell. 2. Ferrel Cell. 3. Polar Cell.

2. Hadley Cells:
The circulation cell closest to the equator is called the Hadley cell. The largest cells extend
from the equator to between 30 and 40 degrees north and south, and are named Hadley
cells, after English meteorologist George Hadley. Winds are light at the equator because of
the weak horizontal pressure gradients located there. The warm surface conditions result
in locally low pressure. The warm air rises at the equator producing clouds and causing
instability in the atmosphere.
This instability causes thunderstorms to develop and release large amounts of latent heat.
Latent heat is just energy released by the storms due to changes from water vapor to liquid
water droplets as the vapor condenses in the clouds, causing the surrounding air to become
more warm and moist, which essentially provides the energy to drive the Hadley cell. The
Hadley Cell encompasses latitudes from the equator to about 30°. At this latitude surface
high pressure causes the air near the ground to diverge. This forces air to come down from
aloft to "fill in" for the air that is diverging away from the surface high pressure.
The air flowing northward from the equator high up in the atmosphere is warm and moist
compared to the air nearer the poles. This causes a strong temperature gradient between
the two different air masses and a jet stream results. At the 30° latitudes, this jet is known
as the subtropical jet stream which flows from west to east in both the Northern and
Southern Hemispheres.
Clear skies generally prevail throughout the surface high pressure, which is where many of
the deserts are located in the world. From the tops of these storms, the air flows towards
higher latitudes, where it sinks to produce high-pressure regions over the subtropical
oceans and the world's hot deserts, such as the Sahara dessert in North Africa. Within the
Hadley cells, the trade winds blow towards the equator, then ascend near the equator as a

3. broken line of thunderstorms, which forms the Inter-Tropical-Convergence Zone (ITCZ).
Ferrel Cells:
In the middle cells, which are known as the Ferrel cells, air converges at low altitudes to
ascend along the boundaries between cool polar air and the warm subtropical air that
generally occurs between 60 and 70 degrees north and south. This often occurs around the
latitude of the UK which gives us an unsettled weather. From 30° latitude to 60° latitude,
the Ferrel Cell takes control. This cell produces prevailing westerly winds at the surface
within these latitudes. This is because some of the air sinking at 30° latitude continues
travelling northward toward the poles and the Coriolis force bends it to the right in the
Northern Hemisphere.
This air is still warm and at roughly 60° latitude approaches cold air moving down from the
poles. With the converging air masses at the surface, the low surface pressure at 60°
latitude causes air to rise and form clouds. Some of the rising warm air returns to 30°
latitude to complete the Ferrel Cell. The circulation within the Ferrel cell is complicated by a
return flow of air at high altitudes towards the tropics, where it joins sinking air from the
Hadley cell.
The Ferrel cell moves in the opposite direction to the two other cells Hadley cell and Polar
cell and acts rather like a gear. In this cell the surface wind would flow from a southerly
direction in the northern hemisphere. However, the spin of the Earth induces an apparent
motion to the right in the northern hemisphere and left in the southern hemisphere. This
deflection is caused by the Coriolis effect and leads to the prevailing westerly and south-
westerly winds often experienced over the UK.

4. Polar Cells:
The smallest and weakest cells are the Polar cells, which extend from between 60 and 70
degrees north and south, to the poles. Air in these cells sinks over the highest latitudes and
flows out towards the lower latitudes at the surface. The two air masses at 60° latitude do
not mix well and form the polar front which separates the warm air from the cold air.
Thus the polar front is the boundary between warm tropical air masses and the colder polar
air moving from the north. The polar jet stream aloft is located above the polar front and
flows generally from west to east. The polar jet is strongest in the winter because of the
greater temperature contrasts than during the summer. Waves along this front can pull the
boundary north or south, resulting in local warm and cold fronts which affect the weather at
particular locations.
Above 60° latitude, the polar cell circulates cold, polar air equatorward. The air from the
poles rises at 60° latitude where the polar cell and Ferrel cell meet, and some of this air
returns to the poles completing the polar cell. Because the wind flows from high to low
pressure and taking into account the effects of the Coriolis force, the winds above 60°
latitude are prevailing easterlies.
Walker Cells:
It was discovered by Gilbert Walker. In contrast to the Hadley, Ferrel and polar circulations
that run along north-south lines, the Walker circulation is an east-west circulation.
The Walker circulation, also known as the Walker cell, is a conceptual model of the air flow
in the tropics in the troposphere. According to this model, parcels of air follow a closed
circulation in the vertical directions. This circulation, which is roughly consistent with
observations, is caused by differences in heat distribution between ocean and land. In

5. addition to motions in the vertical direction the tropical atmosphere also has considerable
motion in the meridional direction as part of, for example, the Hadley Circulation.
Explanation:
Walker determined that the time scale of a year was unsuitable because geospatial
relationships could be entirely different depending on the season. Thus, Walker broke his
temporal analysis into December–February, March–May, June–August, and September–
November. Walker then selected a number of "centers of action", which included areas such
as the Indian Peninsula. The centers were in the hearts of regions with either permanent or
seasonal high and low pressures. He also added points for regions where rainfall, wind or
temperature was an important control. He examined the relationships of the summer and
winter values of pressure and rainfall, first focusing on summer and winter values, and later
extending his work to the spring and autumn. He concludes that variations in temperature
are generally governed by variations in pressure and rainfall. It had previously been
suggested that sunspots could be the cause of the temperature variations, but Walker
argued against this conclusion by showing monthly correlations of sunspots with
temperature, winds, cloud cover, and rain that were inconsistent. Walker made it a point to
publish all of his correlation findings, both of relationships found to be important as well as
relationships that were found to be unimportant. He did this for the purpose of dissuading
researchers from focusing on correlations that did not exist.
Over the eastern Pacific Ocean, surface high pressure off the west coast of South America
enhances the strength of the easterly trade winds found near the equator. The winds blow
away from the high pressure toward lower pressure near Indonesia. Upwelling, the rising of
colder water from the deep ocean to the surface, occurs in the eastern Pacific along South
America near Ecuador and Peru. This cold water is especially nutrient-rich and is stocked
with an abundance of large fish populations. By contrast the water in the western Pacific,
near Indonesia, is relatively warm. The air over Indonesia rises because of the surface low
pressure located there and forms clouds. This causes heavy precipitation to fall over the
western tropical Pacific throughout the year. The air then circulates back aloft towards the
region above the surface high pressure near Ecuador and this becomes the Walker
circulation. The air sinks at this surface high pressure and is picked up by the strong trade
winds to continue the cycle.

6. On some occasions, the Walker circulation and the trade winds weaken, allowing warmer
water to "slosh back" towards the eastern tropical Pacific near South America. You can
think of this as blowing a fan over a bathtub full of water. If the fan blows steadily, the
water at the side farthest from the fan will tend to pile up downwind. If you suddenly slow
the fan down, some of the water that was built up will surge back towards the fan. The
warmer water will cover the areas of upwelling, cutting off the flow of nutrients to the fish
and animals that live in the eastern Pacific Ocean. This warming of the eastern Pacific
Ocean is known as El Niño. The warmer water will also serve as a source for warm, moist air
which can aid in the development of heavy thunderstorms over the mass of warm water.
Circulation Of Atmosphere And Agriculture:
Changes in the Hadley cell and Walker circulation can result in dramatic climate variations
for many regions. In an El Niño winter, for example, the presence of the warm water in the
eastern Pacific shifts the position of the subtropical jet, leading to heavy rainfall in Florida
and southern Georgia. which allows you to look at differences in climate in different years
depending on the El Niño phase.
In a warming climate, the Hadley cell could increase in length and alter the climate of
regions around 30°. For example, many deserts in the northern hemisphere are located
around the 30° latitude, and if the Hadley cell were to increase in length, that could cause
dry conditions to move north of 30°. Ultimately, this would alter the precipitation patterns
of many regions, including the Southeast.