ATMOSPHERIC CIRCULATION : PRESSURE, VERTICAL AND HORIZONTAL DISTRIBUTION
The weight of a column of air contained in a unit area from the mean sea level to the top of the atmosphere is called the atmospheric pressure. The atmospheric pressure is expressed in units of mb and Pascals. At sea level the average atmospheric pressure is 1,013.2 mb or 1,013.2 hPa. Air pressure is measured with the help of a mercury barometer or the aneroid barometer.
Vertical Distribution of Air Pressure :
The columnar distribution of atmospheric pressure is known as vertical distribution of pressure. Air pressure decreases with increase in altitude but it does not always decrease at the same rate. Dense components of atmosphere are found in its lowest parts near the mean sea level.
The higher the density of air, the greater is the air pressure and vice versa. The mass of air above in the column of air compresses the air under it hence its lower layers are more dense than the upper layers; As a result, the lower layers of the atmosphere have higher density, hence, exert more pressure. Conversely, the higher layers are less compressed and, hence,they have low density and low pressure.
Temperature of the air, amount of water vapour present in the air and gravitational pull of the earth determine the air pressure of a given place and at a given time. Since these factors are variable with change in height, there is a variation in the rate of decrease in air pressure with increase in altitude. The normal rate of decrease in air pressure is 34millibars per every 300 metres increase in altitude. The effects of low pressure are more clearly experienced by the people living in the hilly areas as compared to those who live in plains.
Horizontal Distribution of Air Pressure :
The distribution of atmospheric pressure over the globe is known as horizontal distribution of pressure. It is shown on maps with the help of isobars.
Anisobar is a line connecting points that have equal values of pressure. Isobarsare analogous to the contour lines on a relief map. The spacing of isobars expresses the rate and direction of change in air pressure. This charge in air pressure is referred to pressure gradient.
Pressure gradient is the ratio between pressure difference and the actual horizontal distance between two points.Close spacing of isobars expresses steep pressure gradient while wide spacing indicates gentle pressure gradient.
The horizontal distribution of atmospheric pressure is not uniform in the world. It varies from time to time at a given place; it varies from place to place over short distances.
The factors responsible for variation in thehorizontal distribution of pressure are as follows:
(i) Air temperature
(ii) The earth’s rotation
(iii) Presence of water vapour
- Air Temperature:
The earth is not heated uniformly because of unequal distribution of insolation, differential heating and cooling of land and water surfaces.Generally there is an inverse relationship between air temperature and air pressure. The higher the air temperature, the lower is the air pressure.The fundamental rule about gases is that when they are heated, they become less dense and expand in volume and rise. Hence, air pressure is low in equatorial regions and it is higher in polar regions. Along the equator lies a belt of low pressure known as the “equatorial low or doldrums”. Low air pressure in equatorial regions is due to the fact that hot air ascends there with gradual decrease in temperature causing thinness of air on the surface. In polar region, cold air is very dense hence it descends and pressure increases. From this we might expect, a gradual increase in average temperature thords equator. However, actual readings taken on the earth’s surface at different places indicate that pressure does not increase longitudinally in a regular fashion from equator to the poles. Instead, there are regions of high pressure in subtropics and regions of low pressure in the subpolar areas.
(ii) The Earth’s Rotation: The earth’s rotation generates centrifugal force.
This results in the deflection of air from its original place, causing decrease of pressure. It is believed that the low pressure belts of the sub polar regions and the high pressure belts of the sub-tropical regions are created as a result of the earth’s rotation. The earth’s rotation also causes convergence and divergence of moving air. Areas of convergence experience low pressure while those of divergence have high pressure.
(iii) Pressure of Water Vapour: Air with higher quantity of water vapour
has lower pressure and that with lower quantity of water vapour has
higher pressure. In winter the continents are relatively cool and tend todevelop high pressurecentres; in summer they stay warmer than theoceans and tend to be dominated by low pressure, conversely, the oceansare associated with low pressure in winter and high pressure in summer.
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.
1.Hadley Cell :
The circulation cell closest to the equator is called the Hadley cell. 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 30° latitude, some of the air that sinks to the surface returns to the equator to complete the Hadley Cell. This produces the northeast trade winds in the Northern Hemisphere and the southeast trades in the Southern Hemisphere. The Coriolis force impacts the direction of the wind flow. In the Northern Hemisphere, the Coriolis force turns the winds to the right. In the Southern Hemisphere, the Coriolis force turns the winds to the left.
Ferrel cell :
From 30° latitude to 60° latitude, a new cell takes over known as the Ferrel Cell. This cell produces prevailing westerly winds at the surface within these latitudes. This is because some of the air sinking at 30° latitude continues traveling 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 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.
In contrast to the Hadley, Ferrel and polar circulations that run along north-south lines, the Walker circulation is an east-west circulation. 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.
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. 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.
How does this relate to 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.
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.