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Atmospheric Pressure, also called barometric pressure, force per unit area exerted by an atmospheric column (that is, the entire body of air above the specified area). Atmospheric pressure can be measured with a mercury barometer (hence the commonly used synonym barometric pressure), which indicates the height of a column of mercury that exactly balances the weight of the column of Atmosphere over the barometer. Atmospheric pressure is also measured using an aneroid barometer, in which the sensing element is one or more hollow, partially evacuated, corrugated Metal disks supported against collapse by an inside or outside spring; the change in the shape of the disk with changing pressure can be recorded using a pen arm and a clock-driven revolving drum.
The atmosphere is held on the earth by the gravitational pull of the earth. A column of air exerts weight in terms of pressure on the surface of the earth. The weight of the column of air at a given place and time is called air pressure or atmospheric pressure. Atmospheric pressure is measured by an instrument called barometer. Now a days Fortin’s barometer and Aneroid barometer I are commonly used for measuring air pressure.
Atmospheric pressure is measured as force per unit area. The unit used for measuring pressure is called millibar. Its abbreviation is ‘mb’. One millibar is equal to the force of one gram per square centimetre approximately. A pressure of 1000 millibars is equal to the weight of 1.053 kilograms per square centimetre at sea level. It is equal to the weight of a column of mercury which is 76 centimetre high. The international standard pressure unit is the “pascal”, a force of one Newton per square meter. In practice atmospheric pressure is expressed in kilopascals, (one kpa equals 1000 Pa).
Wind belts on earth
Inter-Tropical Convergence Zone (Doldrums)
Since air is heated and rises at the equator, a zone of low pressure is formed. This zone is referred to as the equatorial trough. Air moves towards the equatorial trough, where it converges and moves aloft as a part of Hadley cell. Convergence occurs in a narrow zone, called the inter-tropical convergence zone (ITCZ).
It is the belt of equatorial calms and winds lie over the equatorial trough of low pressure. Average location of doldrums is 5°N and 5°S from the equator and this belt lies between two trade winds. As the horizontal pressure gradient is weak, therefore winds are Light and variable.
Because of the convergence of winds, convective activity is dominant. Convection becomes vary strong in the late afternoons carrying warm moist air, often forming huge cumulus clouds, which results in heavy thunderstorms.
Because of the enormous amount of latent heat released by these clouds, the atmosphere becomes hot, oppressive and sultry. Since it is the meeting zone of north easterly and south easterly trade winds, it is also called Inter Tropical Convergence Zone (ITCZ) or doldrums.
Trade Wind Pattern
This belt extends roughly from 5° to 30° N & S of equator. Here, at the surface wind flows from poles towards the equator and in the upper atmosphere, flow is towards the poles. These trade winds originate because of the pressure gradient force from sub-tropical high to equatorial low. In the northern hemisphere, the trades are north-easterly and in the southern hemisphere, these are south easterly. These winds are regular (steady) and flow in constant direction.
The trade wind belt is also called Hadley cell after the name of the scientist as it resembles the convective model used by Hadley for the whole earth. The energy to derive this cell is believed to come from the release of latent heat during the formation of cumulonimbus clouds in the equatorial region.
The poleward moving winds in the upper atmosphere in this cell begin to subside between 20°-35°N & S latitudes. The subsidence over here may be due to the radiational cooling, because at upper levels it makes the air heavy and at the same time it begins to converge at higher levels over the middle latitudes around 30°. This convergence (piling up) of air aloft increases the mass of air above the surface.
Because of accumulation of air mass at higher levels, it starts descending around 30° latitude in both the hemispheres. This zone of descending air produces sub-tropical high pressure belts and is also known as ‘horse latitudes’ where like the doldrums the winds are light and variable.
The descending air over the sub-tropical high, is dry and warm. As a result, subsiding air produces clear sky and high temperature. Major deserts of the world like Sahara are located in this region.
Subtropical High Pressure (Westerlies Belts)
These lie between 30° and 60°N & S latitudes in both the hemispheres. The winds move from poleward margins of sub-tropical high pressure belts. While moving to higher latitudes, these winds are deflected and become south-westerlies in northern hemisphere and north- westerlies in southern hemisphere. The westerlies of middle latitudes are more variable than trades both in direction and intensity.
These westerly winds are frequently over-powered by polar air masses and cells of Cyclones-2/”>Cyclones and anti-cyclones are formed in these areas. The surface flow of the westerlies may be interrupted by storms and irregular winds blowing from different directions, but in the upper atmosphere these are steady and blow in westerly direction.
The westerlies prevail throughout the year, but are stronger in winter season, especially over North Atlantic and North Pacific Oceans. This is because of the steep pressure gradient from the Aleution islands and icelandic low pressure areas towards the extremely cold continental interiors where the pressure is very high.
These two semi-permanent lows are the cause of a number of cyclonic storms moving along the westerlies across the globe. In the southern hemisphere, between 40° & 60° latitudes, westerlies are persistent and powerful over water, sailors call them Roaring forties, furious fifties and screening sixties.
Polar easterlies are the winds which move from polar highs towards sub-polar lows. Winds blowing from north pole are not regular. Because the polar high is not considered as quasi (semi)-permanent feature of arctic circulation. However, there are prevailing outflowing winds from the green land.
In winter, the easterly winds are observed from anticyclones of Siberia and Canada. The winds in these areas generally blow from various directions and these are largely controlled by local weather disturbances. But on the poleward side of the depressions (cyclones) that form in the northern Atlantic and northern Pacific, the easterly winds do occur.
The easterly winds in southern hemisphere are well defined and are coherent (semi-permanent) and regular. Easterly winds blow from the anti-cyclonic systems formed over the plateau of eastern Antarctica. The Indian ocean near the Antarctica experiences such easterly winds.
Little is known about the atmospheric motion at upper levels in high latitudes beyond 70° or 75° (i.e. 70, 80 or 90°) in both the hemispheres due to lack of the meteorological information.
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The Earth’s atmosphere is divided into five main pressure belts: the equatorial low, the two subtropical highs, the polar lows, and the subpolar lows. The equatorial low is located around the equator and is characterized by warm, moist air. The subtropical highs are located at about 30 degrees north and south latitude and are characterized by dry, sinking air. The polar lows are located at the poles and are characterized by cold, dry air. The subpolar lows are located between the polar lows and the subtropical highs and are characterized by variable weather conditions.
The Earth’s rotation causes the air in the atmosphere to move in a circular pattern. This pattern is called the Coriolis effect. The Coriolis effect causes the air in the atmosphere to move clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere.
The pressure belts and the Coriolis effect cause the Earth’s atmosphere to be divided into six main wind belts: the trade winds, the westerlies, the polar easterlies, the jet streams, the monsoons, and the hurricanes.
The trade winds are located between the equator and the subtropical highs. They blow from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere. The trade winds are caused by the difference in pressure between the equatorial low and the subtropical highs.
The westerlies are located between the subtropical highs and the polar lows. They blow from the west in both the Northern and Southern Hemispheres. The westerlies are caused by the difference in pressure between the subtropical highs and the polar lows.
The polar easterlies are located between the polar lows and the subpolar lows. They blow from the east in both the Northern and Southern Hemispheres. The polar easterlies are caused by the difference in pressure between the polar lows and the subpolar lows.
The jet streams are fast-flowing bands of air that are located in the upper atmosphere. They are caused by the difference in temperature between the equator and the poles. The jet streams are important because they help to regulate the Earth’s Climate.
The monsoons are Seasonal Winds that blow in tropical and subtropical regions. They are caused by the difference in temperature between the land and the ocean. The monsoons bring much-needed rain to many parts of the world.
Hurricanes and typhoons are tropical cyclones that form over warm ocean waters. They are caused by the difference in temperature between the ocean and the atmosphere. Hurricanes and typhoons are the most powerful storms on Earth.
Tornadoes are violently rotating columns of air that form from thunderstorms. They are caused by the difference in temperature and pressure between the air at the ground and the air in the atmosphere. Tornadoes are the most destructive storms on Earth.
Blizzards are severe snowstorms that can cause widespread damage and loss of life. They are caused by the combination of cold temperatures, high winds, and heavy snow.
Dust storms are large clouds of dust that are blown by strong winds. They can cause respiratory problems and can damage crops and property.
Heat waves are periods of abnormally hot weather that can cause heat stroke, dehydration, and death. They are caused by a combination of high temperatures, high humidity, and lack of wind.
Cold waves are periods of abnormally cold weather that can cause frostbite, hypothermia, and death. They are caused by a combination of low temperatures, high winds, and lack of sunlight.
Droughts are periods of abnormally dry weather that can cause crop failures, wildfires, and water shortages. They are caused by a combination of low rainfall, high temperatures, and high winds.
Floods are periods of abnormally high water levels that can cause widespread damage and loss of life. They are caused by heavy rains, melting snow, or dam failures.
El Niño and La Niña are climate patterns that occur in the Pacific Ocean. El Niño is characterized by warmer-than-average sea surface temperatures, while La Niña is characterized by cooler-than-average sea surface temperatures. El Niño and La Niña can have a significant impact on weather patterns around the world.
Climate Change is a long-term change in the Earth’s climate. It is caused by the emission of greenhouse gases into the atmosphere. Greenhouse gases trap heat in the atmosphere, which causes the Earth’s temperature to rise. Climate change is causing a number of changes to the Earth’s climate, including rising sea levels, more extreme weather events, and changes in plant and animal life.
1. What is the difference between high pressure and low pressure?
High pressure is an area of the atmosphere where the air is sinking. This sinking air is warm and dry, which can lead to clear skies and fair weather. Low pressure is an area of the atmosphere where the air is rising. This rising air is cool and moist, which can lead to cloudy skies and Precipitation.
2. What are the four main wind belts?
The four main wind belts are the trade winds, the prevailing westerlies, the polar easterlies, and the jet streams. The trade winds are the winds that blow from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere. The prevailing westerlies are the winds that blow from the west in the mid-latitudes. The polar easterlies are the winds that blow from the east in the polar regions. The jet streams are fast-flowing bands of air that occur in the upper atmosphere.
3. What causes the wind to blow?
The wind blows because of the difference in air pressure between two areas. Air moves from areas of high pressure to areas of low pressure. This is because the air Molecules in an area of high pressure are closer together than the air molecules in an area of low pressure. The closer together the air molecules are, the more pressure they exert. The less close together the air molecules are, the less pressure they exert.
4. What are the effects of the wind?
The wind has many effects on the Earth. It can cause erosion, it can transport heat and moisture, and it can power wind turbines. Erosion is the process of wearing away the Earth’s surface. The wind can erode the Earth’s surface by carrying away loose Soil and sand. The wind can also transport heat and moisture. The wind can carry warm air from the tropics to the poles, and it can carry moist air from the oceans to the land. Wind turbines are machines that use the wind to generate electricity.
5. How can we use the wind?
We can use the wind to generate electricity, to power sailboats, and to fly kites. Wind turbines are machines that use the wind to generate electricity. Wind turbines are made up of blades that are turned by the wind. The blades turn a Generator, which produces electricity. Sailboats are boats that are powered by the wind. Sailboats have sails that catch the wind and push the boat forward. Kites are toys that are flown in the air by the wind. Kites have strings that are attached to a person’s hand. The wind pulls on the kite, and the person holds onto the string to keep the kite in the air.
Which of the following is not a type of wind?
(A) Trade winds
(B) Westerlies
(C) Polar easterlies
(D) MonsoonsWhich of the following is the most common type of wind?
(A) Trade winds
(B) Westerlies
(C) Polar easterlies
(D) MonsoonsTrade winds blow from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere.
(A) True
(B) FalseWesterlies blow from the west in the Northern Hemisphere and from the east in the Southern Hemisphere.
(A) True
(B) FalsePolar easterlies blow from the east in the Northern Hemisphere and from the west in the Southern Hemisphere.
(A) True
(B) FalseMonsoons are seasonal winds that blow from the land to the sea in the summer and from the sea to the land in the winter.
(A) True
(B) FalseThe Earth’s rotation causes the trade winds to curve towards the equator.
(A) True
(B) FalseThe Earth’s rotation causes the westerlies to curve towards the poles.
(A) True
(B) FalseThe Earth’s rotation causes the polar easterlies to curve towards the equator.
(A) True
(B) FalseMonsoons are caused by the difference in temperature between the land and the sea.
(A) True
(B) False