ATMOSPHERIC CIRCULATION WEATHER SYSTEMS

1 ATMOSPHERIC CIRCULATIONAND WEATHER SYSTEMSE arlier Chapter 9 described the unevendistribution of temperature over thesurface of the earth. Air expands whenheated and gets compressed when cooled. Thisresults in variations in the atmosphericpressure. The result is that it causes themovement of air from high pressure to lowpressure, setting the air in motion. You alreadyknow that air in horizontal motion is pressure also determines whenthe air will rise or sink. The wind redistributesthe heat and moisture across the planet,thereby, maintaining a constant temperaturefor the planet as a whole. The vertical rising ofmoist air cools it down to form the clouds andbring precipitation. This chapter has beendevoted to explain the causes of pressuredifferences, the forces that control theatmospheric CIRCULATION , the turbulent patternof wind, the formation of air masses, thedisturbed WEATHER when air masses interactwith each other and the phenomenon of violenttropical PRESSUREDo you realise that our body is subjected to alot of air pressure.

2 As one moves up the airgets varified and one feels weight of a column of air contained ina unit area from the mean sea level to the topof the atmosphere is called the atmosphericpressure. The ATMOSPHERIC pressure isexpressed in units of milibar. At sea level theaverage ATMOSPHERIC pressure is 1, Due to gravity the air at the surface isdenser and hence has higher pressure. Airpressure is measured with the help of amercury barometer or the aneroid your book, Practical Work inGeography Part I (NCERT, 2006) and learnabout these instruments. The pressuredecreases with height. At any elevation it variesfrom place to place and its variation is theprimary cause of air motion, wind whichmoves from high pressure areas to lowpressure Variation of PressureIn the lower atmosphere the pressuredecreases rapidly with height.

3 The decreaseamounts to about 1 mb for each 10 mincrease in elevation. It does not alwaysdecrease at the same rate. Table givesthe average pressure and temperature atselected levels of elevation for a : Standard Pressure and temperature atSelected LevelsLevelPressure in mbTemperature CSea Level1, 17. 310 vertical pressure gradient force is muchlarger than that of the horizontal pressuregradient. But, it is generally balanced by anearly equal but opposite gravitational , we do not experience strong CIRCULATION AND WEATHER SYSTEMS85 Horizontal Distribution of PressureSmall differences in pressure are highlysignificant in terms of the wind direction andpurposes of comparison.

4 The sea level pressuredistribution is shown on WEATHER shows the patterns of isobarscorresponding to pressure SYSTEMS . Low-pressure system is enclosed by one or moreisobars with the lowest pressure in the system is also enclosed by oneor more isobars with the highest pressure inthe Distribution of Sea Level PressureThe world distribution of sea level pressure inJanuary and July has been shown in and Near the equator the sea levelpressure is low and the area is known asequatorial low. Along 30 N and 30o S arefound the high-pressure areas known as thesubtropical highs. Further pole wards along60oN and 60oS, the low-pressure belts aretermed as the sub polar lows. Near the polesthe pressure is high and it is known as the polarhigh.

5 These pressure belts are not permanentFigure : Distribution of pressure (in millibars) JanuaryFigure : Isobars, pressure and wind SYSTEMS inNorthern Hemispherevelocity. Horizontal distribution of pressure isstudied by drawing isobars at constant are lines connecting places havingequal pressure. In order to eliminate the effectof altitude on pressure, it is measured at anystation after being reduced to sea level for2022-23 FUNDAMENTALS OF PHYSICAL GEOGRAPHY86 Pressure Gradient ForceThe differences in ATMOSPHERIC pressureproduces a force. The rate of change of pressurewith respect to distance is the pressuregradient. The pressure gradient is strong wherethe isobars are close to each other and is weakwhere the isobars are ForceIt affects the speed of the wind.

6 It is greatest atthe surface and its influence generally extendsupto an elevation of 1 - 3 km. Over the seasurface the friction is ForceThe rotation of the earth about its axis affectsthe direction of the wind. This force is calledthe Coriolis force after the French physicist whodescribed it in 1844. It deflects the wind to theright direction in the northern hemisphere andin nature. They oscillate with the apparentmovement of the sun. In the northernhemisphere in winter they move southwardsand in the summer Affecting the Velocityand Direction of WindYou already know that the air is set in motiondue to the differences in ATMOSPHERIC air in motion is called wind. The windblows from high pressure to low pressure.

7 Thewind at the surface experiences friction. Inaddition, rotation of the earth also affects thewind movement. The force exerted by therotation of the earth is known as the Coriolisforce. Thus, the horizontal winds near theearth surface respond to the combined effectof three forces the pressure gradient force,the frictional force and the Coriolis force. Inaddition, the gravitational force : Distribution of pressure (in millibars) July2022-23 ATMOSPHERIC CIRCULATION AND WEATHER SYSTEMS87to the left in the southern hemisphere. Thedeflection is more when the wind velocity ishigh. The Coriolis force is directly proportionalto the angle of latitude. It is maximum at thepoles and is absent at the Coriolis force acts perpendicular to thepressure gradient force.

8 The pressure gradientforce is perpendicular to an isobar. The higherthe pressure gradient force, the more is thevelocity of the wind and the larger is thedeflection in the direction of wind. As a result ofthese two forces operating perpendicular to eachother, in the low-pressure areas the wind blowsaround it. At the equator, the Coriolis force iszero and the wind blows perpendicular to theisobars. The low pressure gets filled instead ofgetting intensified. That is the reason why tropicalcyclones are not formed near the and WindThe velocity and direction of the wind are thenet result of the wind generating forces. Thewinds in the upper atmosphere, 2 - 3 km abovethe surface, are free from frictional effect of thesurface and are controlled mainly by thepressure gradient and the Coriolis force.

9 Whenisobars are straight and when there is nofriction, the pressure gradient force is balancedby the Coriolis force and the resultant windblows parallel to the isobar. This wind is knownas the geostrophic wind (Figure ).Table : Pattern of Wind Direction in Cyclones and AnticyclonesPressure SystemPressure ConditionPattern of Wind Directionat the CentreNorthern HemisphereSouthern HemisphereCycloneLowAnticlockwiseClockwi seAnticycloneHighClockwiseAnticlockwiseT he wind CIRCULATION around a low iscalled cyclonic CIRCULATION . Around a highit is called anti cyclonic CIRCULATION . Thedirection of winds around such systemschanges according to their location indifferent hemispheres (Table ).The wind CIRCULATION at the earth s surfacearound low and high on many occasions isclosely related to the wind CIRCULATION at higherlevel.

10 Generally, over low pressure area the airwill converge and rise. Over high pressure areathe air will subside from above and diverge atthe surface ( ). Apart fromconvergence, some eddies, convectioncurrents, orographic uplift and uplift alongfronts cause the rising of air, which is essentialfor the formation of clouds and : Geostropic WindGeneral CIRCULATION of the atmosphereThe pattern of planetary winds largely dependson : (i) latitudinal variation of atmosphericheating; (ii) emergence of pressure belts; (iii)the migration of belts following apparent pathof the sun; (iv) the distribution of continentsand oceans; (v) the rotation of earth. The patternof the movement of the planetary winds iscalled the general CIRCULATION of theatmosphere.