8 PRESSURE BELTS AND THEIR SEASONAL VARIATIONS

Dr. Lubna Siddiqui

epgp books

     Keywords

 

Air pressure, pressure gradient, pressure belts, pressure distribution

 

Introduction

 

Learning Objectives

 

Understanding Air Pressure

 

Measurement of Air Pressure

 

Pressure Gradient

 

Variation in the Atmospheric Pressure:

 

Diurnal Variation:

 

Seasonal or Annual Variation:

 

Factors Affecting Air Pressure

 

Relationship between Air Temperature and Air Pressure

 

Relationship between Altitude and Air Pressure

 

Relationship between Moisture and Air Pressure

 

Distribution of Atmospheric Pressure

 

Vertical Distribution of Atmospheric Pressure

 

Horizontal Distribution of Atmospheric Pressure

 

Equatorial low pressure belt

 

The sub-tropical high pressure belt

 

The sub-polar low pressure belt

 

The polar high pressure belt

 

The Equatorial Low Pressure Belt

 

Sub-Polar Low Pressure Belts:

 

Polar High Pressure Belts:

 

Factors Controlling Pressure Belt System

 

Thermal Factor:

 

Dynamic Factor:

 

Changing Seasons and Pressure Belts

 

Relationship between Pressure Belts and Winds

    Summery and conclusions

 

Introduction

 

Atmosphere is an envelope of gasses circling the earth. It is mainly made up colourless and odorless gases of Nitrogen, Oxygen, Carbon-dioxide and many more. About these gases you have already studied in the lesson Structure, Composition and Importance of Atmosphere. This atmosphere is traced up to many hundreds of km above the earth surface. Technically, pressure can be defined as the force exerted by the atmosphere at a given place and time. Generally it is expressed in millibar (mb). The atmospheric pressure is determined by the mass of a column of air lying above a certain level/ surface.  The increase or decrease  in pressure  of the  air is determined by various factors. The pressure belts have developed in a sequential manner over the  globe but their existence is again determined by the seasons. In this module we will study about the atmospheric pressure belts and their seasonal variations.

 

Learning Objectives

 

After studying this module, you will be able to:

 

define air pressure;

 

define pressure gradient;

 

explain the diurnal and seasonal variations of the air pressure; discuss factors affecting air pressure;

 

establish the relationship of temperature, altitude and moisture with air pressure; explain vertical and horizontal distribution of air pressure belts;

 

give details about the pattern of planetary pressure belts and describe the pressure belt shifting with changing seasons.

 

Understanding Air Pressure

 

Atmospheric pressure is a force of dry air exerted on a particular place/surface. The column of dry air lying above that place/surface is responsible to pressurize as it has its own mass. Since the air above is exerting pressure at the lower level, it is also known as barometric pressure. It is known so because, it is first measured by barometer. The atmospheric pressure keeps on declining very rapidly with increasing height from the sea/earth surface. The rapid decline in pressure with increasing height is due to reduction in the height of the air column. In the lower atmosphere/ air near the sea/ earth surface is denser due to compression by air lying above. But with increasing height the intermolecular space increases, and thus, lesser pressure is observed. The most common unit to measure the atmospheric pressure is millibar (mb) as mentioned before.

 

Measurement of Air Pressure

 

The first atmospheric pressure measuring instrument was invented by E. Torricelli in 1643. In Torricelli’s barometer, mercury was used. If an 84 centimeters tube filled with mercury is taken and inverted into a mercury pot without allowing the air to enter, the mercury will fall from the top. The fall will be different at different altitude from sea level (Figure 1). At sea level, the level would be upto 76 centimeter (760 millimeter) from the pot’s mercury level. It is equivalent to 29.92 inches. One inch of mercury pressure is equivalent to about 33.8639mb at sea level. If the air pressure is calculated at the sea level at this rate, it yields to 1013.25mb. This recording of air pressure is said to be the average sea level atmospheric pressure. This standard air pressure or normal air pressure is recorded at 45° latitude at 15° C temperature. The air pressure varies from about 982mb to 1033mb over the earth surface. The pressure of air applied at sea level is equal to 1034gram (1.034kg) on an area of just one square centimeter. The highest sea level air pressure, so far, has been recorded at Irkutsk in Siberia on 14 January1893. It was found to be 1075.2mb at sea level. The lowest sea level air pressure was recorded on 24 September 1958 at Ida in the Mariana Island in eastern part of North Pacific Ocean. It was found to be 877mb in an eye of a very destructive typhoon.

Figure 1: Torricelli Mercury Barometer

 

Source: http://slideplayer.com/slide/9361080/28/images/8/Mercury+Barometer.jpg

 

The mercury barometer mentioned above is not very convenient to handle and carry. Therefore, for measuring air pressure, aneroid barometer is used. This barometer is easy to carry as it is smaller in size, light in weight and has less coverage space. It is fully secure in case of shock and vibration. It can be used in climbing mountain, on the ship, in aviation or while travelling. It is fixed in a metallic frame. The aneroid barometer is made up of special high grade elastic material within which partial void/ vacuum space is created. When the atmospheric pressure changes, this change is reflected in the elastic material by compression or swelling. When the height is lower, the air column is greater and hence, higher air pressure is recorded. In this case, the flexible/ elastic material is compressed and its result is reflected by the dial of the barometer. In this way, we take the reading. When we go to the higher elevation with the aneroid barometer, the air column is reduced. Smaller column exerts lesser pressure and due to this reason, the flexible/ elastic material gets swollen. Its effect is again recorded on the dial of the instrument by the needle. Apart from the barometer the Figure 2 shows the thermometer (temperature measurement) as well as hygrometer (relative humidity measurement).

 

Figure 2: Aneroid Barometer

 

Source:https://upload.wikimedia.org/wikipedia/commons/thumb/c/c4/Aneroid_barometer_J2.jpg/124 9px-Aneroid_barometer_J2.jpg

 

The air pressure is shown by isobar on the map. Isobar is drawn on the map by joining all the places which have same air pressure. Therefore, an isobar shows the same air pressure value wherever it goes. The air pressure is different in different seasons over the same geographical area. The isobar map of the Indian Subcontinent can be seen from the Figure 3. From this figure, it is quite evident that the pattern of isobars/ air pressure changes.

 

Figure 3: Isobars over Indian Subcontinent

 

Source:https://sites.google.com/site/didatticageo/_/rsrc/1355902121128/documentazione/quiz/fig 17.jpg

 

Pressure Gradient

 

The atmospheric pressure gradient refers to the change in the pressure per unit distance between two places along a line on an isobar map of any area. The maximum pressure gradient is observed along a line perpendicular to the isobar. It is more when the isobars are closely spaced, but when it is widely spaced, the pressure gradient is lesser (Figure 4).

 

Figure 4: Pressure Gradient

Pressure gradient can be computed by getting the difference between the isobaric values of two places as well as the distance between the same. Thus, the isobaric difference divided by the distance gives the pressure gradient. The larger numerator (isobaric value) divided by smaller denominator (distance) shows greater/strong gradient. When the case is reverse, the pressure gradient is gentle/weak. Suppose you get the air pressure at place one to be 1110mb and at another place it is 1000mb. The difference between them is 10mb. If the distance between the same two places is 100km, the air pressure gradient would be 10mb per 100km. In another words, it is 1mb per 10km. Later on we will study that the greater pressure gradient generates greater wind speed while the lesser air pressure gradient is responsible for producing lesser wind speed.

 

Variation in the Atmospheric Pressure

 

The atmospheric pressure varies fromplace to place andfrom time to time. On the basis of time, diurnal and seasonal variations can be observed.

 

Diurnal Variation: The periodic pressure change is observed on daily basis. Two highs and two lows of air pressure is seen each day. The high point occurs at 10am and 10pm while the low pressure occurs at 4pm and 4am (Figure 5). This is also called as semidiurnal observation in pressure variation. It is because of difference of 12 hours between them. The mean daily changes in the air pressure can be found out by calculating the average hourly observed pressure for a long time. The temporary effect of atmospheric disturbance is free from the mean value of the daily pressure. Insolation, heating, cooling and radiation are the factors for the diurnal changes in the air pressure.

 

Figure 5: Diurnal Atmospheric Pressure at Mauna Loa Observatory

 

Source:https://wattsupwiththat.files.wordpress.com/2016/02/average-daily-cycle-pressure-mauna-loa-observatory.jpg?w=720

 

Seasonal or Annual Variation: The amount of insolation received in a particular region varies from one season to another season. Because of this variation, seasonal or annual variation in the atmospheric pressure is found. However, in high latitude and polar region, variation in the pressure is not that great in comparison to tropical region. In the tropical region, the annual variation in the atmospheric pressure is larger than the other part of the world. Generally, high pressure is observed during the cold season and low pressure is observed in warm season.

 

Factors Affecting Air Pressure

 

There are three important factors affecting the air pressure. They are temperature of the air, altitude and moisture. Briefly known as TAM, temperature (T), altitude (A) and Moisture (M). Let us discuss their relationships with air pressure in brief.

 

Relationship between Air Temperature and Air Pressure

 

The relationship between air pressure and air temperature is inversely correlated. Inverse relation shows that the increase in one variable will cause the other to fall and vice versa. When temperature of air is increased, it causes the volume of air to expand. Expansion of air is caused by increase in intermolecular space. Air has its own mass. Once the mass remains the same but the volume is increased, it lead to decrease in density. Lower dense air has tendency to rise. Rising air upward tend to create a partial/ temporary vacuum. Therefore, the area where the temperature is higher, the air pressure is observed to be lower (Figure 6).

 

Figure 6: Relationship between Air Temperature and Air Pressure

 

Source: http://poweranddata.info/wp-content/uploads/Air-High-Pressure-Low-Pressure-POWERandDATA.info_.png

   When the temperature of a certain column of air decreases, the pressure is increased. Lowering of temperature of the air is witnessed as the contraction of that air. Since the mass of the air remains the same, the density of the air is increased. Increased density causes to attract the air from the surrounding columns. It leads to accumulate greater mass of air where the low temperature is witnessed. Accumulation of more mass of air exerts greater pressure. Therefore, higher air pressure is caused due to fall in the air temperature.

 

Relationship between Altitude and Air Pressure

 

Both altitude and air pressure are inversely correlated. Increase in height causes the air pressure to fall whereas the fall in latitude is reflected in the increase in air pressure. As mentioned in the beginning of this module, air pressure is the force exerted by the air parcel lying above a certain level/ surface. When there is increase in altitude, the air parcel lying above an elevated surface has smaller column of air. Smaller column has smaller mass of air. Therefore, at the elevated surface the pressure is less. It is the other way round when the altitude is lessat lower elevation, a bigger column of air lays. Bigger column has greater mass of air. Greater mass of air exerts greater pressure. Therefore, at lower surface/ at sea level, the air pressure is more. It is also a fact that the air (gas) is easily compressible. Greater column of air compresses the air at the lower level. Hence, the density of the air is greater at lower elevation. This causes to increase the air pressure at lower level/ surface (Figure 7A and 7B).

 

Figure 7A: Relationship between Altitude and Air Pressure

 

Figure 7B: Relationship between Altitude and Air Pressure

 

    Relationship between Moisture and Air Pressure

 

Moisture in the atmosphere is the water in the form of vapour. Addition of water vapour in the atmosphere causes the air pressure to fall. You must have observed that the water vapour is going upward while cooking something in the kitchen. It is going upward because, it has less density in comparison to the surrounding air. Smaller volume of water can generate large volume of water vapour. It is happening due to expanding intermolecular space of water when it turns into vapour. This is reflected in lessening of density. Lesser density of vapour mixed in the air lead to lesser mass of the air column. That is why, greater amount of vapour is the cause of lowering in air pressure.

 

The vapour holding capacity of the air is directly and positively correlated with the temperature of the air. When there is high temperature, the water vapour can be held by air is large in comparison to the lower temperature. You must have observed that the damped towel you spread after taking bath is quickly dried up in the summer in comparison to winter. It is not the same case in rainy season. When sufficient amount of water vapour is already available in the air, its further water/ vapour retaining capacity is reduced. Hot air, in general, is dry and air has greater capacity to retain moisture in itself. It is also called as ‘hungry air’. Moisture in the air is temperature dependent. Already available amount/ percent of vapour in the air is also a determining factor. It is said that ‘higher the air temperature, higher is the capacity of the air to hold vapour and vice versa’. This statement is completely in congruence with the Figure 8.

 

Figure 8: Relationship between Temperature and Relative Humidity

 

Source: https://upload.wikimedia.org/wikipedia/commons/4/41/Relative_Humidity.png

 

There are several methods of measuring humidity about which you will be reading later in the modules related to it. One of them is relative humidity. It is expressed in percent. The percentage is calculated based on the amount of water vapour available at a certain temperature in a certain volume of air and the maximum amount of water vapour that could be held at the same temperature in the same volume of air. When the vapour is available to its fullest capacity, the relative humidity is 100 percent. This level of vapour is known as dew point. The relative humidity can be calculated on the basis of the following formula:

 

   Distribution of Atmospheric Pressure

 

Atmospheric pressure varies from place to place and season to season. Its distribution over the globe is not uniform. Variation is seen in both perspectives – vertical and horizontal.

 

Vertical Distribution of Atmospheric Pressure

 

As mentioned earlier, the air is compressible. Therefore, its density is greater in lower layers as compared to the upper layers of the atmosphere. Thus, the atmospheric pressure decreases with increasing height. The vertical distribution of atmosphere is influenced by temperature, water vapors and altitude. Since all these factors vary significant, the rate of decrease in the air pressure with increasing altitude also varies.In the higher altitude, atmosphere becomes thin and intermolecular space is more. However, on an average, vertically pressure decreases by 34mb/300 meters of the altitude in the atmosphere. This rate is confined only in the lower level of the troposphere. With increase in height, the decreasing rate in atmospheric pressure is reduced very drastically. It means, slight increase in elevation from the sea level, greater change in atmospheric pressure is seen. But at higher altitude, the greater increase in elevation shows lower change in pressure. These statements are very vivid from the Table 1.

 

Table 1: Normal decrease in air pressure with the increase in height

 

   Horizontal Distribution of Atmospheric Pressure

 

The distribution of atmospheric pressure on the earth is explained across the latitude. This is considered as horizontal distribution of atmospheric pressure. The distribution of pressure belts is very distinct and classifiable.Based on the characteristics of different belts, they are grouped into four: They are:

 

Equatorial low pressure belt

 

Sub-tropical high pressure belt

 

Sub-polar low pressure belt

 

Polar high pressure belt

 

Last three have two cases of each – northern and southern hemisphere. In fact the first one has also two cases but both north and south cases of equatorial low forms a single belt. That is why, it is call as one.

 

Equatorial Low Pressure Belt: This belt extends from 10° north to 10° south latitude. It is a thermally induced belt because here the temperature remains very high throughout the year due to the vertical sun’s rays. Consequently the air is warmed up. Warm air has lower density. Being lighter, it is uplifted and calm condition prevails. Since, there is almost an absence of horizontal movement of wind, the calm condition is termed as ‘doldrum’. The winds converging from both hemisphere’s high pressure belts results into a zone of convergence. It is known as inter tropical convergence zone (ITCZ). Strong convectional rainfall occurs at the late afternoon and this results adiabatic cooling at this time of highest diurnal temperature. The different air pressure belts can be observed from the Figure 9.

Figure 9: ITCZ and Equatorial Low Pressure Belt

 

The risen air from the equatorial low reaches to the upper troposphere and dragged towards poles. By reaching in the tropics, air descends from 200to 350latitudesin both the hemispheres. It is caused by cooling of the air. Cool air is heavier and hence, it subsides. Subsidence of air in the tropics causes to additional air accumulation. Therefore, high pressure belt is created here.

 

Figure 10: Air Pressure Belts

 

   Sub-Tropical High Pressure Belts: These belts are extended from 20° to 35° latitudes in both hemisphere. These belts are situated over tropic of Cancer and tropic of Capricorn. Since, there is subsidence of air from the upper troposphere in this zone, a high pressure belt is developed here. From this high pressure belt, the wind moves equatorward to fill the temporarily created vacuum/ gap produced by rising air at the low pressure zone. Hence, an atmospheric cell is created by rising air at equator – moving up – getting drifted towards pole – getting subsided due to cooling

 

– becoming heavier – climbing down at sub-tropical high and finally moving towards equator to fill the gap created by rising air. This cell (circular motion) is known as Hedley cell (Figure 9 and 10). Due to subsiding dry air, most of the deserts are found in these pressure belts but in the western margin of the continents. A calm and feeble wind is created in this region which is known as ‘horse latitude’. In early day sailing vessel with cargo of horses was very difficult under such calm conditions. The horses were thrown into the sea to reduce the load of the ship.

 

Sub-Polar Low Pressure Belts: These belts are found between 50° to 70° latitudes in both the hemispheres. These belts are induced due to ascend of air as a result of convergence of wind coming from sub-tropical high pressure belts (westerlies) and polar high pressure belts (easterlies). The air moving from sub-tropical high to sub-polar low – rises above – gets cooled – diverted towards equatorward and descends at sub-tropical high – makes a cell (circulation motion) known as Ferrel’s cell (Figure 10). During winter season, because of high contrast of temperature between land and sea, this belt is broken into two low pressure centers in northern hemisphere – one in the vicinity of the Aleutian Island, and other between Iceland and Green Land. During the summer season, the variation is less. Therefore, more regular low pressure belt develops.

 

Polar High Pressure Belts: High pressure prevails over both the polar regions due to excessive cold condition. The cold climatic condition itself is caused by slanting sun’s ray at the poles. In these pressure zones, thermal factor is more important than dynamic factor. The air coming from polar region – rises up at the sub-polar low – finally pushed towards pole and descends at the polar high. This also makes a cell known as Polar cell (Figure 10).

   

Factors Controlling Pressure Belt System

 

There are two main factors controlling the pressure belt system over the globe. One is thermal factor and another is dynamic.

 

Thermal Factor: As mentioned before, equatorial region receive intense solar energy throughout the year. Polar regions are extremely cold throughout the year due to less effective and inclined solar rays. More solar energy in the equatorial region causes the air to expand and thus low air pressure is created. On the other hand at poles, due to extreme cold conditions the air is very cold and hence, high air pressure is observed. Therefore, the creation of equatorial low and polar high are caused by thermal factor. That is why, they are called as thermally induced pressure belts.

 

Dynamic Factor: The equatorial low as well as polar highs are explained by the thermal factors. But sub-tropical highs and sub-polar lows are not coming into the line of thermal explanation. Their description can be made only through dynamic angle. The warm air uplifted from the equatorial low is pushed towards pole and due to cooling it subsides over tropics. It is considered as a dynamic factor of the development of subtropical highs. In the same way subsided air at the tropics causes the high pressure. Due to this wind blows from the subtropical highs to equatorial low as well as towards pole. Since pole has thermally induced high pressure, wind blows from there as well. Both winds meet in the sub polar low around 50 to 70 degree latitudes in both the hemispheres. Therefore, sub polar lows also have dynamic origin.

 

Changing Seasons and Pressure Belts

 

The earth’s axis is inclined by 66030’from the horizontal plane. On this inclined axis, it is revolving around the sun. Due to this reason, the orbit around the sun is elliptical in shape. We know it very well that the sun’s rays are vertical on equator on 21 March and 22 September. This situation is known as equinox. Equinox means the day and night are equal throughout the globe. The wind coming from both the hemispheres converges at equator. It is inter tropical convergence zone (ITCZ). After 21 March, the northern hemisphere starts tilting towards the sun. By every passing day, northward departure of the sun becomes more and more apparent.

 

Vertical sun rays in the northern hemisphere is marked by increasing energy. This energy heats the areas/ air. Therefore, ITCZ migrates towards north along the thermal equator. Since the ITCZ is temperature induced low pressure (already discussed before) the migration of ITCZ is marked by the shifting of all pressure belts northward. Though the maximum northward departure of sun is seen by 22 June, the effective maximum average energy received in the northern hemisphere is observed in July. The northward maximum departure of ITCZ is seen upto 250 from equator (Figure 11).

 

Land is getting heated much quickly as well as greater temperature is recorded there, but the same is not with water body/ oceans. Greater effective energy is accumulated over the land, ITCZ is also have the tendency to depart more over the land. Look at the Figure 11. Over the huge landmass of Africa and Asia, ITCZ has departed the most. It is not the same with the case of Americas. In South America, the landmass is smaller to the north of equator. Greater water body is seen. The thermal equator is not going northward. Therefore, ITCZ has also migrated at the minimum. Look at the Figure 11.

 

Figure 11: Departures of Inter Tropical Convergence Zone (ITCZ)

 

Source: https://bam.files.bbci.co.uk/bam/live/content/zg9h34j/large

 

Study the given Figure 12. It is showing average air pressure and wind pattern for the month of July. Watch the position of high pressure and low pressure shown on this map. Also compare with the previous illustration showing the idealized pressure belts in Figure 10. You will find that not only the ITCZ has shifted northward, but entire pressure belts and wind pattern have shifted simultaneously.

 

Figure 12: Average Climatic Conditions in July over the Globe

 

Source: http://www.goes-r.gov/users/comet/tropical/textbook_2nd_edition/media/graphics/sthp_winds_july.jpg

 

After 22 June, the retreat is seen and sun again shines vertically over the equator by 22 September. Further onward the southern hemisphere is inclined towards the sun. The same case is repeated in the case of southern hemisphere what it has been discussed above. By 21 December, sun shines vertically over the tropics of Capricorn. After this it starts northward migration and reached over the equator again by 21 March. In brief, ITCZ shifts towards south as the sun energy is effective. As was the case in northern hemisphere, it is turning southward maximum on the bigger landmass. Over the sea, its turn is less (Figure 13).

 

You can study the map given in the Figure 13. This figure shows clearly about the shifting of the ICTZ as well as pressure belts and wind pattern. You may even compare the map of January (Figure 13) with map of July (Figure 12). The apparent changes in the positions of pressure belts can very well be observed

 

Figure 13: Average Climatic Conditions in January over the Globe

 

Source: http://www.goes-r.gov/users/comet/tropical/textbook_2nd_edition/media/graphics/sthp_winds_jan.jpg

 

Relationship between Pressure Belts and Winds

 

Pressure belts and winds are directly related. We have dealt the pressure gradient previously in this module. In that context, it was mentioned that the pressure gradient is steep when the isobars are closely spaced and less when they are placed widely. The pressure gradient reflects the intensity of the winds. You will be studying about the winds and related phenomena in another module in detail. But here, it would not be wise to deal in details. You have already studied the above Figures 12 and 13. In brief, the high air pressure belts exhibits additional amount of available air in comparison to the low air pressure belts. Therefore, the air used to blow from the belt of addition to the belt of shortage. In another words, the winds are blowing from high air pressure to the low air pressure belts.

 

The winds are blowing over the rotating earth. The paths of the winds are not straight but are in a curvilinear. It happens due to Coriolis effects. The Coriolis effects are caused due to rotation of the earth. The blowing of the winds in the northern hemisphere is counterclockwise while in the southern hemisphere it is clockwise. The details about it will be given the respective module.

 

Summary and Conclusions

 

Air has its own mass. Any object with mass exerts pressure over the resting surface. The resting surface could be anywhere. It may be at the bottom of that mass or somewhere in itself. Greater mass have greater pressure whereas less mass will have less pressure. Therefore, the pressure is highest at the bottom intermediary in between and the lowest at the top. Based on this principle, the maximum air pressure is observed at the lowest level of the earth surface. Baring a few spots which is below the sea level, in general, it is believed that the sea level air pressure is the highest. The density of the mass of the air is dependent upon the temperature and water vapour as well apart from the altitude. Both temperature and water vapour have inverse correlation with the air pressure. Therefore, increase in temperature and vapour cause the air pressure to fall because both make the air lighter and less dense. But when the temperature and vapour is reduced, it causes to increase the air pressure as it is reflected in increasing the density of the air. Over the earth, a very well developed air pressure system is observed. An idealized situation has been discussed in the content above. The earth is a rotating and revolving body. Due to these motions, the planetary air pressure system are migratory.

 

 

you can view video on PRESSURE BELTS AND THEIR SEASONAL VARIATIONS

 

References

  • Barry, R.G. and R.J. Chorley (2016) Atmosphere, Weather and Climate, Routledge: New York.
  • Chandrasekar, A. (2010): Basics of Atmospheric Science; PHI Learning Private Limited.
  • Critchfield, H.J. (2011) General Climatology, Phi Publication.
  • Fredrick, K.L and Edward J.T: Atmosphere; Prentice-hall of India PVT, New Delhi.
  • Lal, D.S: Climatology, C.S Jian for Chaitanya Publishing.
  • Miller, A. and R.A. Anthes (1980) Meteorology, Columbus Publication: Ohio.
  • Siddhartha, K. (2014) Atmosphere, Weather and Climate; A text book on Climate; Kisalaya Publication Pvt. Ltd.
  • Strahler, A.N. (1965) Introduction to Physical Geography, Willey: New York.

Web links:

  • http://sciencing.com/relationship-between-moisture-temperature-4007.html
  • http://vsgc.odu.edu/src/Conf09/UnderGrad%20Papers/Ojo%20-%20Paper.pdf
  • http://www.agriinfo.in/default.aspx?page=topic&superid=1&topicid=403
  • http://www.altitude.org/air_pressure.php
  • http://www.cssforum.com.pk/css-optional-subjects/group-vii/geography/14066-geography-one-factors-affecting-horizontal-vertical-distribution-temperature.html
  • http://www.drishtiias.com/upsc-exam-gs-resources-Pressure-Belts-of-the-World
  • https://water.usgs.gov/edu/watercycletranspiration.html
  • https://www.britannica.com/science/atmospheric-pressure
  • https://www.livescience.com/39315-atmospheric-pressure.html
  • https://www.nationalgeographic.org/encyclopedia/altitude/
  • https://www.nationalgeographic.org/encyclopedia/atmospheric-pressure/