3 ATMOSPHERIC HEATING AND COOLING

Dr. Ramashray Prasad

Introduction

Learning Objectives

Source of Heat

Mechanisms of Heating and Cooling

Partial absorption of solar radiation by atmosphere

Conduction

Terrestrial Radiation

Advection

Latent Heat

Latent Heat of Condensation

Latent Heat of Vapourization

Latent Heat of Sublimation

Latent Heat of Freezing/ Melting

Expansion and Compression of the Air

Lapse Rate

Environmental Lapse Rate

Adiabatic Lapse Rate

Dry Adiabatic Lapse Rate

Wet Adiabatic Lapse Rate

Factors Affecting Heating and Cooling of Atmosphere

Latitude

Altitude and Nature of Earth’s Surface

Differential Heating and Cooling of Land and Water

Nature of Ocean Currents vs Heating and Cooling

Transparency of the Sky

Slope Aspects

Summery and Conclusions

Multiple Choice Questions

Answers to the MCQs

References

Web Links

Introduction

You must have observed while taking tea that it is very hot when poured in the cup to sip. Can you dare to sip immediately? Probably not. Hopefully, you would wait for a few minutes to get the heat of the tea lowered. What happens when you leave the tea for a few minutes? The place where you are sitting to sip the tea is cooler than the tea. The tea is not so hot now, as it was when poured in the cup. Where the heat escaped? As a matter of fact, heat released from the cup spreads to your surroundings. Take another example, why do you prefer to drink cold waterfrom your refrigerator during hot weather? Suppose, you are putting the refrigerated cold water on your table for sometime, say an hour or so. What will happen to the cold water after an hour? Will it remain at the same low temperature or its temperature will rise? If the temperature is rising, what is the process to warming? Its condition will be equal to room temperature. In both the above examples, the heat/ temperature of tea and cold water is altered. In the first case, the heat is lowered while in the second case, it is increased. All depends on the heating/ cooling conditions of the objects and its surroundings. The radiant energy coming from the sun is the main cause of heat on our earth. The atmosphere is primarily heated by the long wave radiation emitted by the earth surface. Heating and cooling of the atmosphere are affected by several regional and global factors. In this module, an attempt is being made to understand the heating and cooling process of atmosphere within which we are living in.

Learning Objectives

After studying this module, you will be able to:

name the source of heat for the earth and its atmosphere,
explain the mechanism of heating and cooling of the atmosphere, explain the terms radiation, conduction, convection, advection etc., discuss the mechanism of latent heats and their release,
elaborate upon the distribution of energy surplus and deficit, describe different types of lapse rates, and;
explain various factors affecting the heating and cooling of atmosphere.

Source of Heat

The source of heat on the earth and for earth’s atmosphere is almost completely governed by the energy received from the sun. Little heat reaches to the surface and atmosphere by earth’s cooling, hot springs, and volcanic eruption. Volcanic eruption is confined to certain belts/ pockets on the globe. Primarily these sources are very meager keeping the size of whole of the earth as well as huge mass ofsurroundingatmosphere. It is the solar energy which is responsible for the distribution of heat/ energy all over the globe. The distribution of this energy is again affected by numerous factors. Those factors will be dealt later in this module.

Mechanisms of Heating and Cooling

Heating and cooling of the atmosphere is performed by following processes:

Partial absorption of solar radiation by atmosphere Conduction

Terrestrial radiation Convection

Advection

Latent heat of condensation

Expansion and compression of the air

Partial absorption of solar radiation by atmosphere

The solar radiations are coming to the earth surface directly from the sun. It is in the form of short-wave radiation. They are so energized that atmospheric gases are unable to trap them. But the presence of some dust particles and water vapour in the lower level of troposphere are capable of holding some energy directly coming from the sun.

About 20% of the total incoming solar energy is trapped by the dust particles and vapour in the atmosphere. Approximately, half of the total absorbed energy is done so within two km of the height from the earth’s surface. It is this layer where the amount of dust particles, fire shoots/ smokes and water vapour are available in large quantities. However, the quantity of these particles decreases sharply with increasing height.

You must have also observed that when the patches of clouds are there between you and the sun in day time, you feel cool even when the temperature is very high. It is so, because the concentrated water vapour (clouds) acts as an intervening obstruction by holding some part of solar energy between sun and the earth. In this process, the air is heated where it contains vapour, dust particles and fire shoots. But the heating of air over certain height through these obstructionsdoes not increase the surface temperature in that particular region. Since, these areas are receiving relatively less energy on the surface, the surface is cool as otherwise it would have been. Because of more cloud cover in equatorial regions, relatively less energy is received at the earth surface in these regions. On the other hand, the sky is clear over subtropical belt, hence the energy received in the surface areas of these belts is quite high.

Conduction

The literal meaning of the term conduction is passing on something by a medium without any perceptible movement by itself. It is the transfer of something from one part to the other without any physical movement.

You must have observed, while vegetable is cooked in a pan and a metallic spatula is used to turn the vegetable, it gets heated up while its handle is outside the pan. It happens so, because the heat received by the pan from the burner and the spatula is touching the pan. The received heat of the pan is transmitted to spatula which is warmed up and thus it burns our hand if left for a little more time in the pan.

Air is a very poor medium of heat conduction. It is a very slow process of transferring heat in a mass of air. By this method, air is heated, but its importance is not that great. Because, the air is in the gaseous state and its particles (molecules or atom) are not very solidly compacted. A very thin layer which is very close to the earth surface is heated by conduction method (Figure 1). Once the air atom is heated, it becomes lighter and less dense and ultimately moves upward. Therefore, conduction has a very insignificant role in heating the atmosphere. It is almost negligible in comparison to other methods. Practically speaking, majority of the meteorologists and climatologists prefer to neglect the conduction method of atmospheric heating.

Figure 1: Atmospheric Heating Methods

Source: http://www.srh.noaa.gov/jetstream/atmos/heat.html

Terrestrial Radiation

The terrestrial radiation is the most important method of atmospheric heating. Out of the total solar electromagnetic radiations reaching at the top layer of the atmosphere, approximately 49% reaches to the earth’s surface. Out of this, 5% is reflected back to the space without heating the atmosphere. About 20% is absorbed by the atmospheric substances including water vapour. Therefore, 45% (49-5=45%) plus 20% (total 65%) is available for heating the atmosphere (Figure 2).

Figure 2: Solar Energy at the Earth and in its Atmosphere

Source: https://i.pinimg.com/736x/18/49/48/184948e5fedb1895388ec7b1026b8fd0–greenhouse-effect-greenhouse-gases.jpg

All the above mentioned energy is reaching the earth surface in the form of short wave electromagnetic radiations from the sun. The heated earth radiates back the same but in the form of long wave electromagnetic or infrared radiations. The atmosphere gains the heat radiated through long waves from the earth surface (Figure 2 and 3). Most of the short wave radiations are not being trapped by the atmospheric gases, as they are not capable of.

Terrestrial radiations are a continuous affair all 24 hours throughout the year. During day time when sun is in the sky, the solar short wave incoming energy is greater than the energy lost from the earth surface (land and water). Since there in no incoming solar energy during nights, the day time received energy is radiated back to the atmosphere. There may be addition or subtraction of energy on day to day basis depending upon the seasons, but on annual basis, the incoming and outgoing energy is balanced and a static temperature of the earth is maintained.

Greenhouse gases and water vapour are transparent for incoming radiation but they trap the outgoing long wave radiations. You must have observed that the cloudy day keeps us cool while cloudy night makes us warm. It does not make any difference, whether the taken examples are of summer or winter day and night. The reason is that the cloud reduces the incoming radiations and traps the outgoing radiations. It behaves like greenhouse gas. It has very important role to make the earth livable otherwise in the absence of greenhouse effect, the earth’s average temperature would have gone down to minus 170 C.

Figure 3: Heating of Earth and its Atmosphere

Source: http://igbiologyy.blogspot.in/2014/03/120-greenhouse-gases-and-global-warming.html

It is also very true that there is a gradual decrease in temperature with increasing altitude within the troposphere. Greater temperature is recorded at the ground surface as the earth is heated first and then the heating of atmosphere starts. The addition of heat is distributed and finally results into a cooling. The process of cooling at one level is the cause of heating at another level. It is nothing but the transfer of energy from one to another. In the same way, heating is possible because the energy is again distributed or gained from where it has more. All the time, the energy is transferred from the high level of concentration to the lower level.

Convection

The earth’s surface is heated with incoming solar energy. The air in contact with the surface is in gaseous form. Earth’s surface heating results in heating of the air in its contact. But the air becomes less dense by heating. It further results into rising of the warmed/ expanded air molecules upward. Upward moving air molecules in large quantity create a convection. The removed air by expansion at the lower/ ground level results into creation of low pressure. Therefore, nearby air from relatively cool air starts moving to fill the gap created by upward lifting of air. The heat is also transferred upward with vertically moving air. This convection may occur at a local level as well as at much larger regional level. The occurrence of Hadley cell, Ferrel’s cell and Polar cell are examples of atmospheric convection. Therefore, convection transfers the heat energy received from the sun to the surface and from the surface to the atmosphere. The Figure 4 very clearly explains this.

Figure 4: Convectional Heat Transfer in the Atmosphere

Advection

The meaning of advection is transfer of something from one place to another especially in horizontal direction. Atmosphere is a huge body of air and it has differences in terms of its pressure depending on several affecting factors. Due to varying pressure at local, regional and global level, atmospheric gases are continuously on move. A local level horizontal movement of air (advection) can be seen from Figure 5. The monsoonal air current movement is the example of regional advection while planetary permanent wind system signifies the global advection. All of them are transferring the heat from one area to another. Transfer of energy is not only done by atmospheric advection, but also by hydrospheric advection. There is movement of water also from the low latitude to high latitude in the form of ocean currents. The ocean currents also redistribute the received energy from high concentration to low concentration zones. It is also a fact that the ocean currents are also affected by the advective movements in the atmosphere.

Figure 5: Process of Advection and Horizontal Air Movement

Once the air is on move horizontally, physical transfer of air, associated heat and other related properties of the mass of air are transferred. In this process, the heating of the atmosphere is the result. It is caused by the differences in air pressure and temperature. In fact, these two characteristics of air are primary factors in the movement of the mass of the air.

If the net radiation at global level is calculated for the atmosphere, it is established facts that there is positive budget between incoming and outgoing energy between 400 north and 35 0 south latitudes. This fact is very clear from the Figure 6. It is also apparent that there is negative budget beyond 400 north and 350 south latitudes. Overall the total budget of the earth is almost neutral. The positive received radiation is diverted to the higher latitude areas by advection. Therefore, higher latitude areas are releasing more energy to space than they receive as it is transported from low latitude areas. Contrary to this, the low latitude areas are releasing less energy than they directly receive from the sun as remaining is transported to the high latitude areas. The same information is very clearly presented in Figure 6.

Figure 6: Surplus and Deficit Zones of Radiations

Source: http://www.physicalgeography.net/fundamentals/images/rad_balance_ERBE_1987.jpg

Latent Heat

Heat absorbed or released due to change of the state of any matter is known as latent heat. During this process, there is no change of temperature of that matter. In another words, it is the heat that is required to change the matter to a higher state of matter. You may be well aware, when water changes from one state to another, for example, water vapour to liquid water and liquid water to solid water (ice), it absorbs or releases heat. The energy involved in this process is known as latent heat, popularly meant for ‘hidden’ heat. Water vapour transports the energy from one region to another.

When water is heated, vapour is generated and in this process heat is absorbed. The same vapour when condenses, releases the absorbed heat during vapourization. Generally, it is measured in calories. One calorie is the aggregate amount heat needed to raise the temperature of one gram of water by one degree Celsius.

Suppose one gram of ice is melted, it releases 80 calories of heat and turns into water. If the same one gram of water is evaporated, it requires 540 calories. Therefore, the said calories are known as latent heat. In the same way, when the condensation is taking place, conversion of vapour into one gram water releases 540 calories and it reaches to the atmosphere and heats it. When one gram water turns into ice, 80 calories is released what it was consumed while melting the ice. Sublimation is the process in which ice turns into vapour (it releases 540+80=620 calories) or vapour turns into ice, (it releases 540+80=620 calories). In both the cases, latent heat is either released or utilized Figure 7.

Figure 7: Latent Heat: Absorption and Release

Source: https://qph.ec.quoracdn.net/main-qimg-36a6e3973562f9f51f02b4c061f940e2-c

From the above discussion, it is quite apparent that the latent heat plays a significant role in heating and cooling of the atmosphere. When the heat is utilized in turning the state of matter from lower level of heat to higher level, cooling is evident. But when it is in descending order, i.e., from higher level of heat to lower level, it releases the latent heat and heating of the surrounding atmosphere is the result.

You may even feel cool when the sweat is removed from your skin either by moving fan or the use of air conditioner. Removal of sweat by transpiration/ evaporation is associated with latent heat, and hence we feel comfortable. When condensation takes place, it leads to release of the same latent heat and causes the air to warm up at the level (generally upper level of atmosphere) of condensation. The same thing happens with freezing and sublimation.

Latent Heat of Condensation: It is the amount of heat energy released to the atmosphere when condensation takes place. As mentioned above, when one gram vapour is changed to water 540 calories is released, it is called latent heat of condensation, because it is reaching to the atmosphere due to the process of condensation.

Latent Heat of Vapourization: It is the amount of heat energy needed to evaporate water and it is taken from the surroundings. When one gram water turns into vapour, 540 calories is utilized. This amount of heat is called latent heat of vapourization.

Latent Heat of Sublimation: It is the amount of heat required to change solid (ice) directly into vapour without turning into liquid (water) or vice versa. This amount is 620 calories for changing one gram of ice into vapour or one gram of vapour to ice.

Latent Heat of Freezing/ Melting: It is the amount of heat required to change one gram of water into ice or one gram of ice into water. This amount is 80 calories per gram of water/ ice.

Expansion and Compression of the Air

We have already discussed that the air pressure keeps on declining progressively with increase in height. Consequently, the mass of air is lesser with increase in height. The mass is greater downward. It is due to this reason, any parcel of air, if it rises, going upward is expanded. Because the rising air is entering in the zone of less dense air, it results in expansion. Therefore, the expansion is a natural event in this case. Contrary to this process, when any parcel of air descends, it enters in the zone of more dense air. This results into the compression of the descending air. Rising air expands and the intermolecular space is expanded and it causes the cooling in the air as well. It is also known as adiabatic cooling. That means, the cooling is caused by simply expansion of the volume of air. There is no exchange of heat with the rising air and its surroundings.

Lapse Rate

Lapse rate is observed change in temperature when moving upward in the troposphere. It is generally counted as drop in temperature with per km ascent in the atmosphere. We know that the mountains are colder than plains if the latitude remains the same. It is due to the lesser effectiveness of the long wave radiation with height.

Environmental Lapse Rate: It is a simple drop in temperature. We are simply climbing the height and we feel the drop in temperature. It is known as environmental lapse rate. The average environmental lapse rate is 6.50C per km. This rate is applicable only in the lower atmosphere (troposphere). It happens due to two reasons, fall in atmospheric pressure and decrease in greenhouse gases (carbon dioxide and water vapour)

Adiabatic Lapse Rate: In adiabatic lapse rate, the parcel of the air is lifted and the lifting air is forced to expand. Expansion leads to drop in temperature. In this case, air itself is rising whereas in environmental lapse rate the air was static but someone is going up and feels the declining temperature. Adiabatic lapse rate is categorized into two – dry and wet.

Dry Adiabatic Lapse Rate: When the airis dry or having very less moisture in it and it rises, the temperature will fall more sharply. Since there is less possibility of condensation, the decline in temperature is at higher side and it is about 100C per km (Figure 9). In this case the air is almost stable and in general represents high pressure conditions.

Figure 9: Dry and Wet Adiabatic Lapse Rate

Source: https://i0.wp.com/www.nandyradiotelephony.com/wp-content/uploads/2015/09/CloudFormation.gif

Wet Adiabatic Lapse Rate: When the air is moist, it is unstable. The capacity to hold more moisture in the air is less. There is greater possibility of condensation with rise of air parcel. Once it is fully saturated and the condensation takes place, the latent heat of condensation is released. The released heat is utilized to warm the surrounding air. Therefore, the decrease in temperature is lowered. On an average, the drop in temperature is about 50C per km (Figure 9).

Factors Affecting Heating and Cooling of Atmosphere

The earth is a sphere and the atmosphere is encircling around it. The distribution of energy on the earth surface and in atmosphere is varying to a great degree particularly with respect to latitude. The distribution of heat is affected by several factors important among them are:

Latitude

Altitude and nature of earth’s surface

Differential heating and cooling of land and water Nature of ocean currents

Transparency of the sky Slope aspects

Latitude

The light energy of the sun reaches to the earth’s surface to a maximum limit to 1800 of angle. You know it very well that the earth is a sphere. The angle of incidence keeps on reducing poleward from the equator. Reduction in angle of incidence reduces the energy received per unit of the area. It happens because a beam with vertical or near to vertical will spread to a smaller area while greatly inclined rays will spread over a large area. Therefore, energy received by vertical or near to vertical rays will be greater in comparison to the greatly inclined rays (Figure 10). Sun rays are vertical in equatorial region while in polar region, it is greatly inclined. That is why, the low latitude areas are warmer and high latitude areas are colder. On the other hand, high latitude areas are much colder because of less effective heating.

Figure 10: Decreasing Effectiveness of Solar Energy with Latitudes

Source: https://upload.wikimedia.org/wikipedia/commons/thumb/e/e6/Oblique_rays_04_Pengo.svg/800px-Oblique_rays_04_Pengo.svg.png

Altitude and Nature of Earth’s Surface

You know it very well that, in general, the mountains are cool and the plains are warm/ hot if the latitude is the same. Plain possess thick layer of atmosphere and the mountain has a small thickness of it. Air is greatly compressed on the plains in comparisons to the mountains. The atmosphere is normally warmed by the longwave terrestrial radiations. Hence, low altitude area has more temperature than the high altitude areas.

The nature of the rock also affects the atmospheric heating and cooling. The areas possessing bare rocks have more intense heating by the sun’s energy. That type of area also radiates back more and more received energy and the result is quick heating of the air laying there. On the other hand, the area possessing more and more vegetative cover, the heating is moderate as some of the energy is released from the earth surface in the form of latent heat with evapotranspiration. Vapourazitation form any area keeps it cool. That is why, the forested or grasslands are cooler than the hot deserts. Its effects are also seen on the air/ atmosphere of the area.

Differential Heating and Cooling of Land and Water

The earth surface is covered by land and water bodies. However, the absorption of heat energy to both of these surfaces are markedly different. The land is an opaque to the incoming solar radiation while water is translucent (Figure 11). Whatever the heat energy reaches to the land body, it is utilized to heat a thin layer of the land surface while the same amount of heat energy reaching to the water surface is penetrating to much deeper depths. Apart from that, evaporation also reduces the effectiveness of heat on water surface as a part of its energy is removed by latent heat of evaporation. That is why, we find equalizing climatic condition on the sea coast while the interior part of the continent reflect harsh climate i.e. very cold or very hot. Accordingly the heating and cooling of the air of that region is affected.

Figure 11: Differential Heating of Land and Water

Source: http://www.shsu.edu/~dl_www/bkonline/131online/CourseGraphics/GraphicsMaster/G030.gif

Nature of Ocean Currents: Heating and Cooling

Low latitude areas are warmer while the high latitude areas are colder. The temperature of the ocean water is also affected by the temperature distribution over the globe. Ocean currents are flowing under the influence of planetary wind system as well as the regional shape of the sea coasts. Since the ocean currents are very important medium of heat transfer through advection from low latitude to the high latitude. Depending upon the nature and temperature of the ocean water, currents are categorized into two – warm and cold. Wherever those currents are reaching, they are influencing the areas accordingly. That is way, the north-western European coasts are warmer but the north-eastern coast of North America is chilled during winter though both of them lie on the same latitude.

Transparency of the Sky

Apart from the gases, several other minute suspended particles and water vapour constitute the atmosphere. Though the gases are almost uniformly distributed, but other substances are varying at local and regional level. Their availability and quantity is season dependent also. These substances are obstructing, reflecting and absorbing the solar radiations and affecting the heating and cooling of the atmosphere.

Slope Aspects

Slope of the any region or mountain has direct bearing on the heating or cooling of the air. The south facing slope of northern hemisphere and north facing slope of southern hemisphere receive more energy than their counterpart. Therefore, they are warmer and support more rain resulting in dense vegetation cover. When it is the reverse case, it is colder, less moist and retarded growth of vegetation. The examples can be had from the Himalayas (Figure 12).

Figure 12: North-South Slope Aspects

Eastern and western slope also have bearing on the heating and cooling of the air along them. Eastern facing slope receive less energy as the temperature is not that effective. Therefore, it is cool on the eastern slope. On the western facing slope, more temperature is recorded and hence, it is warmer. This type of example can be had from the two slopes of the Rockies and the Andes (Figure 13).

Figure 13: East-West Slope Aspects

Summery and Conclusions

The major source of heat on the earth as well as on the atmosphere is sun. Sun’s energy reaches directly to the earth by shortwave radiations. Once the earth is heated, it releases received energy through longwave radiations. This radiation heats the atmosphere as it is capable of catching the energy, which it is not the case with the shortwave radiations. The atmospheric heating is possible by different ways. Important among them are absorption of incoming solar radiations, conduction, terrestrial radiations, convection, advection, latent heat transfer and heating and cooling by expansion and contraction. Heating and cooling of earth’s atmosphere may keep on changing on day-to-day or seasonal basis, but on annual basis it is neutral. It means, the total incoming and outgoing radiations are balanced and the entire earth and its atmosphere maintain uniform temperature conditions.

The heating and cooling of the earth and its atmosphere is controlled by several factors. The important among them are the latitude. Since the earth is a sphere, the angle of incidence of sun rays differs with changing latitudes. Therefore, the distribution of energy on the earth is not uniform. It is more in the low latitudes and less in high latitudes. Depending on the distribution of insolation, the heating and cooling of the atmosphere is affected. Other controlling factors are altitude, distribution of land and water, ocean currents, transparency of the sky and the slope. All of them are affecting the temperature conditions on the atmosphere.

you can view video on ATMOSPHERIC HEATING AND COOLING

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.

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