Climatology/Temperature of the Atmosphere

Mechanisms of Heating and Cooling of Atmosphere
Heating and cooling of theatmosphere 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.

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. 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.

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.energy  is  reaching  the  earth  surfacein  the  form  of  shortwaveelectromagnetic radiations from the sun. The heated earth radiates back the same but in the form of long  wave  electromagnetic  or  infrared  radiations.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).warming.htmlIt 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.

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  airin  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  createa  convection. 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.

Advection
The meaning  of  advection  is  transfer  of  something  from  one  placeto  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.The monsoonal air current movement is the example of regional advection while planetary permanent wind system signifiesthe global advection. All of them are transferring the heat from one area to another.

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.one example; 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,  popularlymeant for ‘hidden’ heat.

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

Expansion and Compression of the Air
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. 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.

Factors Affecting Heating and Cooling of Atmosphere
The erth 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
 * Distribution of land and water
 * Nature of ocean currents
 * Transparency of the sky
 * Slope aspects Latitude

Latitude
The light energy of the sun reaches to the earth’s surface to a maximum limit to 180 degree of angle.the low latitude areas are warmer and high latitude areas are colder.high latitude areas are much colder because of less effective heating.

Altitude and Nature of Earth’s Surface
The atmosphere is normally warmed by the longwave terrestrial radiations. Hence, low altitude are a 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.

Distribution of land and water
The earth surface is covered by land and water bodies.The land is an opaque to the incoming solar radiation while water is translucent.Whatever the heat energy reaches to the landbody, 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.

Ocean currents
Heating and Cooling Low latitude are as are warmer while the high latitude are as 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 these a coasts. Since the ocean currents are very important medium of heat transfer through advection from low latitude to the high latitude.

Transparency
Apart from the gases, several other minute suspended particle sand 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.

Slope Aspects
Slope of the any region or mountain has direct bearing on the heating or cooling of theair.The south facing slope of northern hemisphere and north facing slope of southern hemisphere receive more energy than their counterpart.

Horizontal Temperature Distribution
The energy  coming  from  the  sun  is  not uniform all  through the  globe .equatorial  region  is  hot  and  its  temperature  is  high  throughout  the  year. Generally, from equator  to  polewards, temperature  keeps  on  declining. The lowest  temperature  is  at  and near  the  pole.To represent distribution of temperature isotherms are used. Isotherm is an imaginary line joining the  places with  same temperature. If we draw the isotherm of certain time over the  world  map, we would be in a position to have spatial pattern of the distribution of temperature.We study about two position:--
 * 1) when  the  sun  is  overhead  at  Tropic  of  Cancer
 * 2) when the sun is overhead at Tropic of Capricorn

Sun Overhead on the Tropic of Cancer(July)
The sun  is  overhead at  the Tropic  of  Cancer  by  the  end  of third  week  of  June  (June  21st)  at 23.50N. Entire northern  hemisphere  witnesses  bright  sun,  greater  insolation  leading  to  high temperature throughout. But the maximum average monthly temperature is not recorded in June, but it  so  in  July.July  is  taken  to  study ideal  summer  month  for  northern hemisphere. The highest  temperature  is  recorded  over large  chunk  of  area  comprising Sahara desert  of  northern  Africa  and  desertic  parts  ofwest  central  Asia. This belt  runs  from  Sahara desert, via Arabia to Thar. The high temperature zone is extended to the Indo-Gangetic plain as well asTibetan plateau. This zone attracts the monsoon winds as it has intense low pressure due to high temperature. This low pressure zone is characterized by inter tropical convergence zone (ITCZ). During northern hemisphere summer days, the isotherms turns towards northward over land as it is hotter thanwater. The condition is reversed on the oceans as the water bodies are not that hot as the land is. Hence, the isotherms turns southward on the oceans of the northern hemisphere. Another low pressure system is developed over the north western Mexico and south western USA due to more intense record of temperature.

Sun Overhead on the Tropic of Capricorn (January)
The sun is overhead at the Tropic of Capricorn by the end of third week of December(December 21st)at 23.50S. Entire southern hemisphere witnesses bright sun, greater insolation leading to high temperature  throughout. But maximum  average   monthly   temperature   is   not   recorded   in December,  but  it is so  in  January.Therefore, case  of  January  is  taken  to  study ideal  summer month for southern hemisphere. Both major continents –South America and Africa are tapered towards south. The maximum  mean  temperature  of  January  is  about  320C  over  a  small  area  of  western Australian  desert. Over South  America  and  Africa,  it  is  around  270C. The area  bounded  by  the 270C isotherm is wide over continents as well as on the Indian Ocean. The temperature  gradient  is  increasing. It is greater in  the  northern  hemisphere particularly over the large landmass of Asia and North America. The lowest temperature around -400C is  recorded  on  the  polar  region  of  Canada,  Iceland  and  Asian  Siberia. Over the  northern oceans, isotherms are turned towards pole whereas on landmass their bends are towards south. It is because of the transport of heat from equatorial region to poleward through prevailing winds and ocean currents.

Inversion of Temperature
In certain  conditions,  temperature  is  not  always  declining  with  increasing  altitude but  it  rises. This situation is known as inversion of temperature. The term, inversion, means opposite to the normal. Since normal is fall intemperature with altitude, under inversion, it rises with increasing height.It happens when the air near surface is cooler than upper air.

Ideal Conditions for Temperature Inversion:--
Temperature inversion takes place only under certain conditions.
 * There has to be long and cool nights so that earth radiates received solar energy.
 * There has tobe clear sky so that terrestrial radiation escapes.
 * There has to be calm and stable air so thatvertical motion in the air is absent.

Types of Temperature Inversion
there are several types of inversion of temperature.
 * Ground inversion,
 * Valley inversion,
 * Subsidence inversion and
 * Frontal inversion

Ground Inversion
Ground inversion occurs when the surface is cooled rapidly by earth radiation under clear sky. In this way, temperature  above  the  ground  is  still  warmer  than  air  near  the  ground. When temperature near surface reaches todew level, the possibility of fog formation increases. Ground inversion is  very  common  in  the  higher  latitude  areas  or  during  winter  in  the  plain  even  inthe tropical regions.

Valley inversion
Valley inversion takes place  on  the  rolling  topography,  particularly  in  hilly  areas. In such situation, mountainslope becomes cool in the night and the air with its contact gets cooler. Cool air creeps  downward  along  the  slope  and  occupies  the  valley.The  warm  air  of  the  valley  is pushed up and thus the inversion of temperature is evident.

Subsidence inversion
Subsidence inversion takes place mostly insubtropical high pressure belts orleeward side of the mountain where  air  subsides. In either  of the  cases, subsiding  air  gets  warmed  in  thisprocess while the  lower  level  preexisting  air  is  cooler. The warming  is  achieved  about  100C  per  km of descending air.

Frontal Inversion
This type  of  inversion  of  temperature  takes  place  under  the  frontal formation  of  two  different  air  masses. Whencold and heavier  air  mass  under cuts warm  sector occupied by warm air mass, the warm air is lifted up. The ground is occupied with cold air, and thus, inversion is observed.