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 .

The greenhouse effect of solar radiation on the Earth's surface caused by greenhouse gases

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^{[check spelling]} 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:--

when the sun is overhead at Tropic of Cancer
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.

Winter solstice occurs in December for the northern hemisphere, and June for the southern hemisphere.

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.