Theories of Geography Part 12 – Energy, temperature and pressure in Atmosphere

Insolation Energy Budget

Atmosphere and Insolation

Sun is the major source of energy for the entire earth system. The earth does receive very small proportions of energy from other stars and from the interior of the earth itself (volcanoes and geysers provide certain amount of heat energy). However, when compared with the amount received from the sun, these other sources seem insignificant.

The energy emitted by the sun which reaches the surface of the earth is called Insolation. The sun, a mass of intensely hot gases, with a temperature at the surface be 6000°C emits  radiant  energy in the form of waves, which consists  of very short wave-length x-rays, gamma rays, and ultraviolet  rays; the visible light rays and the longer infrared rays. The earth receives only about one two-thousand- millionth of the total insolation poured out by the sun, but this is vital to it; the amount received at the outer limit of the atmosphere is called Solar Constant. Thus Solar Constant is the rate per unit area at which solar radiation is received at the outer limit of the atmosphere.

Effects of the Atmosphere on Solar Radiation

When the sun’s energy passes through the atmosphere several things happen to it. Around one fourth of this energy is directly reflected back to clouds and the ground. Around 8 percent is scattered by minute atmospheric particles and returned to space as diffuse radiation. Some 20 percent reaches the earth’s surface as diffuse radiation after being scattered. Approximately 27 percent reaches the earth’s surface as direct radiation and 19 percent is absorbed by the ozone layer and by water vapour in the clouds of the atmosphere.

On an average, 47 percent of the solar energy arriving at the outer limits of the atmosphere eventually reaches the surface, and 19 percent is retained in the atmosphere. This 19 percent of direct solar radiation that is retained by the atmosphere is locked up in the clouds and the ozone layer and is thus not available to heat the troposphere. The warmth of the atmosphere is due to the 47 percent of incoming solar energy reaching the earth’s surface (that is, both land and bodies of water) and in the transfer of heat energy from the earth back to the atmosphere through such physical processes such as Long-Wave Radiation, Conduction and Convection. Some related phenomena such as advection and Latent Heat of Condensation also contribute to the warmth of the atmosphere.

Radiation as method of Heat Energy Transfer

Radiation is the process by which most energy is transferred through space from the sun to the earth. Radiation is given off by all bodies including earth and human being. The hotter is the body, shorter are the waves.

We can simply say that the radiation from Sun comes to earth in the form of smaller waves and earth being cooler body, gives off energy in the form of long-wave. These are then radiated back to the atmosphere. This Long-Wave Radiation from the earth’s surfaces heats the lower layers of the atmosphere. It is evident that the atmosphere is primarily heated from below by radiation from the heated Earth surface.

As we discussed above, the most important cause of atmospheric temperature is the energy received from the sun. The atmosphere of the earth does not heat up directly as solar radiation is in the form of short waves and air cannot absorb the short waves. The earth absorbs the short wave energy and then radiates in the form of long wave terrestrial radiation that can be absorbed by the air. So, air heats up when comes in contact with the surface of the earth.

Conduction as Method of Heat Transfer

Conduction is the means by which heat is transferred from one part of a body to another or between two touching objects. Heat flows from the warmer to the cooler (part of a) body in order to equalize temperature. Conduction actually occurs through molecular movement, with one molecule bumping into another. The Atmospheric conduction occurs at the interface of (zone of contact between) the atmosphere and the earth’s surface. However, it is actually a insignificant method of heat transfer in terms of warming the atmosphere since it affects only the layers of air closest to the earth’s surface. This is because air is a very poor conductor of heat.

Convection as Source of Heat Transfer

When the pockets of air near the surface are heated, they expand in volume, become less dense than the surrounding air, and therefore rise. This vertical transfer of heat through the atmosphere is called convection, and is the same type of process by which heated water circulates in a pan while heating. The currents set into motion by the heating of a fluid (liquid or gas) make up a convectional system. Most vertical transfer of heat within the atmosphere & Oceans occurs via Convection and is a major cause of clouds and precipitation.

Advection as Source of Heat Transfer

Advection is the horizontal heat transfer within the atmosphere. Obviously the wind is the transfer agent of advection. Wind brings about the horizontal movement of large portions of lower atmosphere. This advection transports warmer or accounts for a major proportion of the lateral heat transfer that takes place within the atmospheric system.

Latent Heat of Condensation

A proportion of the solar energy is used to change liquid water from rivers, lakes, and oceans to water-vapour. The solar energy used to do this is then stored in the water-vapour as latent or potential energy. Later the water-vapour in the atmosphere may change to form liquid water again through a process called CONDENSATION. The energy released through this process is known as the Latent Heat of Condensation. Like other means of heat transfer in the earth system, latent heat of condensation plays a major role in warming of the atmosphere and in addition, is a source of energy for STORMS.

Earth’s Albedo

The ratio between the total solar radiation falling (incident) upon a surface and the amount reflected without heating the earth, is called ALBEDO (expressed as a decimal or as a percentage).  The earth’s average albedo is about 0.4 (40 percent); that is, 4/10 of the solar radiation is reflected back into space. It varies from 0.03 for dark soil to 0.85 for a snow-failed. Water has a low albedo (0.02) with near-vertical rays, but a high albedo for low-angle slanting rays. The figure for grass is about 0.25. Over-pastured land and bare soil are more reflective of solar radiation than are crops and vegetation. A desert is much more reflective than a savanna or forest. If economic pressure on soil and vegetation increases, and drought then occurs, the effect overall is to increase the albedo of the surface.

Earth’s Energy Budget

Earth’s Energy Budget can be discussed in terms of incoming heat energy and outgoing heat energy. These are as follows:

Incoming Heat Energy

This is made of:

  • Solar radiation (99.97%)
  • Geothermal energy (0.025%)
  • Tidal energy (0.002%)
  • Fossil fuel consumption (about 0.007%)
  • Minor Sources: remains part

Outgoing Heat Energy

  • The average albedo (reflectivity) of the Earth is about 0.3, which means that 30% of the incident solar energy is reflected into space, while 70% is absorbed by the Earth and reradiated as infrared. This 30% of the incident energy is reflected, consisting of 6% reflected from the atmosphere, 20% reflected from clouds and 4% reflected from the ground (including land, water and ice). The remaining 70% of the incident energy is absorbed, out of 51% is absorbed by land and water, and then emerges in the following ways:
  • 23% is transferred back into the atmosphere as latent heat by the evaporation of water, called latent heat flux
  • 7% is transferred back into the atmosphere by heated rising air, called Sensible heat flux
  • 6% is radiated directly into space
  • 15% is transferred into the atmosphere by radiation, then reradiated into space
  • 19% is absorbed by the atmosphere and clouds, including:
  • 16% reradiated into space
  • 3% transferred to clouds, from where it is radiated back into space.

The above figures are the averages for the whole earth over a year’s time.

Balance in Earth’s Heat Budget

For any particular location, the factors discussed may not be balanced, and adjustments must be made within the entire earth system. Some places have a surplus of incoming solar energy over outgoing energy loss in their budget, while others have a deficit. The main cause of these variations is the differences in latitude, and the seasonal fluctuations.

We know that the amount of insolation received is directly related to the latitude. The tropical zone where insolation in high throughout the year; more solar energy is received at the earth’s surface and in the atmosphere than can be emitted back into space. In the arctic and Antarctic zones there is so little insolation during the winter, when the earth is still emitting long-wave radiation, that there is a large deficit for the year. Places in the mid latitude zone have lower deficits or surpluses, but only at about latitude 38° is the budget balanced. It is the heat energy transfer within the atmosphere that prevents a situation whereby the tropical zones get hotter and hotter and the arctic and Antarctic zone get colder and colder.

Atmospheric Temperature and Pressure

Distribution of Temperature

Temperature differs from one part of the world to the other. Since Insolation is the basic source of energy for the atmosphere, the distribution of insolation would determine the temperature of the earth. Thus latitude, altitude, distance from sea, features of the surface, nature of the landscape are some important factors that affect the distribution of temperature. Since, the insolation is highest at equator; temperature should be highest at the equator and lowest near the poles, however actually it is not. Highest temperature on earth is recorded at a few degrees north of equator. Altitude is the second major control of temperature of a place. The temperature depends upon albedo of the surface also. One major factor affecting the distribution of the temperature of Earth is distribution of Land and Oceans. Since there is more land in Northern Hemisphere and more waters in Southern hemisphere and there is a big difference between the specific heat of land and water; the loss of heat from the continents is bigger than the oceans. The continents get heated faster and get cooled faster in comparison to the Oceans. This is the reason that the temperatures of the Oceans are moderate while that of continents is extreme. The moderating effect on temperature of the land due to proximity of the seas is called Maritime influence. The increasing effect on temperature of the land at interior of the continents is called Continental Influence.

Three Broad Temperature Zones

The earth can be generally divided into three broad temperature zones viz. Torrid Zone, Temperate Zone and Frigid zone.

Torrid Zone

Torrid Zone is the tropical region. The temperature remains high. Sun is directly overhead at least once during the year. In the Northern Hemisphere, the overhead Sun moves north from the equator until it reaches 23.5 °North (Tropic of Cancer) for the June solstice after which it moves back south to the equator. The year is consequently divided nearly into four equal parts by the two times at which the sun crosses the equator (Equinoxes) and those two at which it attains greatest declinations (Solstices). The Torrid Zone forms the hottest region of the world with two annual seasons namely a dry and a wet season. This zone includes most of Africa, southern Asia, Indonesia, New Guinea, northern Australia, southern Mexico, Central America and northern South America.

Temperate Zones

Temperate zones are the mid latitudinal areas, where the temperature is moderate. There are two temperate areas viz. North and South. In the two Temperate Zones, consisting of the tepid latitudes, the Sun is never directly overhead, and the climate is mild, generally ranging from warm to cool. The four annual seasons, Spring, Summer, Autumn and Winter occur in these areas. The North Temperate Zone includes Great Britain, Europe, northern Asia, North America and northern Mexico. The South Temperate Zone includes southern Australia, New Zealand, southern South America and South Africa.

Frigid Zones

The two Frigid Zones, or polar regions, experience the midnight sun and the polar night for part of the year – the cliff of the zone experiences one day at the solstice when the Sun doesn’t rise or set for 24 hours, while in the centre of the zone (the pole), the day is literally one year long, with six months of daylight and six months of night. Please note that the Frigid Zones are not the coldest parts of the earth, and are covered with ice and snow. The coldest temperature on earth has been recorded a few degrees below the 90°N.

Patterns of Global Isotherms

The global distribution of temperature can be represented with the help of isotherms. Isotherms are the lines that join the places with the identical temperatures. Please note that isotherms are drawn after correcting the temperature of a place to the sea level so that the differences due to altitude can be minimized. The Isotherms on the earth run parallel to the latitudes.

Due to the difference between the specific heat between water and land, at any latitude, the temperature over the landmass is higher in summer and lower in winter in comparison to the seas. Here we discuss about the global isotherms drawn in the month of January and July. As shown in the picture, Isotherms for the month of July bend towards Northward while moving from Sea to Land.

For the Month of January, the isotherms bend towards south while moving from sea to land. The only thing you have to note about Isotherms is that water in the South Atlantic and Pacific is absorbing greater amounts of energy during January and the land is rapidly heating and radiating energy. Please also note that due to difference in the specific heat, both highest and lowest temperatures are observed in the interiors of the continents.

Vertical Distribution of Temperature

The vertical distribution of temperature on earth is also unequal. As we studied above in detail that in troposphere, the temperature falls uniformly with height as per the Environmental Lapse Rate. The normal value of this Lapse Rate is 6.4°C per kilometers. When a parcel of air rises upwards and cools this is known as adiabatic cooling. This adiabatic cooling is the result of the expansion of air as it is lifted upwards. When the air descends, it gets warmed and this is called adiabatic warming.

Inversion of the Temperature

In the mountain valleys, the temperature of the air is found increasing with increasing altitude. Thus there is an inversion of the temperature. This is because during the night, the quick radiation from the upper exposed slopes of the mountains causes the surface and air over it to cool rapidly. This cooler air is denser and gets drained by the valley slopes and displaces the warmer air toward up. So, when we go up in a valley, the temperature seems to getting increased. This phenomenon is also called drainage inversion.

Mean Thermal Equator

Thermal equator is a global isotherm having the highest mean annual temperature at each longitude around the globe. Thermal equator does not coincide with the geographical equator. The highest absolute temperatures are recorded in the Tropics but the highest mean annual temperatures are recorded at equator. But because local temperatures are sensitive to the geography of a region, and mountain ranges and ocean currents ensure that smooth temperature gradients (such as might be found if the Earth were uniform in composition and devoid of surface irregularities) are impossible, the location of the thermal equator is not identical to that of the geographic Equator.

Further, we know that the Earth reaches perihelion (the minimum distance from the Sun in its orbit) in early January and is at aphelion (maximum distance) in early July. During winter season of the respective hemispheres, the angle of incidence of the sun’s rays is low in tropics. The average annual temperature of the tropical regions is therefore lower than the observed near the equator, as the change in the angle of incidence is minimum at equator.

The thermal equator shifts towards north and south with north south shift in the position of vertical rays of the sun. However, annual average position of the Thermal equator is 5° N latitude. The reason is that highest mean annual temperature shifts towards northwards during the summer solstice to a much greater extent than it does towards south at the time of winter solstice.

Daily variation of Temperature

Sun is at the highest point at noon but the highest temperature does not occur at 1200 hours because the atmosphere does not get the heat directly from the Sun. It receives heat from the earth’s surface slowly and that is why maximum temperature is generally attained by 1400 hours (2.00p.m.). The daily minimum temperature at a place does not occur at about 0400 hours (4.00 p.m.) in the morning because radiation of heat continues upto the sun rise.

Here are some notable observations on daily temperature ranges:

  • Daily temperature range is low in clouded areas because the clouds obstruct the receipt and loss of insolation.
  • The sky is clear in hot desert’s areas. Insolation is received without obstruction in the day and lost without obstruction in the night. This causes high temperature range in deserts.
  • Ice or snow absorbs less and reflects the insolation more. Hence, the daily temperature range is low is snow bound areas.
  • The air is thin in areas of high altitude. There is great loss of insolation in the night. There is no obstruction in the receipt of insolation in the day. Such places have a high temperature range.
  • There is a higher temperature range in than interior areas of continents than at seas because the sea heats and cools slowly but the land heats and cools rapidly.
  • Warm and cool winds also disturb the temperature range.

Annual temperature range

On Equator

The duration of the day or night is the same in equatorial countries. The sun’s rays are vertical all through the year. Hence, there is no worthwhile difference between the summer and winter seasons. This is the reason that the lowest annual temperature range is found in equatorial areas.

On Poles

Towards the poles, the duration of the day and the inclination of the sun rays go on increasing. It causes a lot of difference between the temperatures of the two seasons. Hence, towards the poles, the annual temperature range goes on increasing. Near Oceans Near the seas and oceans, the equalizing effect of sea water makes the winter less cold and the summer less hot. This reduces the annual range of temperature near the seas. The equalizing effect of the sea water cannot reach land areas, away from the seas. The countries like Mongolia and Tibet which are situated far into the interior of the continent have a high annual range of temperature. The ocean currents near the coasts also affected the temperature range. Due to the warm gulf stream, the winter of western Europe is less cold than what it Europe is less cold than what it should have been without the gulf stream. This reduces the annual temperature range.

The shifting attitude of ocean currents has a lot of effect on the annual temperature range. For example, the weather and seasons have to undergo greater changes on the eastern coasts of Indian and Australia due to the shifting of ocean currents. It increases the annual temperature range on these coasts as compared to that on the opposite side coasts.

Impact of Winds

The prevailing winds also have a greater effect on the annual temperature range. Winds from the land blow in Arabian countries and therefore increase the annual range of temperature. Winds from the oceans and seas blow into Western Europe and reduce the annual temperature range. The variation in the annual temperature range in west and east European countries is due to land and sea winds. The effect of winds from the ocean has a far smaller effect in Eastern Europe than in Western Europe. It is why the annual temperature range is higher in eastern than in Western Europe.

Atmospheric Pressure and Pressure Belts

Air has weight and a column of air extending vertically over a given area on earth’s surface exerts pressure. The atmospheric pressure is measured as a force per unit of area and most common unit of measuring the air pressure is millibar. The instrument used for measuring pressure is Barometer. Some barometers are calibrated to show pressure in mercury inches. At constant temperature of 0°C and latitude of 45°, 1049 millibar is equal to 31 inches of mercury. Barograph is used to take continuous readings of air pressure.

Measuring Atmospheric Pressure

Bar is a unit of pressure equal to 100 kilopascals and roughly equal to the atmospheric pressure on Earth at sea level. Other units derived from the bar are the megabar (symbol: Mbar), kilobar (symbol: kbar), decibar (symbol: dbar), centibar (symbol: cbar), and millibar (symbol: mbar or mb). Bar is neither an SI unit nor a CGS unit. 1 bar is 1% smaller than the atmosphere (symbol: atm), which now is defined to be 1.01325 bar exactly. One millibar is also equal to 1000 dynes per cm².

Pressure Belts of Earth

The distribution of pressure on earth is uneven. Usually pressure is inversely related to the temperature and pressure reduced with altitude. The major factors are earth’s rotation and ascent and descent of air to affect distribution of pressure.

Creation of the Pressure Belts

Due to high amount of insolation over the equator, the air ascends and this air rising in the equatorial region descends at around 30° north and south latitudes. This means that the air at the equatorial region is thrown away from the earth and air at the Polar Regions is pulled towards earth. This implies that there is a low pressure is on equator and there is a high pressure area on poles. This gives rise to two belts of high pressure on Polar Regions each and one belt of low pressure on equator. The air that descends at 30°N and 30°S also created two belts of high pressures in the subtropical regions of both the hemispheres.  Further, the rotation of the earth pulls the air at Polar Regions causes a rarification of air pressure at sub-polar regions. This also produces two belts of low pressure around 60°N and S latitude. This means that there are 7 belts of pressure as shown in the below graphics.

The planetary distribution of pressure, in the 7 belts is determined by two major factors viz. thermal factor and dynamic factor. Please note that equatorial belt of low pressure and polar belts of high pressures are due to the thermal factor while, the subtropical belts of high pressure and subpolar belts of low pressure are primarily due to earth’s rotation or dynamic factors.

Intertropical Convergence Zone / Doldrums

The pressure belt between the 0° to 5°N and S is called Equatorial Low Pressure Belt. This belt is characterized by intense heating, with expanding air and ascending convectional currents. Because the air is largely moving upward, surface winds are light and variable. This region is known as the doldrums.

The term doldrums has been used by the sailors as it has been marked by erratic weather patterns with stagnant calms and violent thunderstorms. Doldrums are belt of calms and variable windsoccurring at times along the equatorial trough. Doldrums are characterised by:

  • Low atmospheric Pressure
  • High Humidity
  • Thunderstorms

The same area is also called the Intertropical Convergence Zone (ITCZ) or Doldrums. This is the area encircling the earth near the equator where winds originating in the northern and southern hemispheres come together. Please note that the location is not precisely defined as location of the Intertropical convergence zone varies over time. Over land, it moves back and forth across the equator following the sun’s zenith point. Over the oceans, where the convergence zone is better defined, the seasonal cycle is more subtle, as the convection is constrained by the distribution of ocean temperatures. Sometimes, a double ITCZ forms, with one located north and another south of the equator. When this occurs, a narrow ridge of high pressure forms between the two convergence zones, one of which is usually stronger than the other. Between 10° and 15° North and South, there are high pressure belts, where air is comparatively dry, light and calm. This region is beneficial to the maritime trade.

Subtropical High / Horse Latitudes

Horse Latitudes or Subtropical High are subtropical latitudes between 30 and 35 degrees both north and south. This region, under a ridge of high pressure receives little precipitation and has variable winds mixed with calm. The air is comparatively dry and calm. This is also the region of descending air current and is marked by some cyclonic activities. The consistently warm, dry conditions of the horse latitudes also contribute to the existence of temperate deserts, such as the Sahara Desert in Africa, the southwestern United States and northern Mexico, and parts of the Middle East in the Northern Hemisphere; and the Atacama Desert, the Kalahari Desert, and the Australian Desert in the Southern Hemisphere.

Other Belts

30°-60°North and South Belt region is of temperate low pressure belt or anti-trade wind area. It is marked by cyclones and anticyclones. 60°North and South are the two Temperate Low Pressure belts which are also called zones of convergence with Cyclonic activity. The 90° North and South are called Polar High belts.

January 14, 2018

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