Geography One - TERRISTRIAL HEAT BALANCE

[Bhalla Changa]

TERRISTRIAL HEAT BALANCE

INTRODUCTION

The sun provides 99.97% of the energy required for all the physical processes that take place on the earth and in the atmosphere. As a result of absorbed insolation, different types of radiant heat or radiation flow through out the earth-atmosphere system, inputs and outputs of radiation are balanced at the planetary surface.
A remarkable feature of the planet earth is that despite many changes, irregular receipts and the divergence in the flow of solar radiation reaching the earth and its atmosphere, the temperature average for the earth as a whole have varied only within relatively narrow limits over the billions of years since first life form appeared on earth. There have been several periods with warmer and colder temperatures than at present. But it is unlikely that the average temperature of the planet have ever varied more than 20 to 30 oF.

OPEN ENERGY SYSTEM OF ATMOSPHERE

The main reason for the narrow variations in global temperature range is that the earth and its atmosphere constitute an “open energy system” in which the output of energy must balance the input within narrow limits and in which mechanisms exist to correct imbalances in the energy flow or flux. The most careful measurements reveal that the same amount of radiation leaves the earth as enters in the form of solar radiation with only slight and temporary exceptions.

Short and Long Wave Radiations:- We may regard radiation as a transmission of energy in the form of electromagnetic waves. The wavelength of the radiation is the distance between two successive wave crests. This wave length varies in different types of radiation, and is inversely proportional to the temperature of the body that sent it out: the higher the temperature at which the radiation is emitted, the shorter the wave length of the radiation. The sun has a surface temperature of about 6000 oC (11,000 oF). Thus radiation coming from the sun is short wave radiation and that emitted from the earth is long wave radiation.

Net Radiation:- One can observe that the outgoing radiation is dominated by the long wave radiation emitted by the earth. The amount leftover, when all the incoming and outgoing radiation flows have been tallied, is the net radiation. This net radiation balance totals about one-forth of the short wave radiation originally arriving at the atmosphere. There is usually a difference between an area of land and sea at the same latitude. The difference in net radiation over land and sea is greatest in low latitudes and smallest, or even negative, at high latitudes. Net radiation may well be the single most important factor affecting the earth’s climate. For example, net radiation is far the most significant factor determining the evaporation of water.
The amount of water available and the quantity of net radiation together can largely explain the distribution of vegetation across the land surfaces of the earth, from the dense forests of the equatorial tropics to the sparse mosses and lichens of the sub-arctic environmental zones. Furthermore, net radiation is vital in shaping the heat balance of the earth.

GROUND OR TERRISTRAIL RADIATIONS

The ground or ocean surface possessing heat derived originally from absorption of the sun rays continually radiates this energy back into the atmosphere, a process known as Ground Radiation or Terrestrial Radiation. This infra red radiation consists of wavelengths longer than 3 or 4 microns and is referred to as “long wave radiation”.

COUNTER RADIATION

Of the long wave energy radiated from the ground, a portion is radiated back to the earth surface, a process called “Counter Radiation”. The lower atmosphere with its water vapor and CO2 acts as a blanket which returns heat to the earth and helps to keep surface temperatures from dropping excessively during the night or in winter at middle and high latitudes. Thus our atmosphere is actually heated from below, not directly by the sun above. This basic process of atmosphere heating is called Greenhouse Effect.

MECHANISMS OF ENERGY FLUX

Reflection of incoming radiation – Earth’s Albedo

Just as the moon light consists of reflected solar radiation bouncing off the barren lunar surface, so does the earth shines with reflected radiation. This energy does not enter the earth’s atmospheric system of energy in any way and it is reflected back into space unchanged. The best reflecting surfaces by far are the upper surfaces of the clouds and it is estimated that for the earth as a whole some 21% of the incoming radiation is reflected in this way.
Another 5% is reflected back into the space after having been reflected back and forth (Defused Radiation) among the surface of tiny particles in the air. Only about 6% of the Short Wave Radiation is reflected from the earth’s surface. Good reflecting surfaces on the earth include bare ground, snow and ice, the wind rippled surfaces of water bodies, city streets and roof tops.

Absorption and disposition of short wave solar radiation

Of the 68% of solar radiation that is not reflected, only about 18% is absorbed directly by the atmosphere, mainly by water vapors and water droplets in clouds, also by molecules, dust particles, CO2 etc. Some of the solar radiation that is absorbed is used to evaporate cloud droplets, but most of it is altered into “Sensible or Thermal Heat”, which in turn is re-radiated as infra red radiation. In any case the 18% enters the atmosphere sector of the global energy system.
The 50% of radiation that is absorbed by the earth plays much more diverse role than that in the atmosphere. A large amount of it evaporates water, and most of the rest produces Sensible Heat that is ultimately re-radiated like that in the atmosphere.

Absorption and disposition of long wave thermal radiation

The surface of the earth and atmosphere both transfer the thermal radiation in the infra red range following the production of Sensible Heat. While the earth’s surface absorbs short wave and thermal radiation alike, the atmosphere is far more efficient in absorbing infra red radiation than the incoming solar radiation, largely because of the important absorption bands. In fact, the atmosphere absorbs all of the Terrestrial Radiation except for a narrow band of a wave length between 8 and 12 microns. This window accounts for the 8% of the thermal radiation that passes directly from the earth to outer space without entering the atmospheric sector.
Perhaps the puzzling figure of 137% within the atmospheric sector of the system can be explained by the fact that same energy can be exchanged several times between the atmosphere and the earth’s surface, with an energy exchange taking place each time. This multiple exchange is termed as “Back Radiation”. The disposition of this 137% is indicated with 60% passing into outer space and 77% being transmitted back to the earth’s surface. Much of this 77% keeps us warm at night when there is no solar input.
The input of the atmospheric sector comes first from the absorption of the solar radiation i.e., 18%. Another 90% is received from terrestrial radiation much of which again is Back Radiation. Some sensible heat is shifted into atmosphere by air currents (9%) while the condensation of water vapors in the cloud zone adds another 20%.

INCOMING SOLAR RADIATION

Total radiation at top of atmosphere-------------------------------100%
Defused Reflection to space by particles-----------------------------5%
Reflection from clouds to space-------------------------------------21%
Direct short wave reflection from earth’s surface----------------------6%
Total reflection loss to space by earth’s atmospheric system----------32%
Absorbed by water vapor, molecules, dust, CO2 and clouds (15+3)----18%
Absorbed by earth’s surface-----------------------------------------50%
Total absorption by earth-atmosphere system------------------------68%
Sum of absorption and reflection------------------------------------100%

OUTGOING SOLAR RADIATION

Infra red radiation from earth’s surface-------------------------------98%
Lost to space--------------------------------------------------------8%
Absorbed by atmosphere--------------------------------------------90%

Infra red radiation emitted by atmosphere (18%+90%+9%+20%)------137%
Radiation lost to space-----------------------------------------------60%
Absorbed by earth’s surface as counter radiation----------------------77%

Effective (net) outgoing radiation from earth’s surface-----------------21%
From atmospheric system--------------------------------------------47%
Net outgoing radiation from earth atmospheric system-----------------68%


LATITUDINAL HEAT BALANCE

There are some great inequalities that are found in the global energy system. Surpluses and deficiencies in the net radiation balance not only occur in different places horizontally and vertically, but also change daily and seasonally in the same place. Yet the system functions; the patron of temperature does not change much; the output of radiation generally matches the input within extremely narrow limits; and the inequalities in the net radiation balance are compensated.
In order to maintain global energy balance, there must be several types of energy transfer and transport mechanisms to carry surplus energy from the major areas of accumulation (the heat sources), to those portions of the earth’s atmosphere system that are more efficient in draining off surplus energy in the form of outward radiation (the heat sinks). The major heat sources are low latitude oceans, while the major sinks are the snow and ice covered surfaces of the winter hemisphere continents and the upper part of the lower atmosphere.
Some of the energy transfer mechanisms are:

Winds

Wind is horizontal air flow and is capable of transporting both sensible and latent heat from the heat sources of the low latitudes to the cooler high latitudes.
(Heat used to change the form of water is known as latent heat i.e., solid, liquid or gas. Heat required to change the state of ice to water is known as latent of fusion and from water to vapor, latent heat of vaporization. )

Ocean Currents

A circulation system, roughly similar in principle to the hot water heating system in a home, exists in the world oceans. It is not nearly as efficient or effective as air circulation, but it carries surplus heat away from the low latitude oceans towards heat sinks at high latitudes.

[young hopeful]

infra red emitted from th atmosphere 137 %????the earth n atmosphere collectively recieve 68 %.out of this 8% is sent in to the outer space.out of the remaining 60 % ,,,where does this 137 % comes from please help some 1

[SYEDA SABAHAT]

hmm! be hopeful young boy it is quite confusing but dont worry dear when i am here.i will reply you after research.ok