Geography One - INSOLATION
 

[B]INSOLATION[/B]

Our sun, a star of about medium mass and temperature has a surface temperature of 6,000 oC or 11,000 oF. The highly heated incandescent (radiant) gas that comprises the sun’s surface emits a form of energy known as Electromagnetic Radiation. This form of energy transfer can be thought of as a collection or spectrum of waves of a wide range of lengths travelling at a uniform velocity of 186,000 miles/sec or 300,000 km/s. The energy travels in straight line radially outwards from the sun and requires about 9.33 minutes to travel 93 million miles or 150 km from sun to earth.
This amount of heat received from the sun is known as INSOLATION. Insolation for a day is the total amount of heat received from the sun during the day. The expression “Intensity of Insolation” has nearly the same meaning as the common phrase “strength of the sun” or “strength of the sun’s rays”. If we say that at a certain time, the intensity of insolation at one place is twice than at another, it means that during equal and very short periods of time, the former place receives twice as much heat as the latter.
This radiation from sun is made up of three parts; the visible white light that we see when sun shines and the less visible ultra violet and the infra red rays. The visible light which is white, has greatest influence on our climate. The ultra violet rays effect our skin and cause sun burn when our bare body is exposed to them for too long a period. The infra red rays can penetrate even dust and fog and are widely used in photography. Only the part of sun’s radiation which reaches the earth is called “Insolation”.
What matters is the effect of the atmosphere upon the incoming solar radiation. It is estimated that of the total radiation that reaches to us, 35% reaches the atmosphere and is directly reflected back to space by dust, clouds and air molecules. Another 14% is absorbed by the water vapor, CO2, and other gases. Its interception by the air causes it to be scattered and defused so that the visible rays of the spectrum U-V and infra red gives rise to the characteristic blue sky that we see above us. The remaining 51% reaches the earth and warms the surface. In turn the earth warms the layers of air above it by “Conduction” (direct contact), and through the transmission of heat by upward movement of air currents or “Convection”. This “Radiation” of heat by the earth continues during the night, when insolation from the sun cannot replace it. The earth’s surface, therefore cools at night.
The measure of the intensity of insolation per unit area received at the outer limit of the atmosphere is called the “Solar Constant”, which is calculable. This is equal to nearly 2 gm-cal/cm2/min, i.e., a radiation received each minute on a surface area of a cm2 could raise the temperature of 1 gm of water through 2oC or alternatively equivalent to 1 KW/m2.
1 gm-cal/cm2 constitutes a unit measuring heat energy termed as “Langley”. Therefore, we can say that, the solar constant is equal to 2 Langleys/min. In English heat units it is equal to 10 BTU/ft2/hr.

[B]FACTORS AFFECTING INSOLATION[/B]

[B]Introduction[/B]

The earth both rotates on its own axis, which is responsible for the alternation of days and nights, and also revolves in its elliptical orbit around the sun for a period of 365.25 days, which is responsible for the seasons. Because the earth is a sphere, only one point on the earth presents a surface at right angle to the sun’s rays. In all directions away from this point, the earth’s curved surface becomes turned at a decreasing angle with respect to the rays until the circle of illumination is reached.
Therefore, throughout the earth’s surface the temperature varies and this variation basically depends on two factors, viz.
a. the angle at which sun’s rays strike; and
b. the length of time of exposure to the rays.
These factors are varied with latitude and by the seasonal changes in the path of the sun.

[B]Factors[/B]

[B][I]1. Latitude[/I][/B]
The intensity of insolation is greatest where the sun’s rays strike vertically, as they do at the noon at the latitude equal to the sun’s declination ranging between the Tropic of Cancer and Capricorn. With diminishing angle, the same amount of solar energy spreads over a greater area of ground surface. Hence, on the average, the polar areas receive least heat per unit area. This fact helps to explain the general distribution of average temperature over the globe from a maximum at low latitudes to a minimum near either poles.

[B][I]2. Incidence of sun’s rays[/I][/B]
The incidence of sun’s rays depends upon the distance from the sun and its elevation. While the rays of sun are perpendicular at one place, they are slanting at the other. The greater the distance the more slanting are the rays. The more slanting are the rays, the greater distance through atmosphere they will have to cover, and over a greater distance would they be spread. On the other hand, the perpendicular rays have to pass through a smaller portion of the atmosphere and spread over a smaller extent of the surface. Hence the regions receiving direct and perpendicular rays of the sun are warmer than others. That is why, the equator is the warmest region and everyday, noon is warmer than morning or evening.

[B][I]3. Influence of the atmosphere[/I][/B]
Although the air seems so transparent, it does not allow the rays of the sun to pass through it without loss. The atmospheric layers to be crossed also influence the energy of the sun through reflection and absorption. The absorption decreases the heat and this decrease is accentuated (intensified) as the mass of the atmospheric traverse (obstruction) increases. By and large, the chief elements of the atmosphere that impedes (obstructs) the solar rays are the drops of water, dust particles, water vapor, salt and smoke (CO2). These are most numerous in the lower layers of the atmosphere. Hence, insolation is intense on the desert areas where the atmosphere is the clearest and on higher altitudes where these impediments are not to be found in the atmosphere.

[B][I]4. Duration of sunlight[/I][/B]
Insolation has a direct relationship with the duration of the sunlight or the hours when the sun is shining. Because the earth is tilted at an angle of 66.5o and the orbit of the earth is an ellipse, the circle of illumination is ever shifting in its position due to rotation and revolution. The result is that duration of sunlight varies with latitudes and seasons. The distribution of insolation also differs accordingly, from latitude to latitude and from season to season. The seasonal change is, however, the least at the equator and the insolation is the maximum at the equinoxes.

[B][I]5. Nature of surface[/I][/B]
Heat affects land and water differently, and therefore the distribution of temperature is greatly influenced by the distribution of continents and oceans. In general, the land masses heat more quickly in the summer and cool more quickly in the winter than the seas and there are several reasons for this difference. They are:

[U]a. Specific Heat:[/U] The specific heat of a substance is defined as the number of calories required to raise the temperature of 1 gm of that substance through 1oC. A calorie represents the amount of heat required to raise the temperature of 1 gm of water through 1oC. The specific heat of sea water is 0.94 and of granite is 0.2. In other words, water must absorb nearly five times as much heat in order to increase its temperature by the same amount as earth. Land surface, therefore, heats up more rapidly and intensely than does water surface. On the other hand, land cools more rapidly when the source of heat is cutoff.

[U]b. Penetration of sun’s rays:[/U] The rays of the sun penetrate more deeply into water than into earth. The daily variations of temperature are not perceptible below a depth of about 3 ft. in earth. In water they may sometimes be observed at 60 ft. beneath the surface. The seasonal variations disappear at a depth of 60-80 ft. in earth, 300-600 ft. in water. Thus the sun rays falling on water are engaged in heating a much larger volume of water than when they fall on land. The temperature reached is, therefore, not so high; but when the sun sets there is a larger quantity to cool and the cooling is slower.

[U]c. Mobility of water: [/U]Water is mobile and land is not. When the sun’s rays fall on the land, their effect is practically limited to the area on which they fall. In water, convection currents are setup and the heat is partly carried away. Consequently, when the sun’s rays fall on water they warm not only the area on which they fall but also the surrounding areas.

[U]d. Evaporation:[/U] When the sun shines on water, a considerable amount of the heat is used in evaporating water and not raising its temperature. Infact, evaporation has the after effect of reducing the temperature. On dry land, there is no such loss.

[U]e. Reflection:[/U] Much of the amount of heat that falls on the water is reflected and does not raise its temperature. Land surfaces are poor reflectors and little heat is lost in this way. Water reflects a high proportion of the light while opaque earth absorbs more.
The ratio between the total solar radiation falling upon a surface and the amount reflected is expressed as a decimal or a percentage, known as the “Reflection Coefficient” or “Albedo”. Albedo is derived from the Latin word albus, meaning white. The earth’s average albedo as seen from the space is about 0.4, i.e., 4/10th of the solar radiation is reflected back into space. Water has a low albedo, i.e., 0.02 with near vertical ray, though a high one for low angle slanting rays. Grass has an albedo of about 0.25.

[U]f. Clouds in the sky:[/U] On the whole, sky is cloudier over the ocean than over the land. The clouds not only obstruct the rays of the sun, but they also hinder loss of heat in radiation. Their effect is therefore, to retard both the heating and the cooling of the water.
All these factors tend to make water heat more slowly than the land.

[B]6. Winds[/B]
The effect of wind on temperature is to transport temperature prevailing in areas over which they blow, either from land or from sea. Onshore winds in the tropics, blowing from over the cooler oceans, tend to modify temperatures on the coastal margins. On the other hand, onshore winds, such as Westerlies, may in winter carry mild temperatures from over the oceans on to the continental margins. Local winds may produce the rapid temperature changes.

[B]7. Ocean Currents[/B]
Ocean currents share with winds the ability to transport temperatures. Where onshore winds blow over these currents, they convey similar temperature conditions to land margins. Specially important are the cold water coasts of the western sides of the continents in the tropics and sub-tropics.