Radiation Balance and Factors affecting Insolation
Earth's Radiation Balance
Earth’s Radiation Balance deals with the balance maintained in the Earth by incoming solar radiation and outgoing radiation. Balance can happen only if the amount of heat received in the form of insolation equals the amount lost by the earth through terrestrial radiation
While passing through the atmosphere some amount of energy is reflected, scattered and absorbed (of 100%)
Roughly 35 units are reflected back to space even before reaching the earth’s surface. Of these, 27 units are reflected back from the top of the clouds and 2 units from the snow and ice-covered areas of the earth and 6 units scattered by space. The reflected amount of radiation is called the albedo of the earth
The remaining 65 units are absorbed, 14 units within the atmosphere and 51 units by the earth’s surface
Terrestrial Radiation/Outgoing Solar Radiation
The earth radiates back 51 units in the form of terrestrial radiation
Of these, 17 units are radiated to space directly and the remaining 34 units are absorbed by the atmosphere (6 units absorbed directly by the atmosphere, 9 units through convection and turbulence and 19 units through latent heat of condensation).
48 units absorbed by the atmosphere (14 units from insolation +34 units from terrestrial radiation) are also radiated back into space
The total radiation returning from the earth and the atmosphere respectively is 17+48=65 units which balance the total of 65 units received from the sun. This is termed the heat budget or heat balance of the earth
This explains, why the earth neither warms up nor cools down despite the huge transfer of heat that takes place
Factors affecting Insolation
Solar radiation received by the Earth system, known as insolation (for incoming solar radiation), is the main source of energy on our planet
Out of the total radiation received maximum is the Infrared radiation followed by visible and Ultravoilet component of radiations.
Insolation & Temperature
The seasonal variations in temperature that we experience are due primarily to fluctuations in insolation
Heavy cloud cover, for instance, will keep more solar radiation from reaching Earth’s surface than will a clear sky. However, cloud cover is irregular and unpredictable, and it affects total insolation to only a minor degree over long periods of time
It is generally believed that the distance of Earth from Sun affects the insolation; but the fact is it has very less impact on the insolation.
The perihelion of any orbit of a celestial body about the Sun is the point where the body comes nearest to the Sun. It is the opposite of aphelion, which is the point in the orbit where the celestial body is farthest from the Sun
Earth is about 147.1 million kilometers from the Sun at perihelion around January 3, in contrast to about 152.1 million kilometers at aphelion around July 4. If it affects the temperature must be highest at perihelion but it happens in January
What causes variations in insolation??
Two major phenomena that vary regularly
for a given position on Earth as our planet rotates on its axis and revolves around the sun: the duration of daylight and the angle of the solar rays
The amount of daylight controls the duration of solar radiation, and the angle of the sun’s rays directly affects the intensity of the solar radiation received.
Together, the intensity and the duration of radiation are the major factors that affect the amount of insolation available at any location on Earth’s surface
Earth’s rotation and revolution : Insolation
Earth rotates every 24 hours on its own axis but takes a year to revolve around the sun. This creates the difference in insolation at a particular geographical location
a location on Earth will receive more insolation if
- the sun shines more directly
- the sun shines longer,
- or both
The intensity of solar radiation received at any one time varies from place to place because Earth presents a spherical surface to insolation.
Therefore, only one line of latitude on the Earth’s rotating surface can receive radiation at right angles, while the rest receive varying oblique (sharp) angles
Solar energy that strikes Earth at a nearly vertical angle renders more intense energy but covers less area than an equal amount striking Earth at an oblique angle
Sun and Earth
The intensity of solar radiation falling on a location depends on the distance travelled by radiation. Some part of Earth received sun’s vertical rays having more intensity however other part will receive Sun’s oblique rays thus less energy and heating.
Latitudinal variation of insolation
The intensity of insolation received at any given latitude can be found using Lambert’s Law. Lambert developed a formula by which the intensity of insolation can be calculated using the sun’s zenith angle (that is, the sun angle deviating from 90° directly overhead)
Using Lambert’s Law, one can identify, based on latitude, where greater or lesser solar radiation is received on Earth’s surface
In addition, the atmospheric gases act to diminish, to some extent, the amount of insolation that reaches Earth’s surface. Because oblique rays must pass through a greater distance of atmosphere than vertical rays, more insolation will be lost in the process.
German scientist and mathematician August Beer established a relationship to calculate the amount of solar energy lost as it comes through our atmospheric gases. Beer’s Law, as it’s called, is strongly affected by the thickness of the atmosphere through which the energy must pass
Air mass number
The Air Mass can be defined as the mass of air; thus is understood as the thickness of atmosphere that radition need to travel. More is the air mass number less will be the insolation.
Example if angle θ is 30 degree 90-30 = 60 = 1/1/2 = 2
θ decides how far is sun from the overhead and the amount of insolation received
AM = 1 (1000 Kw/m2)
AM = 1 .5 (800 Kw/m2)
Seasons : Insolation
Many people assume that the seasons must be caused by the changing distance between Earth and the sun during Earth’s yearly revolution. As noted earlier, the change in this distance is very small
For people in the Northern Hemisphere, Earth is actually closest to the sun in January and farthest away in July. This is exactly opposite of that hemisphere’s seasonal variations.
Seasons are caused by the 23½° tilt of Earth’s equator to the plane of the ecliptic and the parallelism of the axis that is maintained as Earth orbits the sun
Reason for 23½° tilt
Giant impact hypothesis
Long, long ago, it is thought that something big hit Earth and knocked it off. So instead of rotating with its axis straight up and down, it leans over a bit.
That big thing that hit Earth is called Theia. It also blasted a big hole in the surface. That big hit sent a huge amount of dust and rubble into orbit. Most scientists think that that rubble, in time, became our Moon.
As Earth orbits the sun, its tilted axis always points in the same direction. So, throughout the year, different parts of Earth get the sun’s direct rays
The seasons are decided based on to which direction the tilt is headed; towards sun or away from it. Based on that three major positions that decided insolation are
- Summer solstice
- Winter solstice
About June 21, Earth is in a position in its orbit so that the northern tip of its axis is inclined toward the sun at an angle of 23½°. In other words, the plane of the ecliptic (the 90° sun angle) is directly on 23½° N latitude.
This day during Earth’s orbit is called the summer solstice in the Northern Hemisphere. Thus, June 21 is the longest day, with the highest sun angles of the year in the Northern Hemisphere, and the shortest day, with the lowest sun angles of the year, in the Southern Hemisphere
Sometimes it is the North Pole tilting toward the sun (around June) and sometimes it is the South Pole tilting toward the sun (around December).
It is summer in June in the Northern Hemisphere because the sun's rays hit that part of Earth more directly than at any other time of the year. It is winter in December in the Northern Hemisphere, because that is when it is the South Pole's turn to be tilted toward the sun
Earth’s axis points neither toward nor away from the sun (imagine the axis is pointed at the reader); the circle of illumination passes through both poles, and it cuts Earth in half along its axis
Summer and Winter Condition
This is how the angle of inclination affect the illumination.