Insolation - Notes | Geography for UPSC 2024 (Pre & Mains) PDF Download

 Page 1


 
 
 
INSOLATION 
? The earth’s surface receives most of its energy in short wavelengths. The 
energy received by the earth is known as incoming solar radiation 
which in short is termed as insolation.  
? As the earth is a geoid resembling a sphere, the sun’s rays fall 
obliquely at the top of the atmosphere and the earth intercepts a very 
small portion of the sun’s energy. 
? The solar output received at the top of the atmosphere varies slightly 
in a year due to the variations in the distance between the earth and 
the sun. 
 
SOLAR CONSTANT: 
The rate at which energy reaches the earth's surface from the sun. 
On an average the earth receives 1.94 calories per sq. cm per minute 
at the top of its atmosphere. 
 
NOTE: 
The effect of this variation in the solar output is masked by other factors 
like the distribution of land and sea and the atmospheric circulation. 
Hence, this variation in the solar output does not have great effect on daily 
weather changes on the surface of the earth. 
 
VARIABILITY OF INSOLATION ON EARTH SURFACE  
FACTORS: 
1) The rotation of earth on its axis 
2) Angle of inclination of the sun’s rays  
Page 2


 
 
 
INSOLATION 
? The earth’s surface receives most of its energy in short wavelengths. The 
energy received by the earth is known as incoming solar radiation 
which in short is termed as insolation.  
? As the earth is a geoid resembling a sphere, the sun’s rays fall 
obliquely at the top of the atmosphere and the earth intercepts a very 
small portion of the sun’s energy. 
? The solar output received at the top of the atmosphere varies slightly 
in a year due to the variations in the distance between the earth and 
the sun. 
 
SOLAR CONSTANT: 
The rate at which energy reaches the earth's surface from the sun. 
On an average the earth receives 1.94 calories per sq. cm per minute 
at the top of its atmosphere. 
 
NOTE: 
The effect of this variation in the solar output is masked by other factors 
like the distribution of land and sea and the atmospheric circulation. 
Hence, this variation in the solar output does not have great effect on daily 
weather changes on the surface of the earth. 
 
VARIABILITY OF INSOLATION ON EARTH SURFACE  
FACTORS: 
1) The rotation of earth on its axis 
2) Angle of inclination of the sun’s rays  
 
 
3) The length of the day  
4) The distance between the earth and the sun  
5) The transparency of the atmosphere  
6) Sunspots  
7) Land-Sea Differential 
8) Prevailing Winds 
9) Aspects of Slope 
10) Ocean Currents 
 
Rotation of the earth  
The variations in the duration of the day and angle of sun's rays at 
the different latitude . 
Angle of sun's rays  
The higher the latitude the less is the angle they make with the surface of 
the earth resulting in slant sun rays.  
The area covered by vertical rays is always less than the slant rays. 
 If more area is covered, the energy gets distributed and the net energy 
received per unit area decreases.  
Length of the day  
The longer the duration of day the larger is the amount of insolation . 
The length of day is always of 12 hours at the equator because the 
circle of illumination always divide the equator into two equal halves.  
 
Length of day increases poleward with the northward March of the 
sun in the Northern hemisphere while it decreases in the southern 
hemisphere in the meantime  
Page 3


 
 
 
INSOLATION 
? The earth’s surface receives most of its energy in short wavelengths. The 
energy received by the earth is known as incoming solar radiation 
which in short is termed as insolation.  
? As the earth is a geoid resembling a sphere, the sun’s rays fall 
obliquely at the top of the atmosphere and the earth intercepts a very 
small portion of the sun’s energy. 
? The solar output received at the top of the atmosphere varies slightly 
in a year due to the variations in the distance between the earth and 
the sun. 
 
SOLAR CONSTANT: 
The rate at which energy reaches the earth's surface from the sun. 
On an average the earth receives 1.94 calories per sq. cm per minute 
at the top of its atmosphere. 
 
NOTE: 
The effect of this variation in the solar output is masked by other factors 
like the distribution of land and sea and the atmospheric circulation. 
Hence, this variation in the solar output does not have great effect on daily 
weather changes on the surface of the earth. 
 
VARIABILITY OF INSOLATION ON EARTH SURFACE  
FACTORS: 
1) The rotation of earth on its axis 
2) Angle of inclination of the sun’s rays  
 
 
3) The length of the day  
4) The distance between the earth and the sun  
5) The transparency of the atmosphere  
6) Sunspots  
7) Land-Sea Differential 
8) Prevailing Winds 
9) Aspects of Slope 
10) Ocean Currents 
 
Rotation of the earth  
The variations in the duration of the day and angle of sun's rays at 
the different latitude . 
Angle of sun's rays  
The higher the latitude the less is the angle they make with the surface of 
the earth resulting in slant sun rays.  
The area covered by vertical rays is always less than the slant rays. 
 If more area is covered, the energy gets distributed and the net energy 
received per unit area decreases.  
Length of the day  
The longer the duration of day the larger is the amount of insolation . 
The length of day is always of 12 hours at the equator because the 
circle of illumination always divide the equator into two equal halves.  
 
Length of day increases poleward with the northward March of the 
sun in the Northern hemisphere while it decreases in the southern 
hemisphere in the meantime  
 
 
 
Distance between the earth and the sun 
Aphelion 
Perihelion 
Sunspots  
It is the energy radiated from the sun and therefore the amount of 
insolation decreases with the number of sunspots increases and vice 
versa. 
Effects of atmosphere  
The transparency of the atmosphere is determined by factors like 
cloud cover presence of particulate matter mist and haze . 
 
Land-Sea Differential 
Albedo of land is much greater than albedo of oceans and water 
bodies. 
Average penetration of sunlight is more in water – up to 20 metres, 
than in land – where it is up to 1 metre only. Therefore, land cools or 
becomes hot more rapidly compared to oceans. 
 
Prevailing Winds 
Winds transfer heat from one latitude to another. They also help in 
exchange of heat between land and water bodies. 
 
The oceanic winds have the capacity to take the moderating influence 
of the sea to coastal areas. 
 
Aspects of Slope 
Page 4


 
 
 
INSOLATION 
? The earth’s surface receives most of its energy in short wavelengths. The 
energy received by the earth is known as incoming solar radiation 
which in short is termed as insolation.  
? As the earth is a geoid resembling a sphere, the sun’s rays fall 
obliquely at the top of the atmosphere and the earth intercepts a very 
small portion of the sun’s energy. 
? The solar output received at the top of the atmosphere varies slightly 
in a year due to the variations in the distance between the earth and 
the sun. 
 
SOLAR CONSTANT: 
The rate at which energy reaches the earth's surface from the sun. 
On an average the earth receives 1.94 calories per sq. cm per minute 
at the top of its atmosphere. 
 
NOTE: 
The effect of this variation in the solar output is masked by other factors 
like the distribution of land and sea and the atmospheric circulation. 
Hence, this variation in the solar output does not have great effect on daily 
weather changes on the surface of the earth. 
 
VARIABILITY OF INSOLATION ON EARTH SURFACE  
FACTORS: 
1) The rotation of earth on its axis 
2) Angle of inclination of the sun’s rays  
 
 
3) The length of the day  
4) The distance between the earth and the sun  
5) The transparency of the atmosphere  
6) Sunspots  
7) Land-Sea Differential 
8) Prevailing Winds 
9) Aspects of Slope 
10) Ocean Currents 
 
Rotation of the earth  
The variations in the duration of the day and angle of sun's rays at 
the different latitude . 
Angle of sun's rays  
The higher the latitude the less is the angle they make with the surface of 
the earth resulting in slant sun rays.  
The area covered by vertical rays is always less than the slant rays. 
 If more area is covered, the energy gets distributed and the net energy 
received per unit area decreases.  
Length of the day  
The longer the duration of day the larger is the amount of insolation . 
The length of day is always of 12 hours at the equator because the 
circle of illumination always divide the equator into two equal halves.  
 
Length of day increases poleward with the northward March of the 
sun in the Northern hemisphere while it decreases in the southern 
hemisphere in the meantime  
 
 
 
Distance between the earth and the sun 
Aphelion 
Perihelion 
Sunspots  
It is the energy radiated from the sun and therefore the amount of 
insolation decreases with the number of sunspots increases and vice 
versa. 
Effects of atmosphere  
The transparency of the atmosphere is determined by factors like 
cloud cover presence of particulate matter mist and haze . 
 
Land-Sea Differential 
Albedo of land is much greater than albedo of oceans and water 
bodies. 
Average penetration of sunlight is more in water – up to 20 metres, 
than in land – where it is up to 1 metre only. Therefore, land cools or 
becomes hot more rapidly compared to oceans. 
 
Prevailing Winds 
Winds transfer heat from one latitude to another. They also help in 
exchange of heat between land and water bodies. 
 
The oceanic winds have the capacity to take the moderating influence 
of the sea to coastal areas. 
 
Aspects of Slope 
 
 
The direction of the slope and its angle control the amount of solar 
radiation received locally.  
 
Slopes more exposed to the sun receive more solar radiation than 
those away from the sun’s direct rays. 
 
Slopes that are devoid of direct sunlight are usually well forested. 
 
Ocean Currents 
Ocean currents influence the temperature of adjacent land areas 
considerably.