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Insolation

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Insolation can be described as the incoming solar energy that reaches the Earth’s atmosphere and surface.  This energy is released from the sun in short waves and travels through space until it reaches the earth’s outer atmosphere (the Thermosphere) from where it can either be absorbed, reflected or pass directly through to the Earth’s surface.  Some absorbed radiation reaches the Earth’s surface as diffuse radiation but in total only 46% of the energy received at the outer atmosphere is received by the Earth’s surface.  This is known as the solar energy cascade but it is not constant over different periods of time nor is it constant for different locations across the globe.

Figure 1 below shows that the amount of Insolation received by the Earth’s surface does indeed vary.  At the Equator there is a year round gain of Insolation and this region gains the most Insolation of all of the locations on the globe.  The amount of Insolation at the Equator also varies little throughout the year. As you progress north and south of the Equator the amount of Insolation received by the Earth’s surface varies seasonally and decreases in quantity towards the Poles.  The Northern Hemisphere receives its maximum amount of Insolation between March and September, and for the Southern Hemisphere it is between September and March. There are times of the year when Polar Regions receive no Insolation at all.

The principle reason for these patterns is the way in which the Earth travels around the sun each year.  This is shown in Figure 2.

Earth's current orbit

The Earth is tilted at an angle of 23.5° relative to the sun and this has a marked impact upon the amount of Insolation received by different places and at different times of the year.  On December the 21st (Mr Gs birthday!) the North Pole is tilted away from the sun, and the maximum overhead sun is located at the Tropic of Capricorn.  Hence the North Pole receives no insolation at this time and the Tropic of Capricorn its maximum amount.  3 months later, the Earth has moved into a position where the maximum overhead sun is over the Equator, which receives the most insolation.  The Northern and Southern hemispheres are in spring and autumn respectively.  On June the 21st, the Earth has continued its journey so that the maximum overhead sun is over the Tropic of Cancer (23.5° N), and the Northern Hemisphere experiences its maximum amount of insolation received and summer. This accounts for the seasonality of Insolation, but what about the total amount received by different attitudes?

            The total amount of insolation received by the latitudes of the Earth does not vary a great deal, but the curvature of the Earth ahs a vital role to play in determining how much Insolation is received by different places.  Figure 3 below shows this.

As you can see above, because of the curve of the Earth a sunbeam of Insolation hitting the Earth at higher latitudes has to spread out over a larger surface area than one reaching the Equator.  Thus lowering the amount of Insolation per km2 in more Northerly and Southerly latitudes.  In addition, because the insolation hits the atmosphere at a greater angle in more northerly and southerly latitudes it has more atmosphere to pass through (despite the lower atmosphere being thicker at the Equator than it is at the Poles because of available heat energy).  The result of this is that at higher latitudes there is greater chance of scattering and absorption which lowers the amount of radiation received.

            The curvature of the Earth and its tilt also combine to affect the length of Day and Night in higher latitudes. As shown on Figure 3, in December the maximum overhead sun is over the Tropic of Capricorn and because of this and the Earth’s curve day length shortens in Northern latitudes and goes to 0 hours at the North Pole.  This means that during this period these areas receive very little Insolation during these short days.