# Insolation

• It all starts with the Sun, where the fusion of hydrogen creates an immense amount of energy, heating the surface to around 6000°K; the Sun then radiates energy outwards in the form of ultraviolet and visible light, with a bit in the near-infrared part of the spectrum.
• By the time this energy gets out to the Earth, its intensity has dropped to a value of about 1370 W/m2 —as we just saw this is often called the solar constant (even though it is not truly constant — it changes on several timescales)
• Earth intercepts only one in two billion partsof solar radiation. This intercepted  radiation  is called
• Insolation is the proportion of solar energy received or intercepted by earth.
• Some heat within the core and mantle is transferred to the surface and ocean bottoms through volcanoes, springs and geysers. But this heat received at the surface form interiors of the earth is negligible compared to that received from sun.
• Earth receives Sun’s radiation (heat) in the form of short waves which are of electromagnetic nature.
• The earth absorbs short wave radiation during daytime and reflects back the heat received into space as long-wave radiation during night.
• The insolation is not constant over the surface of the Earth — it is concentrated near the equator because of the curvature of the Earth.
• But, the situation is complicated by the fact that the Earth’s spin axis is tilted by 23.4° relative to a line perpendicular to the Earth’s orbital plane, so that as Earth orbits around the Sun, the insolation is concentrated in the northern hemisphere (the northern hemisphere summer) and then the southern hemisphere (winter in the northern hemisphere). This tilt of the spin axis, also called the obliquity, is the main reason we have seasons.