The Sun emits energy in the form of electromagnetic radiation. We see this radiation as light, feel it at infra-red wavelengths as 'heat' and become tanned by its ultraviolet rays. Other radiation is also emitted as X-rays and radio waves. All of these are electromagnetic waves and are part of the electromagnetic spectrum. The property of these waves that differs is the wavelength (different wavelengths are in different parts of the spectrum). X-rays have a very short wavelength, radiowaves have a very long wavelength. The proportion of each type of radiation emitted by the sun is shown in the diagram below.

A = Angstrom unit (10^-10m)

As can be seen, the energy spectrum is peaked in the visible region (i.e.the largest portion of the sun's light is visible). But there are also significant amounts of ultraviolet and infrared. The atmosphere absorbs wavelengths containing long radiowaves, x-rays and some infra red and ultraviolet.

The diagram below shows the radiation received per day in different latitudes and during different times of the year at the outer edge of the atmosphere (before absorption). This would be the radiation arriving at the Earth's surface were there no atmosphere.

Units are in Langleys per day (1 Langley=1 cal/cm²)
Diagram is for the Earth/atmosphere system

Note the Sun appears to swing between the tropics but this is purely a consequence of the tilt of the Earth's axis of rotation. At the vernal and autumnal equinoxes every where has equal day and night and the Sun is overhead at the equator.

The angle of the Earth's surface with respect to the sun means that the amount of solar radiation for a given area of the Earth's surface (insolation) is less at higher latitudes.

This is countered during the summer months by the 24 hour sunshine at the poles.

The tilt of the Earth's angle of rotation means that the length of day varies with latitude - shortening towards the poles in winter and lengthening towards the poles in summer. For the northern hemisphere, midwinter occurs on Dec 21st, in the southern hemisphere, midwinter occurs on Jun 21st. The net effect of the tilt of the earth's axis is that it gives a seasonal variation in insolation and ultimately leads to an energy defecit at high latitudes. The diagram below shows the variations in the Earth's yearly average heat budget with latitude.

Note that at the poles, more radiation is emitted at the poles than is absorbed. This is due to the reflection of radiation from high albedo (eg. white snow) surfaces coupled with the glancing angle at which the radiation encounters the surfaces. At lower latitudes, more heat is absorbed than is emitted due to the large amount of cloud acting as an insulator. As there is no overall temperature change there must be a overall transfer of energy between low and high altitudes.