The vast majority of energy in nature comes from the Sun. While the Earth itself does provide an amount of geothermal energy, everything else comes from solar radiation. How much solar radiation an individual location receives depends on a number of factors, including the angle of incidence of the Sun, cloud cover, composition of the atmosphere and the albedo of the surface itself. Of these, the angle of incidence is perhaps the most important as it is this that determines seasonal variation.
When energy comes from the Sun it has to first penetrate the atmosphere. Some energy is absorbed by the air or particles contained in the air; some energy is reflected back into space; some energy makes it all the way to the surface.
Different wavelengths of air are reflected and absorbed by different elements – this is why, for example, the ozone layer is so important as O3 particles are absorb over 90% of the Sun’s harmful ultraviolet wavelengths.
Depending on the angle of the Earth, the rays will have a different amount of atmosphere to pass through before hitting the surface. If the rays are coming from straight above i.e. the Sun is at meridian, then they have to pass through the minimum amount of atmosphere in order to penetrate through to the surface. This means that the minimum amount of energy will be absorbed by the rays.
If the rays are coming from an angle however, they will have to pass through a greater mass of air and more of the energy will be absorbed or reflected. This has the effect of lessening the amount of energy available at the surface from the Sun.
Other factors, such as cloud cover or the presence of volcanic ash or other particles in the air may also increase the absorptive or reflective effective of the atmosphere and decrease the amount of energy that gets through.
Surface energy is also a product of surface absorption. Snow has a high albedo and reflects a lot of energy back into the atmosphere – snowy areas have lower surface energy than rocky areas, which have a relatively low albedo.
As well as reflective/absorptive properties of the surface the angle at which the Sun’s rays strikes also plays a part. If a surface is tilted directly towards the Sun then the energy will hit it straight on and will be concentrated in the smallest possible area.
If the surface is tilted however, the area exposed to the Sun will appear smaller when looked at from the POV of the arriving rays. Because of this a smaller amount of rays will be spread over the area, reducing concentration dramatically. Lower concentrations mean lower surface energy.
Thus the angle of incidence of the Sun affects the surface energy levels in two ways – firstly it increases the amount of atmosphere that the Sun’s rays must pass through in order to reach the Earth; secondly it decreases the concentration of those rays when they reach the surface.
Uneven terrain will mean that the effects of the second factor are more local than the first, but overall these two effects account for why we experience the seasons of summer, autumn, winter and spring.