Why is the sun more intense at high altitude

The amount of heat energy received at any location on the globe is a direct effect of Sun angle on climate, as the angle at which sunlight strikes Earth varies by location, time of day, and season due to Earth's orbit around the Sun and Earth's rotation around its tilted axis. Seasonal change in the angle of sunlight, caused by the tilt of Earth's axis, is the basic mechanism that results in warmer weather in summer than in winter.Change in day length is another factor.

Presents a case when sunlight shines on Earth at a lower angle (Sun closer to the horizon), the energy of the sunlight is spread over a larger area, and is therefore weaker than if the Sun is higher overhead and the energy is concentrated on a smaller area.

It's commonly known that the sun is more intense at higher altitudes. However, contrary to popular belief, the sun is not more intense due to being closer to the sun, but the angle of incidence of the sun. The angle of incidence is steeper, and therefore the density of the sun's  radiant energy is concentrated to a high density in a smaller area. This makes the sun's energy more intense