Why is it cold when the sun is closer? Mountain tops are cold primarily due to the decrease in atmospheric pressure with altitude, which leads to a lower air density and reduced capacity for heat retention. Although higher elevations are slightly closer to the sun, the difference is negligible relative to the Earth–sun distance (~150 million km). As altitude increases, the thinner atmosphere contains fewer gas molecules to absorb and re-radiate infrared radiation from the Earth's surface. Additi