All locations on earth except latitudes near the equator experience seasonal temperature changes. Generally the farther a point is from the equator, the greater those changes are. The changes are a consequence of the earth's orbital motion about the sun, coupled with the tilt of earth's axis of rotation with respect to its orbital plane. These combine to produce a change in the daily amount of radiative energy a location receives from the sun. There are three reasons that the earth's orbital motion causes the daily amount of energy a location receives from the sun to vary:
Variations in solar altitude (the sun's angular distance above the horizon): For example, at Provo's latitude (40°15' N) the rate of noontime solar energy reaching the top of the atmosphere is 2.16 times greater on the date of the summer solstice than on the date of the winter solstice (at the surface the ratio is even larger because a lower sun requires the sunlight to pass through more atmosphere to reach the surface causing greater atmospheric attenuation). The ratio is greater than 2.16 at other times of day, generally increasing as the time separation from apparent noon increases.
Variations in the length of the day: At Provo's latitude the sun spends about 15 hours above the astronomical horizon on the date of the summer solstice, compared with 9 hours on the date of the winter solstice, a ratio of 1.67.
Variations in the distance to the sun: The sun is about 3% closer at perihelion (~January 3) than at aphelion (~July 4). This means the rate at which solar energy reaches the earth is about 6% greater near the northern winter solstice than near the northern summer solstice. This tends to make northern seasonal changes more moderate and southern changes more extreme. (However, because the southern hemisphere is more ocean-dominated than the northern hemisphere [~80% to ~60%], southern seasonal temperature fluctuations are smaller than northern seasonal fluctuations.) Because of precession, this difference oscillates back and forth between favoring the southern hemisphere and favoring the northern hemisphere, with an oscillation period of about 26,000 years. (This is the only variation of the three which occurs on the equator. It is small enough and of short enough duration that it has virtually no effect on equatorial temperatures.)
