Sky watch: Summer's seasonal temperature change

A change of seasons is coming in two weeks.

Officially, the summer solstice arrives this year on June 21 at 7:28 a.m. This time marks the exact moment when the sun reaches its farthest northward point over the Earth, and signals the onset of summertime in the Northern Hemisphere.

On this day, sky watchers will notice the sun rising farthest to the northeast, arching to its highest point around noontime and setting farthest to the northwest in the evening. From the other side of the Earth's equator - in the Southern Hemisphere - viewers now see the sun moving low across their daytime sky. For them, June 21 marks the onset of winter.

While it is true that our distance from the sun changes during the year, it is not this that causes our seasonal temperature variations. In fact, we're about 6 million miles farther from the sun during July than during January. No, our warming temperatures arise mostly from how long the sun remains in our daytime sky.

During the summer months, the sun appears higher in our sky. It rises in the northeast, swings very high overhead around midday, and sets in the northwest. In most places in the United States, the sun appears in the sky for about 15 hours or so and is below the horizon only about nine hours each night. This means that very little of the heat gained during the daytime can escape into space after dark, and temperatures naturally rise over time.

In the winter, most places in the United States see the sun only about nine hours a day. In other words, our atmosphere loses heat during the long nights faster than it can gain it during the short days.

A logical question might be this: Why, then, doesn't the year's hottest day occur on the first day of summer, when the sun is in the sky the longest? That's because our atmosphere takes time - a couple months - to respond to the sun's heat. Like a pot of water on the stove, it gradually heats up.

As a result, our hottest times of the year typically occur around August and September, while our coldest often come in January and February.