Why does the weather change from season to season?

Chicago area closets have outfits for all kinds of weather — swimsuits and T-shirts for hot days, and heavy coats and scarves for chilly ones. That's because our weather shifts dramatically throughout the year. But what actually causes those seasonal changes?

A common guess is that Earth is closer to the sun during summer and farther away during winter. While that may sound reasonable, it's not the real explanation. Seasons exist because of the way Earth spins and tilts.

As our planet travels around the sun, it rotates like a spinning top. Long ago, something massive collided with the young Earth, knocking its spin a bit sideways. Instead of standing perfectly upright, Earth is tilted 23.5 degrees to one side. That tilt is the key.

When Earth moves through certain parts of its orbit, the tilt causes one half of the globe to lean toward the sun. That side gets more direct sunlight and heats up, creating summer in that hemisphere.

Half a year later, the situation flips: the Earth has the same tilt, but is on the other side of the sun, exposing the opposite half of the planet to the sun's stronger rays. That's when the other hemisphere experiences its summer.

Since only one hemisphere tilts sunward at a time, the northern and southern halves of Earth have opposite seasons: summer in one and winter in the other.

And that's why your closet has to keep everything from sandals to snow gear at the ready.