National Weather Service – What Causes the Seasons?

Seasons Cause

This is the initial installment of ours. Ask a Scientist Series from the National Weather Service in Las Vegas. This presentation discusses what makes the seasons occur.

Earth has seasons because its axis is tilted. Earth rotates on its axis as it orbits the sun, but the axis always points in the same direction. Note the red arrow on the right side of the image. The earth’s axis of rotation is tilted 235 degrees from vertical with respect to the plane along which it orbits the sun.

On the day of the northern hemisphere winter solstice, the North Pole has tilted 235 degrees away from the sun. The sun shines more directly on the southern hemisphere and indirectly on the northern hemisphere. This tilt also results in shorter days this time of year for the northern hemisphere and it becomes cooler than the southern hemisphere which is experiencing summer this time of year. Three months later as the earth continues its counterclockwise orbit around the sun, on the northern hemisphere Vernal or Spring Equinox, both the neither north nor the South Pole is tilted toward the sun and the sun is directly overhead at the equator.

So, the sun shines equally on the northern and southern hemispheres. The number of daylight hours roughly equals hours of darkness. By the time we get to late June, the North Pole is now tilted toward the sun and the northern hemisphere receives more direct sunlight than the southern hemisphere and the days are also longer. As a result, the northern hemisphere becomes warmer than the southern hemisphere. This marks the summer season in the northern hemisphere and the winter season in the southern hemisphere.

In late September, on the northern hemisphere, Autumnal of Fall Equinox, once again the sun shines equally on both the northern and southern hemisphere and the days and nights have roughly the same duration. This image illustrates how the angle of the sun on the earth’s surface changes with the seasons at the mid-latitudes of the northern hemisphere. On June 21st the sun angle is very high and is 735 degrees above the horizon at solar noon. The sun does not have to travel through as much atmosphere and is intense.

On the shortest day of the year, December 21st, the sun is very low in the sky, reaching only 265 degrees above the horizon at solar noon. At this low angle, the sun must travel through more of the earth’s atmosphere and is much less intense than in late June. Hopefully, you now have a good understanding of what causes the change of seasons on earth.


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