What Causes the Different Seasons?

Even though the Earth does pass closer to the sun during part of its orbit, this is not what’s responsible for the hot summers and cold winters. If this were the case, then both the northern and southern hemispheres would have their summers and winters at the same time. However as you’ll see below, exactly the opposite is true. When it’s summer in the northern hemisphere, it’s winter in the southern hemisphere.

The Earth rotates about its axis once per day, but the Earth’s axis is tilted, 23.5° from vertical (Actually, the tilt varies from near 22° to 24.5°. The Earth wobbles a bit). For half the Earth’s orbit around the sun, the north pole is tilted towards the sun (summer) while the south pole is tilted away from the sun (winter). For the other half of the orbit, the south pole is tilted towards the sun (summer) while the north pole is tilted away (winter).

When the one of the poles is tilted away from the sun (winter), sunlight must pass through more atmosphere before striking the Earth’s surface. Therefore, less of the sun’s energy is absorbed by the ground and converted to heat which results in cooler temperatures. At the same time, the other hemisphere is tilted towards the sun. During this time, there is less atmosphere the sun’s energy must pass through and as a result, more of its energy reaches the Earth’s surface. The extra sunlight hitting the earth’s surface is converted to heat, which results in warmer temperatures.

Notice in the graphic to the left how the top arrow must pass through about 1/3 more atmosphere as compared to an arrow closer to the equator. This is due to the Earth’s axis being tilted away from the sun. All that energy is reflected back into space rather than being absorbed by the earth’s surface. It doesn’t looks like a lot, but it is.

The second affect the Earth’s tilt has on our seasons is the angle at which the sunlight strikes the surface of the Earth. The greater the angle, the more dispersed and less concentrated the sunlight is on that particular point. As an example, take a flashlight and shine it straight down. Most of the light is focused in a circle on the floor. Now, slowly rotate you wrist so the light shines at an angle. The same amount of light now covers a larger area and is a little less intense in that same spot. This is what happens to the sun’s energy as the Earth is tilted away or towards the sun. When the northern hemisphere is tilted away from the sun (winter) the sun is lower in the horizon resulting in a greater sun angle.

Also important is your location on the Earth as it plays a big role in how much of the Sun’s energy you receive on average and how much your seasonal temperatures may vary from winter to summer. Obviously, the north and south poles average the coldest temperatures anywhere on Earth. This is because they receive the least amount of the sun’s energy due to the extreme sun angle and the amount of atmosphere the sun’s energy must penetrate. In fact, during the winter, the sun never rises! Conversely, in the summer, the sun never sets. You might think that if the sun never sets, it would be incredibly hot, but you must remember at the poles the amount of atmosphere the sun must pass through is at its greatest.

As you move away from the poles and get close to the equator, your seasonal temperatures will vary less and the average temperature will be warmer year round. This is because there is less variation in the sun angle as the Earth orbits the sun and the amount of atmosphere the sun’s energy must pass through is less. This is why the tropics and other places near the equator never experience cold weather (excluding some mountainous regions).

One last thing. The Earth is closest to the sun on January 7th when it’s 91,399,726 miles away. The Earth is furthest from the sun on July 7th when it’s 94,508,727 miles away. The Earth is more than 2 million miles closer to the sun during the Northern Hemisphere’s winter!

Significant Dates

  • Summer Solstice: June 22nd. Longest day of the year.
  • Winter Solstice: December 22nd. First day of Winter. Shortest day of the year.
  • Vernal Equinox: Around March 21st. Marks the first day of Spring.
  • Autumnal Equinox: Around September 21st. Marks the first day of fall.

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