Weather Imagery

Even though the Earth does pass closer to the sun during part of its orbit, this is not what’s responsible for our hot summers or cold winter months. If this were the case, then the northern and southern hemispheres would both 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.

The Earth rotates about its axis once per day. What you might not of known is that 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). This tilt virtually never changes and for half the Earth’s orbit around the sun, the north pole is tilted towards the sun while the south pole is tilted away from the sun. During this time the northern hemisphere has its summer and the southern hemisphere (which is tilted way from the sun) has its winter. For the other half of the orbit, the south pole is tilted towards the sun while the north pole is tilted away. This is when the northern hemisphere has its winter and the southern hemisphere has it’s summer. The tilt of the Earth has two primary affects which create our seasons.

When the north pole 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 southern hemisphere is tilted towards the sun (summer). 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 which in turn is converted to heat. This results in warmer temperatures. See the image below for an illustration. Notice how the top arrow representing the sun’s energy must pass through twice as much atmosphere as compared to an arrow closer to the equator. This is due to the Earth’s axis being tilted away from the sun.

The second affect the Earth’s tilt has on our seasons in 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. The Image below illustrates this point.

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 atmopshere 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 atmopshere the sun’s energy must pass through is less. This is why Hawaii, Florida, Mexico hardly ever see harsh winters (excluding mountainous regions). However, areas further away from the equator such as Montana, New York, and Canada will see lots of very cold wintery days.

So as you can see, the tilt of the Earth’s axis is the reason we have seasons. Not because the Earth orbits closer to the sun for one part of the year as opposed to the other.

Some interesting 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.