Weather school: Five reasons for the seasons

Earth is a spinning top tilted to one side – and is always tilted in the same direction

In our previous Weather School class we looked at incoming solar radiation and the solar constant. With no new or exciting happenings in our local weather, we’re going to continue our look at how the incoming energy from the sun drives the seasons and weather we experience here on Earth.

Net global radiation is the balance between incoming shortwave radiation from the sun and outgoing longwave radiation from Earth, as measured at the top of the atmosphere. The map you see here, developed over a nine-year period, shows that, as expected, there is a very strong latitudinal component to the energy imbalances. We have surpluses of radiation (the Earth gains energy) in the equatorial regions, while there is a deficit poleward of around 36 degrees north and south latitudes (the Earth loses more energy than it receives in these locations).

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The areas with the greatest gains of radiation are over the Pacific and Indian oceans, right along the equator, whereas we see the largest deficit of radiation over Antarctica. Of interest are the negative values over the Sahara Desert, even though it’s in a fairly southern location and this tends to be a hot place, so you would assume it must receive a large amount of radiation. Well, it does, but due to predominantly clear skies and a light-coloured surface, it also loses a large amount of radiation.

Just looking at this simple average picture of global net radiation, we can see the basics of what causes most of the weather around the world. We have a surplus of energy in the equatorial regions, while we have strong deficits in the polar regions. Weather is a result of the Earth trying to even out this imbalance. Unfortunately, it’s not as simple as looking at an average picture; there are plenty of other items we have to look at to really understand the big picture.

The science of seasons

The first thing that we will throw into the mix are seasons — no, not “seasonings.” While weather has often been described as “cooking,” we’re talking seasons — you know — spring, summer, fall and winter? Most of us have at least a fundamental understanding of what causes the seasons, so here is your first little quiz: what are the five reasons for the seasons?

The reasons for the Earth experiencing seasons are revolution, rotation, tilt, axial parallelism, and sphericity – yikes! and I thought it had only to do with the tilt of the Earth!

Let’s first look at revolution, which is Earth’s orbit around the sun. What does this have to do with the seasons? Well, Earth’s revolution, which takes 365.24 days, determines the length of each season. Secondly, we have Earth’s rotation. Without our Earth rotating, the whole planet would basically have six months of daylight and six months of darkness. Due to our rotation, which takes about 24 hours to complete, we have 365 days in a year.

Along with Earth’s rotation, we need to add in axial tilt. To picture this, imagine Earth as a spinning top, doing a large orbit around the sun. Now, instead of picturing that top standing straight up and down, picture it leaning to one side, at an angle of about 23.5 degrees. This means that at one point in Earth’s orbit around the sun, the northern part of the planet is pointing toward the sun, while the southern end points away from the sun. This explains why different parts of the Earth have differing amounts of daylight at different times of the year.

To tie this into the seasons we need to take a look at axial parallelism. While Earth is a spinning top tilted to one side at an angle of 23.5 degrees, it is always (at least in our lifetime) tilted in the same direction (toward Polaris or the North Star). So, as Earth revolves around the sun, the tilt of the planet remains in a constant direction. In December, the southern part of the Earth is pointed toward the sun (summer in the Southern Hemisphere) while the northern part is pointed away (winter in the Northern Hemisphere). Six months later, Earth has revolved halfway around the sun, so now the northern part is pointed toward the sun (summer) and the southern part is pointed away (winter). Oh, and by the way, the distance between Earth and the sun does change a little bit over the course of the year, but this small change does not influence the seasons. Heck, we are closest to the sun in January, when it’s winter in the Northern Hemisphere.

Our final reason for seasons has to do with the fact that the Earth is a sphere. This results in an uneven receipt of incoming solar radiation. Areas around the equator receive more direct sunlight, while northern regions in the winter see the sun’s energy coming in at a steep angle, thus the incoming energy spreads out over a much larger area. In the summer, the sun is higher in the sky, which results in the energy being concentrated over a smaller area.

Next class we’ll continue our look at the seasons, then begin our look at the composition of the atmosphere — but before that, we’ll take a break from Weather School to do our monthly look back and ahead, and see if the long-range forecasts have changed.

About the author

Co-operator contributor

Daniel Bezte

Daniel Bezte is a teacher by profession with a BA (Hon.) in geography, specializing in climatology, from the U of W. He operates a computerized weather station near Birds Hill Park.

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