As we move toward the shortest day of the year — or rather, the least amount of daylight — a question that often comes up is “Why isn’t the third week of December the coldest time of the year, rather than the second to third week of January?” To begin our look at this topic we need to go way back and examine net global radiation and the seasons.

Net global radiation is the balance between incoming shortwave radiation from the sun and outgoing long-wave radiation from the Earth, as measured at the top of the atmosphere. The map below shows these global net radiation balances. Looking at that map you can see that, as expected, there is a very strong latitudinal component to energy imbalances. We have surpluses of radiation (the Earth gains energy) in the equatorial region, while there is a deficit poleward of around 36 degrees north and south latitude — that is, the Earth loses more energy than it receives over the year.

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 tends to be quite hot, so one 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 near the equator and strong deficits in the polar regions. Weather is a result of the Earth trying to even out this imbalance. Unfortunately, it is not as simple as looking at an average picture; there are plenty of other items that we’ll have to look at to really understand the big picture.

All the angles

The first thing that we will throw into the mix are seasons — no, not seasonings. While the weather has often been described as “cooking,” we are 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 only had to do with the tilt of the Earth!

Let’s first look at revolution, which is the Earth’s orbit around the sun. What does this have to do with the seasons? Well, the Earth’s revolution, which takes 365.24 days, determines the length of each season. What doesn’t impact our seasons is the Earth’s distance from the sun. On average, the Earth is about 150 million kilometres from the sun. At its closest point, or perihelion, the Earth is about 147 million km from the sun and this occurs around Jan. 3. The furthest point, or aphelion, occurs around July 4, when the Earth is about 152 million km from the sun. We are actually closer to the sun when it is coldest in the Northern Hemisphere!

Next up is the Earth’s rotation. Without our Earth rotating, the whole planet would basically have six months of daylight and six months of darkness — or even worse, we could be tidally locked to the sun with only one side of the Earth receiving sunshine. Due to our rotation, which takes approximately 24 hours to complete, we have 365 days in a year.

Along with the Earth’s rotation, we need to add in axial tilt. To picture this, imagine that the Earth is 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 one end of the Earth is pointing toward the sun, while the other end is pointing away from the sun. This explains why different parts of the Earth have differing amounts of daylight.

To tie this into the seasons, we need to take a look at axial parallelism. While the 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). As the Earth revolves around the sun, the tilt of the Earth remains in a constant direction. This results in the Southern Hemisphere pointing toward the sun for part of the year and the Northern Hemisphere pointing toward the sun for the other part of the year.

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, as light from the sun strikes different areas at a different angle.

Now we have an idea of why it is coldest in winter and warmest in the summer. The answer to the original question — why the coldest weather on average does not occur when we are receiving the least amount of radiation — is simply that during this period, we are consistently receiving a very low amount of radiation. This low level of energy continues into January and since we are losing much more than we gain, we continue to cool off. Eventually, the amount of solar radiation starts to increase to the point that we slowly begin to warm up, leading us to spring and summer.

Daniel Bezte is the weather columnist for the Manitoba Co-operator. His article appeared in the Dec. 13, 2018 issue.




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