Weather school: Insolation and the solar constant

Equatorial regions get about 2.5 times more incoming solar energy than polar regions

In my last article, we gently stuck our toes into the idea of going back to school with our first weather school class. In that class we discussed how the sun creates energy and then how that energy travels from the sun to Earth. Today we will start to look at how that energy is distributed around the Earth.

If we look at Earth’s distance from the sun and calculate how much of the sun’s total energy the Earth receives, we would discover we only receive about one two-billionths (1/2,000,000,000) of the sun’s total energy output. Even this tiny amount of the sun’s energy is an enormous amount of energy to be added to Earth’s system.

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When climatologists and meteorologists talk about incoming energy from the sun, we use the term insolation. This term is used whether we are talking about the sun’s energy arriving at the top of the atmosphere or at the surface of the Earth. Since our atmosphere can affect the amount of the sun’s energy reaching the surface, scientists like to know how much energy is reaching Earth at the top of the atmosphere. This insolation is called the solar constant.

The solar constant is the average amount of insolation received at the top of the atmosphere when Earth is at its average distance from the sun. We need to use the average distance from the sun because Earth’s orbit is not perfectly round, but is actually slightly elliptical. On average, the Earth is about 150 million kilometres from the sun. At its closest point, called perihelion, around Jan. 3, the Earth is about 147 million km from the sun. The farthest point, or aphelion, occurs around July 4 when the Earth is about 152 million km from the sun. This means it is not the change in distance from the sun that causes us to have seasons, as we are closest to the sun in the winter. More on this in the next issue.

Counting calories

Now back to the solar constant. The solar constant value is 1,372 watts per square metre. While this is the typical way of looking at the solar constant, another way of expressing it would be that it equals about two calories per square centimetre per minute. A calorie is the amount of energy needed to raise the temperature of one gram (that is, about one millilitre) of water by 1 C.

One of the bigger questions that has arisen lately is, just how constant is the energy output from the sun? While this is a hot topic, it is actually a very complicated one to figure out. The best estimates put the variability of the solar constant around 0.1 to 0.2 per cent, or about 1.2 to two watts per square metre. While there is no argument that even a fairly small change in the sun’s energy output can have big effects here on Earth, this small variation has little impact on the total energy reaching Earth. We will hopefully take a deep look at this topic in a future weather class.

So, we now know the Earth receives energy from the sun at a fairly constant rate. If the Earth was a flat object pointing straight at the sun, things would be pretty simple, but we are not flat, we are a sphere (and no, I am not going to get into an argument about flat Earth versus round Earth, the Earth is a sphere!), and this creates all sorts of problems.

Earth’s curved surface results in different parts of the Earth receiving different amounts of insolation. Areas of the Earth that have the sun directly overhead, so the sun’s rays hit perpendicular to the Earth’s surface, will receive the maximum amount of insolation. The farther away from perpendicular the sun’s rays are, the less insolation is received. On Earth, the equatorial regions receive 2.5 times more insolation than at the poles.

Keep in mind we are talking about the amount of insolation received at the top of the atmosphere. If we looked at the amount of insolation received at the surface, we’d find an even greater difference. Since the polar regions have a low solar angle, energy from the sun has to travel through much more atmosphere when compared to equatorial regions. This longer path results in more energy being absorbed and reflected, resulting in even less energy making it to the ground.

Well, I think we will have to stop the class here. In our next class we’ll continue our look at solar radiation by examining global net radiation and exploring the reasons for seasons and their effect on our energy balance.

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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