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.
Because our atmosphere can affect the amount of the sun’s energy reaching the surface, scientists like to know how much energy is reaching the Earth at the top of the atmosphere. This insolation is called the solar constant.
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The solar constant is the average amount of insolation received at the top of the atmosphere when the Earth is at its average distance from the Sun and has a value of 1,361 watts per sq. metre.
We need to use the average distance from the Sun because Earth’s orbit is not perfectly round but rather slightly elliptical.
On average, the Earth is about 150 million kilometres from the Sun. At its closest point, called perihelion, the Earth is about 147 million kilometres from the Sun, which occurs around Jan. 3.
The furthest point, or aphelion, occurs around July 4, when the Earth is about 152 million km from the sun.
One question that keeps popping up is, just how constant is the energy output from the sun?
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 sq. metre.
There is no argument that even a fairly small change in the sun’s energy output can have big effects here on Earth, but that is a topic for a future article.
Now we know the Earth receives energy from the sun at a fairly constant rate, and if the Earth was a flat object pointing straight at the Sun, things would be pretty simple.
However, we are not flat. We are 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 that its rays hit perpendicular to the Earth’s surface, will receive the maximum amount of insolation.
The further away from perpendicular the sun’s rays are, the less insolation is received.
For example, the equatorial regions receive 2.5 times more insolation than at the poles.
If we looked at the amount of insolation received at the surface, we would find an even greater difference.
Because the polar regions have a low solar angle, the energy from the sun has to travel through much more atmosphere when compared to the equatorial regions. This longer path results in more energy being absorbed and reflected, resulting in even less energy making it to the ground.
I think we will have to stop the class here. In our next class, we will continue our look at solar radiation by examining global net radiation and exploring the reasons for seasons and their effect on our energy balance.
