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 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 square metre. We need to use the average distance from the sun because Earth’s orbit is not perfectly round but is 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 km from the sun — 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.
One question that has 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 2.0 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 — that is a topic for future article.
Now we know Earth receives energy from the sun at a fairly constant rate, and if 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 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 the sun’s 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. Since 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. This leads us to the next topic, net global radiation, the seasons, and their impact on Earth’s energy balance.
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. We have surpluses of radiation in the equatorial regions, and a deficit poleward north and south of 36 degrees latitude. The areas with the greatest gains of radiation are over the Pacific and Indian oceans, right along the equator. The largest deficit of radiation is over Antarctica.
By looking at this simple picture of net global 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. Of course, it is not that simple; there are plenty of other items that we must look at to truly understand the big picture.
Seasons change
The first item to look at are seasons: spring, summer, fall and winter. Most of us have a basic understanding of what causes the seasons, but did you know there are five reasons for the seasons:
- Revolution,
- Rotation,
- Tilt,
- Axial parallelism, and
- Sphericity.
Let’s start with revolution, which is the Earth’s orbit around the sun. The Earth’s revolution, which takes 365.24 days, determines the length of each season. Secondly, we have the 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 approximately 24 hours to complete, we have 365 days in a year.
Next up is axial tilt. To picture this, imagine that the Earth is a spinning top that is 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 now means that 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 look at axial parallelism. What this means is that, while the Earth is a spinning top tilted to one side, it is always titled in the same direction. So, as the Earth revolves around the sun the tilt of the Earth remains in a constant direction, this means that for half of the year the southern part of our planet is pointed toward the sun and during the other half, the northern part is pointed towards the sun.
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.
Next issue we’ll finish up our look at the seasons and then begin our look at the composition of the atmosphere.
