As it travels through the atmosphere of the Earth, the light from the Sun interacts with the molecules in the air and are scattered or absorbed, causing the radiation energy that ultimately reaches the ground to weaken. This effect is more significant in early morning or late afternoon than at noon because sunlight has to travel a longer distance in the atmosphere before reaching the ground (Figure 1) and, therefore, has a higher chance of being scattered or absorbed in the atmosphere. This is also why stars appear to be less bright at the horizon than above head.
In solar energy engineering, this effect is called the air mass, which defines the length of the sunlight’s path through the atmosphere (not to be confused with air mass in meteorology, which defines a volume of air that can be as large as thousands of square miles).
Why is the air mass important to predicting solar energy performance of a building?
Let’s consider a high latitude location like Boston. For a south-facing window, the Sun is lower in the sky in the winter, causing more solar energy to shine into the house, compared with the case in the summer. This is known as the projection effect (Figure 2). Does this mean that we get a lot more solar energy from the window in the winter like the cross-sections of the solar beams in Figure 2 seem to indicate? Not so fast.
In Boston, the day in the winter can be as short as 9 hours and the day in the summer can be as long as 15 hours. So even if the projection effect favors the winter condition, the duration of the day doesn’t. Our calculation must consider these two competing factors. After these considerations, the solar energy the window gains in 12 months is shown in Figure 3: It turns out that the window still gets more energy in the winter — the projection effect wins big!
OK, let’s now include the effect of the air mass in the calculation. Big surprise (at least to me when I first saw the results)!
Figure 4 shows that a south-facing window no longer picks up the highest amount of solar energy in the winter. Its solar gain peaks in the spring and fall and the difference between the summer value and the winter value decreases dramatically.
As comparisons, Figures 3 and 4 also show the results for a west-facing and a north-facing window, both peaking in the summer (for different reasons that we will not elaborate here).
Is this finding the universal truth? Not at all! It turns out that the peak solar gain of a south-facing window depends on the latitude. Figure 5 shows the comparison of four locations from Miami to the North Pole. In Miami, the energy gain peaks in the winter and declines almost to zero in the summer. In contrast, at the North Pole (to which anywhere else is south), the energy gain peaks in the summer and is nearly zero for almost six months. The situation of Moscow is between Boston and the North Pole, with the peaks moving more towards the summer and are less distinguishable.
Given Figure 5, the fact that a south-facing window in Boston receives peak solar energy in the spring and fall becomes comprehensible now — by the law of mathematics, the transition from the Equator to the North Pole must be smooth and the energy peak of any latitude in-between must be somewhere between winter (the season of peak energy in the Equator) and summer (the season of peak energy in the North Pole).
Interestingly enough, the peak energy at the North Pole is comparable to that at any other location in Figure 5. Considering that the Arctic has 24 hours of sun in the summer and the projection effect reaches maximum, this result is in fact a good demonstration of the air mass effect. Without the air mass, the North Pole would have gotten three times of solar energy as it does now, making the Arctic a tropical resort in the summer. Our planet would have been quite different.
All these analytic capabilities are freely available in our Energy3D software and all you need to do are some mouse clicks and some thinking. For the air mass calculation, you can choose to use the Homogeneous Sphere Model (default) or Kasten-Young Model. You can also turn the air mass off temporarily to evaluate its effect, just like what I showed in this article — this is a piece of cake in Energy3D but is impossible to do in reality because you cannot turn the atmosphere off!