As I sit here in the Clime-On World Head Quarters, watching rain pour off the roof and snow clog our beloved Sinks Canyon, it is hard to imagine that the hot days of summer are just around the corner. However, there is no escaping our position in this solar system, and because of those astronomical facts, summer will come. It is as inevitable as your project getting down rated in the next guidebook.
Heat, as defined by scientists, is the transfer of energy. If you want to get real technical about it, there are a lot of big words and elaborate descriptions to go with this concept, but in essence, heat in our atmosphere is the transfer of energy from the Sun to Earth. The Sun’s rays vary in intensity as they strike the planet, and this varies the amount of heat different places on Earth receive. There are thousands of factors that go into creating the weather we live with, but none are more influential than the angle of the Sun’s rays as they hit Earth.
The angle that the rays contact Earth varies the amount of energy the planet actually absorbs. This has to do with the shape of the Earth, a sphere, and how the straight line of the Sun’s rays makes contact with that sphere. The Earth revolves around the Sun at a tipped angle of 23.5 degrees, so the Sun’s rays hit a given spot on the planet at different angles on different days of the year. At what is the winter solstice in the Northern Hemisphere, on December 21st/22nd, it is the summer solstice in the Southern Hemisphere and the Sun is directly overhead at 23.5 degrees south, roughly the latitude of Rio de Janeiro. This means that at that latitude, at midday, the Sun’s rays are shining straight down on the planet and are most concentrated. Not only are there more rays, but their energy filters through less atmosphere before reaching the surface of the Earth.
During the northern hemisphere’s winter, the intensity of the radiation from the Sun heats the atmosphere in the southern hemisphere to give that half of the globe its warmest months. The angle of the Sun’s rays are more direct in the south and create summer, while they are less direct in the north and allow for winter. We are in our winter when the southern hemisphere is in its summer, and visa versa. Thus on June 21st/22nd, what we call the summer solstice, the Sun is overhead at its most northern position, and the northern hemisphere is in its summer.
On a daily basis, our understanding of how the Sun’s rays heat an area is intuitive. We all know that on a hot day, at mid-afternoon when the Sun is more directly overhead, it’s far more comfortable to be in the shade. However, at sunset on that same hot day, we can sit on the beach and enjoy the Sun. This is because the angle of the suns rays means we are being hit by fewer rays at sunset, and those rays are being filtered by far more atmosphere than they are when the Sun was overhead.
These diagrams, which are not to scale, show the way the Sun’s rays are affected by the spherical shape of the Earth. Note that in position A the distance between the Sun’s rays is the same. The rays travel in straight lines and when they hit a spherical object they are affected differently depending upon where they reach the sphere. Comparing the rays hitting the Earth at the equator to those striking near the poles, we can see that at B they pass through more atmosphere. We can see that at C they strike the surface of the Earth at a lower angle. Position D shows that though the space between rays is equal when they leave the Sun, they strike Earth with less intensity (farther apart) at the poles than at the equator. In short, the higher latitudes receive less energy from the Sun.