Online Astronomy eText: The Sky
Climatic Zones Link for sharing this page on Facebook

General Concepts
      Each part of the Earth receives approximately the same number of hours of daylight per year -- namely, half a year of full daylight. At the equator, this is delivered in bits and pieces, at the rate of exactly half a day, every single day, throughout the year. At the poles, it is delivered all at once -- half a year of daylight, and then half a year of darkness. And at mid latitudes, it is delivered in greater or lesser amounts, throughout the year -- some days having more than half a day of daylight, but others, half a year later, having less than half a day, and the average, throughout the year, being half a day of daylight per day.
      However, although the different regions receive the same number of hours of daylight, they do NOT receive the same amount of sunshine, for the amount of sunshine which is received, per square foot of ground, depends upon how high the Sun is in the sky.
      When the Sun is nearly overhead, and its light shines straight down, one square foor of sunlight falls on one square foot of ground. But when it is lower, the light spreads out, as shown in the diagram below, so that each square foot of ground receives less than a square foot of sunlight. When the Sun is 30 degrees above the horizon, which is about the highest that it gets, for the Long Beach area, each square foot of ground receives only half a square foot of sunlight. And when the Sun is even lower, as when it is rising or setting, or for people near the Poles, where it is never much more than 20 degrees above the horizon, and averages only half that, the light is spread out by a factor of 3 or more.

When sunlight shines from overhead (on left), one square foot of sunlight falls on one square foot of ground. When it shines at a shallow angle (on right), each square foot of sunlight spreads out over many feet of ground.

      Because of this, regions near the poles, even when the Sun is up all the time, receive only a fraction of the sunlight and heat per day that equatorial regions receive in just the few hours when the Sun is nearly overhead. And, as a result, those regions which are near the equator on any planet, where the Sun is normally very high in the sky, and its light is not spread out, are relatively warm, or tropical, whereas those regions which are near the poles, where the Sun is low in the sky, and its light is considerably spread out, are relatively cold, or arctic.
      The tilt of a planet, and the seasons which result from that, and its distance from the Sun, do not change this zonal effect. On a planet which is far from the Sun, such as Neptune, the entire planet will be very cold, but regions which are near the poles should be a little colder, and regions near the equator, a little warmer. Similarly, on Mercury, the closest planet to the Sun, and which should be the hottest one, as far as solar heating alone is concerned, although it is very, very hot on the day side of the planet, ranging from almost 600 degrees above zero Fahrenheit at aphelion to more than 800 degrees above zero Fahrenheit at perihelion, those temperatures only apply, strictly speaking, to the areas near the equator where the Sun stands nearly overhead at local "noon".
      At Mercury's poles, on the other hand, the Sun's light is spread out over much wider areas, the ground temperature can be far lower. Someone standing in the sunlight, with the Sun's light directly hitting them, would quickly heat up to the same temperature, of many hundreds of degrees above zero, as applies near the equator. But if you were in the shade, so that you were only warmed by reflected sunlight and heat radiated by the surrounding surface, depending upon how nearly horizontal it was, and how close to the pole you were, the temperature might well be no more than a hundred or so degrees above zero Fahrenheit. (Note: this has nothing to do with the possible presence of water ice in deep craters near the poles, which never see the light of day at all, and have temperatures very close to absolute zero, or nearly 460 degrees below zero, Fahrenheit.)
      When you study how seasons work, you will find that the height of the Sun is also important there, as well, for when the Sun stands higher, and is up longer, we receive more sunlight on both accounts, and when it is lower, and up for a shorter period of time, we receive less sunlight on both accounts. However, for climatic zone purposes, we only consider the average amount of sunlight, which is related to the average height of the Sun, not the changing height that it has at different latitudes. Thus, at the poles, the Sun is, on the average, very close to the horizon, and its light spreads out over many square feet of ground, per square foot of incoming radiation. There are days when it is higher, and its light is not as spread out, and others when it is lower, and its light is more spread out, but on the average, it is always considerably spread out, anyway.
      So, to summarize, on every planet, regions near the poles should receive far less light and heat, on the average, than regions near the equator, and regions of intermediate latitude should receive intermediate amounts of light.

Defining the Boundaries of Climatic Zones
      Now, how can we define the boundaries of climatic zones? In other words, such parallels of latitude as the Tropic of Cancer, which defines the northern edge of the tropics on the Earth, and the Arctic Circle, which defines the southern edge of the northern polar, or arctic zone.
      One good way might be to define the arctic zones as extending perhaps a quarter to a third of the way from the poles to the equator, and the tropics as extending perhaps a quarter (22 1/2 degrees) to a third (30 degrees) of the way from the equator to the poles, so that the temperature zones, in between those, cover a third to a half of each hemisphere. And, as it happens, that is more or less the way that we do define the various regions, on the four planets whose axial inclinations, or tilts, are similar to the Earth. For what we do is to define the boundaries of the region according to the amount which the Sun moves north and south in the sky, during the course of the planet's year, which is equal to its axial inclination.
      In the case of the Earth, the tilt is about 23 1/2 degrees, so, going that far from the poles, which are at 90 degrees latitude, we define the position of the Arctic and Antarctic Circles as being 66 1/2 degrees North or South latitude. And going from the equator toward the poles, the Tropics of Cancer and Capricorn are 23 1/2 degrees North or South latitude. Using the same method, on Mars, where the tilt is about 25 degrees, the Arctic Circle would be at 65 degrees latitude, and the Tropics would be at 25 degrees latitude, and for Neptune, where the tilt is about 30 degrees, the Arctic Circle would be at 60 degrees latitude, and the Tropics would be at 30 degrees latitude.
      Of course, on a planet such as Neptune, where the temperature rarely reaches even 300 degrees below zero, let alone higher values, talking about "tropics" may seem a bit of a misnomer, but at least in theoretical terms, the regions near the equator should be somewhat warmer than those near the poles, and insofar as we can, or would want to talk about, such regions, it isn't at all unreasonable to consider them as extending to 30 degrees latitude, as pointed out, above.

Other Considerations
(to be added, later, but basically, in the tropics, the Sun is overhead at least once each year, and in the polar, or arctic regions, it never rises, or never sets, on at least one day during the year)
(also, a discussion of how things would work on a planet with 'no' tilt, like Mercury or Venus, or an extreme tilt, such as Uranus or Pluto, so that using the tilt doesn't give a reasonable size for the various zones)
(note to self -- put in diagram of globe with Arctic Circle/etc and Tropics? and, perhaps, a diagram showing the tilts of the various planets? or, perhaps a link to page on tilts/inclinations, including a definition of what those tilts represent?)