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The Earth contains many different climate types. From the teeming tropical jungles to the frigid polar wastelands, there seems to be an almost endless variety of climatic regions. There are many factors that determine or control the climate in any given place. Perhaps the most obvious and strongest influencing factor is the amount of solar energy input from the sun. The two main influences are the intensity of sunshine and length of day. These vary with latitude and time of year giving rise to what are termed "seasonal changes".
I will use the term "seasonal changes" to mean changes during a calendar year in (1) the intensity of sunshine received and (2) the length of daylight hours at a given location (latitude). The severity of seasonal changes is not the same at all latitudes. Seasonal changes at the equator and throughout the tropics are minor. As you move further away from the Equator, seasonal changes become more severe, and the largest seasonal changes occur at the north and south poles and throughout the arctic and antarctic regions. The explanation for seasonal changes is based on the geometry of Earth's yearly orbit about the Sun relative to the Earth's daily rotation. Surprisingly, most people in the United States (including college graduates) cannot explain it.
If you trace the path of the Earth as it orbits the Sun, it defines a plane called the ecliptic plane. Don't be scared by the terminology. All this means is that the Earth remains in a plane as it orbits around the sun. Each time around takes one year (365 days). The second thing we need to define is the Earth's axis of rotation. The axis of rotation is an imaginary line which extends through the Earth's north and south poles. The Earth spins (or rotates) about this axis, making one complete rotation each day (24 hours). Seasons on Earth occur because the axis of rotation is not perpendicular to the ecliptic plane. Currently, it is 23.5° away from perpendicular. This is what is meant by the statement: "the Earth is tilted on its axis". To understand how this results in seasonal changes, you must realize that the orientation between the ecliptic plane and the axis of rotation does not change as the Earth orbits the Sun. I suggest that you check out the following Earth revolution animation.
Because the Sun is so far away from the Earth and the Earth intercepts only a tiny fraction of all the radiation energy coming from the Sun, it is a fairly good approximation to say that all the energy from the Sun is coming from a single direction. Since the Earth is a sphere this means that at any given instant, there is only one point on the Earth where the Sun's rays are striking perpendicular to the surface. In other words, there is only one place where the Sun is directly overhead or straight up. The further you move away from this one point, the further away you need to look from straight up to see the Sun. In this class, we will use the term solar angle to define the angle between the direction to the Sun and straight up. Therefore, when the Sun is straight up, the solar angle is 0°, and when the Sun is on the horizon, the solar angle is 90°.
Solar angle is important because the energy received from the Sun is most intense where it hits perpendicular to the surface (solar angle = 0°). As the solar angle gets larger, the energy received from the Sun becomes less intense. Again, don't be confused by the terminology. We experience this effect each day. Think of how "strong" the Sun feels at sunrise (when the solar angle is large), compared to noon (when the solar angle is much smaller). At any day and location Solar Noon is defined as the exact middle of the daylight period or when the sun is halfway between sunrise and sunset. At solar noon, the Sun is aligned with true north and south. Note that solar noon does not necessarily occur at 12 PM local time. No matter where you are on Earth, solar noon is when the solar angle reaches its daily minimum. Outside of the tropics, you know that the noon-time sun "feels stronger" in summer as compared to winter. Therefore, the solar angle must be smaller in summer as compared to winter. This seasonal change results from the geometry of Earth-Sun orientation described above. The changes of sun angle with time of year and latitude are quantified below.
The solar declination or sub-solar point is defined as the LATITUDE at which the noon-time sun is directly overhead (solar angle at noon is 0°). This happens at only one latitude each day. The solar declination changes slowly from day to day and ranges from 23.5°N Latitude (called the tropic of Cancer) on the summer solstice to 23.5°S Latitude (called the tropic of Capricorn) on the winter solstice. This latitudinal boundary defines the tropics, which is the region between 23.5°N Latitude and 23.5°S Latitude, where the noontime sun is directly overhead at least one day per year. You should be able answer these questions:
It is simple to compute the solar angle at noon for any latitude and any day of the year (and you will have to do this on the exam). You will be given the solar declination and your latitude. The solar angle is simply equal to the degrees of latitude between your latitude and the latitude of the solar declination. If your latitude is North of the solar declination, then the noon-time sun will be located toward the South, and if your latitude is South of the solar declination, then the noon-time sun will be located toward the North. Much of this is decribed using digrams on this Department of Geography, Okanagan University web page. Note that they use "Solar altitude" to define the sun angle at noon, which is the angle between the ground and the sun. This is different than our definition of Sun Angle, which is the angle between the Sun and the straight up direction. The relationship between the two is given by: (Solar Angle) = (90°) - (Solar Altitude). So a "Solar altitude" of 90° corresponds to our "solar angle" of 0°.
Example. What is the noon-time solar angle in Tucson (latitude = 32°N) on (a)Summer solstice,
(b)Spring Equinox, and (c)winter solstice?
(ANS: (a)8.5 deg& toward the south, (b)32 deg& toward the south,
(c)55.5 deg& toward the south)
The second factor which regulates seasonal changes is the length of daylight. I'm sure most of you know that the number of hours of daylight is longer in the summer as compared to the winter. More hours of daylight translates to more hours of heating from the sun. Therefore, regions outside of the tropics receive more intense sunshine in summer as well as more hours of sunshine. As with solar angle at noon, there are mathematical formulas which one can use to calculate the number of daylight hours at any latitude for any day of the year, but they are complex, and we will not use them.
On the summer solstice, all places north of 66.5°N latitude (called the Arctic circle) have 24 hours of sunshine, while all places south of 66.5°S latitude (called the Antarctic circle) have 24 hours of darkness. (See animation). Converselsy, on the winter solstice, all places north of 66.5°N latitude (called the Arctic circle) have 24 hours of darkness, while all places south of 66.5°S latitude (called the Antarctic circle) have 24 hours of sunshine. (See animation). In fact, this is the defines the Arctic and Antarctic regions, which are regions between 66.5° and 90° latitude where there is at least one day with 24 hours of sunshine and one day with 24 hours of darkness each year.
The following observations about the length of day are a bit harder to show with diagrams.