Climate Introduction

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". Click here for a simple picture of latitude on Earth.

Seasons (Astronomical)

We 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 (see Figure 16). 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.

Intensity of Sunshine -- Solar angle at noon

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 (See Figure 15). 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 (See Figure 14). 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, which happens around June 21 each year, to 23.5°S Latitude (called the tropic of Capricorn) on the winter solstice, which happens around December 21 each year. 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. Other notable dates include, the spring equinox, which happens around March 21 each year, and fall equinox, which happens around September 21. On these days the noon-time sun is directly overhead at the equator. You should be able answer these questions:

1. How many days per year is the noon-time solar angle 0° in Tucson (32° N latitude)?
   (ANSWER: 0)
2. How many days per year is the noon-time solar angle 0° for a city located at 10° latitude)?
   (ANS: 2 days. This is the same answer for anywhere in the tropics, except right on the tropics of cancer and capricorn, where the noon-time solar angle is 0° only one day per year [solstice days]).

You should understand the following observations.

• In the northern hemisphere, outside of the tropics (i.e., north of 23.5° N latitude), the noon-time sun is most intense (smallest solar angle) on the summer solstice. The more days away from the date of the summer solstice, the less intense the solar heating at noon. The reverse is true in the southern hemisphere outside of the tropics.
• In the northern hemisphere, outside of the tropics (i.e., north of 23.5° N latitude), the noon-time sun is least intense (largestest solar angle) on the winter solstice. The more days away from the date of the winter solstice, the more intense the solar heating at noon. The reverse is true in the southern hemisphere outside of the tropics.

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 Physical Geography web site. Note the above web page uses "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 the Solar 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° toward the south, (b)32° toward the south, (c)55.5° toward the south)

Length of Day

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.

• The closer you are to the north and south pole, the greater the number of days with 24 hours of sunshine (and 24 hours of darkness). At 66.5° latitude, there is only one such day per year. At 90° latitude (north and south poles), there is sunshine 24 hours a day for half the year followed by 24 hours a day of darkness for the other half of the year. For example, there are more consecutive days per year with 24 hours of sunshine at 80° latitude than there are at 70° latitude.


• At the Equator (0° latitude), there are 12 hours of sunshine and 12 hours of darkness every day of the year


• On the spring and fall equinoxes, everywhere on Earth has 12 hours of daylight and 12 hours of darkness. This is why these days are called "equinoxes", equal day and night everywhere on Earth.


• In the northern hemisphere, the longest day of the year is the day of the summer solstice, and the shortest day of the year is on the winter solstice. The more days away from the date of the summer solstice, the shorter the day, and the more days away from the winter solstice, the longer the day. (This is reversed in the southern hemisphere).


• At any given latitude, the number of daylight hours changes slowly from day to day, following a seasonal cycle. For example, starting on the spring equinox in Tucson. The length of day increases a little each day from 12 hours to its maximum of about 14 hours on the summer solstice, then decreases a little each day to be 12 hours on the fall equinox. The length of day continues to decrease a little each day from 12 hours to its minimum of about 10 hours on the winter solstice. After the winter solstice the length of day begins to increase slowly again reaching 12 hours on the spring equinox.


• The further away from the Equator you move, the bigger the seasonal changes between the longest and shortest days of the year.

The last two observations are shown in these links: longest day at different latitudes and
length of day during year for different latitudes.

Seasons - (Popular Usage)

The above description of seasonal changes in the intensity and duration of sunshine are probably different from what you are used to. For example, the "summer season" is popularly defined as the season extending from the summer solstice (around June 21) through the fall equinox (around September 21). (See Declination and Northern Hemisphere Seasons) As we have seen, the day of maximum solar intensity (or maximum solar heating) in the northern hemisphere occurs on the summer solstice. You may wonder why summer solstice is not typically the warmest time of the year. The reason is that there is a lag between the maximum solar heating and the warmest time of the year. In the northern hemispere, the warmest temperatures generally occur near the end of July and the beginning of August, even though the maximum heating from the sun happens on the summer solstice (around June 21).

To understand the lag between maximum solar heating and maximum temperatures, you must consider our simple relationship between energy transfer and temperature that we covered in a previous lecture, which states that when energy input is greater than energy output, an object will warm. It takes some time for Earth's surface (land and ocean) to warm up from winter to summer. Even though the Sun's heating is most intense on the summer solstice, energy input to the northern hemisphere remains greater than energy output until about the beginning of August. Therefore, the warmest time of the year in the northern hemisphere does not happen on the summer solstice, but lags the summer solstice by just over one month.

By the way, there is also a lag each day between the time of maximum solar heating (solar noon) and the time of the high temperature, which usually happens in the late afternoon (2:30-4:30 PM). Even though heating is maximum at noon, energy input from the sun remains greater than energy output (mainly in the form of radiation energy lost by the Earth) until late afternoon each day (see this figure showing typical daily temperature changes taken from Ahrens, Essentials of Meteorology, third edition).

Similar arguments can be applied to understand the lag between the day of minimum solar heating in the northern hemisphere (winter solstice around December 21) and the coldest time of the year in the northern hemisphere which occurs around the end of January. In this case, the energy output from the northern hemisphere is greater than the energy input from the sun until the end of January.

Thus the commonly used "summer season" contains the warmest time of the year in the northern hemisphere and the "winter season" contains the coldest time of the year in the northern hemisphere, even though they do not correspond with the astronomical terms "summer solstice" and "winter solstice" (see figure 17).

EXAMPLE: For Tucson, compare the differences in solar heating, average temperature, and trends in average temperature on the Spring Equinox (March 21) with the Fall Equinox (September 21).

ANSWER: The solar heating is the same on both dates. The solar declination is located at the equator, so the solar angle at noon in Tucson is 32° south of straight up. In addition, there are 12 hours of daylight on each date. The average temperature is much warmer on Sept 21 compared with Mar 21 because Sept 21 is much closer to the time of highest temperatures (end of July / beginning of August) than Mar 21. (For Tucson ave high is 74°F on Mar 21 and 93°F on Sept 21.) The trend on Mar 21 is for increasing temperatures as time moves forward (from the lowest temperature of the year (end of Jan) to the highest (end of July)). The trend on Sept 21 is for decreasing temperatures as time moves forward (from highest at end of July to lowest at end of January).