How Do the Seasons Affect Insolation

explained was that the earth moves around the sun at an average speed of about 18.5 miles a second, and that on December 21 we are about 89.8 million miles from the sun, but 95.9 million miles from the sun on June 21. (This is because the sun isn't always in the exact center of our orbit.) Our childish logic might have told us that it should be the other way around: if we were closer in December, it should be warmer then — summer!

What we also may not have understood as very young children, but could grasp later on, was the fact that the earth's axis — about which we go "round and round" — is tilted 23.5 degrees from vertical. (What we may recall most vividly from those early astronomy lessons is that the earth's axis points directly at the north star.) This axial tilt — not our distance from the sun — is what causes changes in the seasons (Figure 13).

As we spin once a day, 365.25 times, we move in a path around the sun one time. (One revolution accounts for a year of what we call "time," and the fact that each year has an extra quarter of a day explains why we need a leap year once every 4 years.) For some reason, as we get closer to the sun, our speed slows down slightly and then quickens again as we get further away. So

Hour Angle Sunrise For Tilted Surface

Tilted Surface

Horizontal Surface

Figure 12. There is a tremendous difference between the energy that is received by a surface that is square to the sun's rays and one that is at an angle. This is why solar collectors that are far from the equator must be tilted to absorb the maximum amount of radiation for as many hours as possible during the day.

Years ago our parents or some elementary school teacher taught us that the earth goes around and around and that the sun shines on us from 93 million miles away. This is why we have day and night. What they may or may not have

Figure 13. The earth spins on a tipped axis as it moves around the sun. This is why we have changes in seasons. On June 21, although we are actually further from the sun, we are tilted toward it — that's if we live in the northern hemisphere. On December 21, we are tipped away from the sun, so its rays don't hit us directly.

Figure 13. The earth spins on a tipped axis as it moves around the sun. This is why we have changes in seasons. On June 21, although we are actually further from the sun, we are tilted toward it — that's if we live in the northern hemisphere. On December 21, we are tipped away from the sun, so its rays don't hit us directly.

Orbiting The Sun

we're not always orbiting the sun at the same precise rate of speed.

On June 21 — the summer solstice — we in the northern half of the world are tilted most toward the sun so it hits us most directly. This is the day when we receive sunlight for the longest amount of time. On December 21, we are tilted farthest away from the sun. Then we get the fewest hours of sunshine and those rays we do get hit us at a pretty flat angle.

On March 21 and September 21 — the spring and fall equinoxes — there are equal amounts of sunlight and darkness. The Tropic of Cancer (23.5 degrees north), that weird parallel on the globe that seems out of spacing with the other latitude lines, marks the point where the sun is directly overhead at noon on June 21. The Tropic of Capricorn (23.5 degrees south) describes the same path that most direct sunlight takes in the

How The Seasons Happen

Figure 14. The Tropic of Cancer indicates the northernmost point on earth where the sun can be directly overhead. This happens on June 21. On December 21 it is directly above the Tropic of Capricorn. This diagram shows the seasonal relationships between the sun and the earth. The normally recommended angle for a fixed solar collector is your latitude plus 10 to 15 degrees.

Figure 14. The Tropic of Cancer indicates the northernmost point on earth where the sun can be directly overhead. This happens on June 21. On December 21 it is directly above the Tropic of Capricorn. This diagram shows the seasonal relationships between the sun and the earth. The normally recommended angle for a fixed solar collector is your latitude plus 10 to 15 degrees.

southern hemisphere on December 21. When it is summer in the southern half of the world, of course, it is winter in the north (Figure 14).

That's how things look, theoretically at least, from outer space. But here on earth we're looking at it from the opposite end, which gives us a very different perspective. What we earth beings see is this: On June 21 the sun appears higher in the sky at noon than it does at any other time of the year. It is higher, in relation to the horizon, and this angle between the horizon and the sun is expressed as the sun's altitude (Figure 15).

Summer Solstice (June 21)

Summer Solstice (June 21)

East West

Time of Day

East West

Time of Day

Figure 15. We see more of the sun in June than we do in December. In the summer it is not only higher in the sky; it is there for more hours.

Figure 15A. The sun's position relative to the horizon is called its altitude. The sun's altitude is expressed in degrees, not in distance.

Summer 71 '/2 "June 21

Figure 15A. The sun's position relative to the horizon is called its altitude. The sun's altitude is expressed in degrees, not in distance.

Summer 71 '/2 "June 21

Sunrise
Figure 16. X marks the spot where the sun is directly overhead at four different times of year.

The sun is at its lowest altitude on December 21 — the winter solstice. On March and September 21 its position in the sky is midway between the other two extremes. We not only see less of the sun in December because it is around for fewer hours, its intensity is diminished because of its low altitude. This is why we have winter (Figure 16).

Don't be confused by insolation maps like the one in Figure 17. Here solar energy is being measured and described in "Langleys" rather than BTU's. The Weather Service is simply using the language of the metric system instead of the old English system. A Langley is the amount of heat it takes to heat one gram of water one degree centigrade. To convert Langleys per day to BTU's just multiply the number of Langleys by 3.69 (Table 7).

One more thing to keep in mind: Langleys are measured on a horizontal surface at any weather

Before you make a commitment to a solar heating system of any kind you should study how much sunlight falls on your area at all times of the year. Solar insolation maps that are even more accurate than U.S. Weather Service maps can be found in a book called World Distribution of Solar Radiation, by Smith, Duffie and Löf. For a copy of this title write to the University of Wisconsin, Engineering Experiment Station, 1500 Johnson Drive, Madison, Wisconsin 53706, and send them a check for $6.00. You may also get more specific climatic information from your nearest Weather Service office. And The Old Farmer's Almanac, although not always precise, is another good source of general weather data (Figure 17).

Langleys Per Day What Kind Map

Figure 17. This map, similar to one from the U.S. Weather Service, shows the average annual insolation in the continental United States. The figures are in Langleys per day, not in BTU's per square foot (1 Langley = 3.69 Btu/ft2). Other maps available from the Weather Service give a monthly breakdown of insolation.

Figure 17. This map, similar to one from the U.S. Weather Service, shows the average annual insolation in the continental United States. The figures are in Langleys per day, not in BTU's per square foot (1 Langley = 3.69 Btu/ft2). Other maps available from the Weather Service give a monthly breakdown of insolation.

station, meaning that a solar collector' angled directly toward the sun could be expected to pick up a great deal more energy at most times of the year than the insolation map suggests (Graph 2).

Before you decide whether or not a solar system is feasible, learn all you can about your local weather idiosyncracies — specifically how many cloud-free days there are each month or how smoggy it gets at various times of the year. (Smog reduces insolation by as much as 15 to 20%.)

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Responses

  • Vittoria
    How does seasons affect insolation received?
    8 years ago
  • liberato dellucci
    Do seasons affect solar radiation?
    7 years ago
  • Larisa
    How do the seasons happen?
    7 years ago
  • evelyn mackenzie
    How do seasons affect solar panels?
    7 years ago
  • Stefan
    How water affects the seasons?
    7 years ago
  • lorenza
    Where is the overhead noon sun on june 22?
    7 years ago
  • nereo
    How do seasons effect solar radiation?
    4 years ago
  • philipp
    How does the season affect the insolation?
    3 years ago
  • Thomas
    How does the time or season of the year affects insolation?
    3 years ago
  • bisrat
    What happens when we receive the most insolation on 21 December?
    3 years ago
  • giuseppina
    When does the northern.hemisphere receive the most insolation?
    3 years ago
  • azzeza
    How does seoson affect insolation?
    2 years ago
  • SILKE
    Does insolation and the sun's path mean the same thing?
    2 years ago
  • Alyssa
    How does insolation and incidence effect the seasons?
    11 months ago
  • milen
    How does latitude and season affect duration of insolation?
    10 months ago
  • GLEN
    How does latitude affect solar incolation?
    10 months ago
  • Asmara
    How seasons affects insolation?
    9 months ago
  • max
    How does ocean pollution affect solar insolation?
    3 months ago

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