1. Insolation and the Seasons

2. What is Insolation?
Insolation =
INcoming
SOLar
radiATION!!
Not to be confused with insulation, which is the stuff you put in your attic to keep your house warm in winter.

3. What is Insolation?
Energy received from the sun. It can be absorbed, reflected or reradiated.

4. Variations of Insolation
The intensity of insolation received is not the same at every point on Earth.
The reasons for this are:
1. Earth’s shape
2. Latitude changes
3. Seasonal changes
4. Time of day

5. Variations of Insolation
1. Earth’s Shape
Earth is a sphere, not flat, therefore at any given time, there is just one place on Earth getting direct (90º) sunlight.

6. Variations of Insolation
2. Change in Latitude
The intensity of insolation is greatest at the equator, since it always gets direct (90º) rays of sunlight.
With each degree in latitude you go up or down from the equator the intensity of insolation DECREASES.

7. Variation of Insolation
3. Seasonal Changes
The angle of insolation changes as the Earth revolves around the sun during it’s day cycle.

8. Notice when the sun is lower in the sky you will have a longer shadow.

10. Variation of Insolation
4. Time of Day
On any given day, the altitude (height) of the sun varies from zero (at sunrise) to a maximum (noon), back to zero again (at sunset).

11. By the way, wear sunscreen
During the spring and summer months, the peak sun burning times are 10 am to 4pm.
WHY?

12. Variation of Insolation
In addition to the angle of insolation changing, the duration of insolation changes as well.
The duration of insolation is the time between sunrise and sunset.

16. Seasons Video

17. Main Reason for the Seasons
Earth is tilted on it’s axis 23.5º

18. What Changes Do We Observe During The Seasons?
1. The sun’s altitude (height) changes with the seasons.
Highest = summer (June 21)
Lowest = winter (December 21)
NEVER directly overhead at our latitude!!

19. What Changes Do We Observe During Seasons?
2. Sunrise and Sunset positions change with the seasons.
South of E/W in fall and winter.
North of E/W in spring and summer.

20. Azimuth of sunrise changes with season
East
North of East
South of East
Dawn

21. What Changes Do We Observe During Seasons?
3. Daylight length – Duration of Insolation
Shortest on Winter Solstice, Dec. 21
12 hours on Equinox
Longest on Summer Solstice, June 21.

22. Reasons for the seasons animation

23. The Summer Solstice June 21 - longest daylight of the year.
Sun at highest altitude at noon.
24 hrs of daylight at North Pole.
Direct sun ray at 23° 30’ north latitude – also known as the Tropic of Cancer.
Most likely to get sun burned.
Sunrise = N of E, Sunset = N of W

24. The Winter Solstice Dec. 21 - shortest daylight of the year.
Sun at lowest altitude at noon.
24 hrs. of darkness at North Pole.
Direct sun ray at 23° 30’ south latitude – also known as Tropic of Capricorn
Not too much chance of sunburn, unless you ski.
Sunrise = S of E , Sunset = S of W

25. The Equinoxes Sept. 21 – Autumnal (Fall) Equinox
March 21- Vernal (Spring) Equinox
12 hrs of daylight, 12 hrs of night.
The word equinox means equal day and equal night.
Direct sun ray at Equator (0º)
Sunrise = E, Sunset = W. This is also called Due East and Due West.

26. Realize that: June and July get approximately the same insolation
May and August get the same insolation
April and September get the same insolation.
Wear Sunscreen.

27. Is Distance Important to Seasonal Change?
Farthest away on July 4, Closest on Jan. 3.
NO!
Earth’s orbit is an ellipse (oval).

29. Water with the appropriate temperature will only be found in tropical regions

30. What dates correspond to each figure?2
1 – December 21, Winter Solstice
2 – March 21, Spring (vernal) Equinox
3 – June 21, Summer Solstice
4 – September 23, Fall (autumnal) Equinox 3 1 4