Variation of Insolation
Insolation can vary
by changes in its
intensity and
duration.
Variation in Intensity and Angle of Insolation
Intensity of Insolation = The
rate at which solar energy is
received by a given area per
unit of time.
Insolation is measured in
joules/m/s.
There are several
factors that can affect
the intensity of
insolation receives by a
certain area on Earth’s
surface.
When insolation strikes
the Earth at 90°
(perpendicular to the
Earth surface), it has
maximum intensity.
90°
Factors that Affect the Intensity of Insolation
Earth’s Shape
Sun’s Rays
Earth is a sphere. Due to its shape, only
one area of the planet can receive
maximum intensity of insolation (Sun’s
rays = 90°).
All other areas of Earth’s surface would
receive less direct insolation.
90°
Factors that Affect the Intensity of Insolation
Earth’s Atmosphere
When the Sun is directly
overhead, the intensity of
insolation is greatest.
The Sun’s radiation passes
through a smaller area of the
atmosphere.
When the Sun is on the horizon
(dusk/dawn), the intensity of
insolation is smallest.
The Sun’s radiation passes
through a greater area of the
atmosphere. The radiation has a
stronger chance of being
reflected, absorbed or scattered in
the atmosphere.
Factors that Affect the Intensity of Insolation
Latitude
90°
During the
Summer
Solstice
(June 21st), the
maximum
intensity of
insolation (90°)
strikes the
Tropic of
Cancer
– 23 ½ ° N
Factors that Affect the Intensity of Insolation
Latitude
90°
During the
Winter Solstice
(December 21st),
the maximum
intensity of
insolation (90°)
strikes the
Tropic of
Capricorn
– 23 ½ ° S
Factors that Affect the Intensity of Insolation
Latitude
90°
During the Fall
and Spring
Equinox (Sept.
23rd and March
21st ), the
maximum
intensity of
insolation (90°)
strikes the
Equator 0°
Factors that Affect the Intensity of Insolation
Seasonal Change
Due to the Earth’s
tilted axis (23 ½ °)
and the its
revolution
around the Sun,
the maximum
intensity of
insolation will
vary between
23 ½ ° N to 23 ½
°S throughout the
year.
Factors that Affect the Intensity of Insolation
Time of Day
Solar Noon
Maximum
insolation will
occur for an
area when solar
noon is reached.
Factors that Affect the Duration of Insolation
Duration of
Insolation = The
length of time
insolation is
received within a
day.
Latitude and Seasonal Change
Factors that Affect the Duration of Insolation
Latitude and Seasonal Change
During the summer solstice, NY state receives
more direct insolation because the Sun is
higher in the sky. Likewise, the apparent path
the Sun takes from sunrise to sunset is greater.
Sun rises North of East and sets North of West
Apparent Pathways of of the Sun from NY State
Summer Solstice = 15 hrs. of daylight
During the winter solstice, NY state receives
less direct insolation because the Sun is lower
in the sky. Likewise, the apparent path the Sun
takes from sunrise to sunset is small.
Sun rises South of East and sets South of West
Winter Solstice = 9 hrs. of daylight
During the equinoxes, NY state receives more
insolation when compared to the winter
solstice, but less when compared to the
summer solstice. Likewise, the apparent path
the Sun takes from sunrise to sunset is greater
when compared to the winter solstice, but less
when compared to the summer solstice.
Sun rises dueEast and sets dueWest
Equinoxes = 12 hrs. of daylight
Relationship of Surface Temperature to Insolation
The temperature of the Earth’s surface at a given
location will change throughout the day ….as well as
throughout the year.
A rise in temperature will occur when the intensity
and/or duration of insolation received is greater than
the energy lost by the earth’s surface radiation.
Yearly Maximum and Minimum Temperatures
In Westhampton Beach, the maximum intensity and duration of
insolation is on June 21st – the Summer Solstice. Yet, the
maximum temperature here is not felt until late July or early
August.
Why???
Westhampton Beach, NY
June 21
81°F
Westhampton Beach, NY
August 3
93°F
It takes time for the change in balance between energy received from the sun (insolation) and energy lost from the
Earth (surface radiation) to occur.
After the summer solstice, the Earth continues to heat up due to a general increase in insolation. It isn’t until a
month later when the balance between incoming insolation and terrestrial radiation is the same.
The result? An increase in surface temperature!
Yearly Maximum and Minimum Temperatures
The same process occurs for when Westhampton Beach
receives its minimum temperatures.
Westhampton Beach, NY
Dec. 21
37°F
Westhampton Beach, NY
February 3
20°F
It takes time for the change in balance between energy received from the sun (insolation) and energy lost from the
Earth (surface radiation) to occur.
After the winter solstice, the Earth continues to lose heat due to a general decrease in insolation. It isn’t until a
month later when the balance between incoming insolation and terrestrial radiation is the same.
The result? An decrease in surface temperature!
Times of Daily Maximum and Minimum Temperatures
In Westhampton Beach, the maximum intensity of insolation
would be at solar noon – when the Sun is highest in the sky. Yet
the maximum temperature of the day would not be at solar
noon, but later in the afternoon.
Again, there is a need for a balance to be reached between
insolation and terrestrial radiation. It takes time for the Earth’s
surface to “heat up” after solar noon and for the surface
temperature to reflect this.