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.

90°

When insolation strikes the Earth at 90°

(perpendicular to the Earth surface), it has maximum intensity.

There are several factors that can affect

the intensity of insolation receives by a certain area on Earth’s

surface.

Factors that Affect the Intensity of Insolation

Earth’s Shape

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.

Sun’s Rays 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 During the Summer Solstice

(June 21st), the maximum intensity of insolation

(90°) strikes the Tropic of

Cancer – 23 ½ ° N

90°

Factors that Affect the Intensity of Insolation

Latitude During the Winter Solstice

(December 21st), the maximum intensity of insolation

(90°) strikes the Tropic of

Capricorn– 23 ½ ° S

90°

Factors that Affect the Intensity of Insolation

LatitudeDuring the Fall

and Spring Equinox (Sept. 23rd and March

21st ), the maximum intensity of insolation

(90°) strikes the Equator 0°

90°

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

Maximum insolation will occur for an area when

solar noon is reached.

Solar Noon

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 InsolationLatitude 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

Summer Solstice = 15 hrs. of daylightApparent Pathways of of the Sun from NY State

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, NYJune 2181°F

Westhampton Beach, NYAugust 393°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, NYDec. 2137°F

Westhampton Beach, NYFebruary 320°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.