HKDSE Geography/M7/Factors Affecting Insolation

Latitude

As one can see here, energy decreases with latitude:

Why?

Angle of the Sun

As one can see from the diagram below, if the sun strikes the ground at a small angle, the solar radiation will be spread across a large area. If the sun strikes the ground at a right angle, the solar radiation will be most intense (this is known as overhead sun):

Therefore, higher latitudes receive less intense insolation:

However, this difference in temperature varies with season because of the tilt of the earth's axis (at around 23.5°). When the earth revolves around, different areas will receive the largest angle of the sun:

• At spring equinox (21/22 Mar) and autumnal equinox (22/23 Sep), the overhead sun is at the equator. The equator is thus the hottest.
• At winter solstice (21/22 Dec), the overhead sun is on the Tropic of Capricorn. The Tropic of Capricorn is hottest. It is winter in the Northern Hemisphere and summer in the Southern Hemisphere.
• At summer solstice (21/22 Jun), the overhead sun is on the Tropic of Cancer. The Tropic of Cancer is hottest. It is summer in the Northern Hemisphere and winter in the Southern Hemisphere.

Daylight Hours

The longer an area is exposed to solar radiation, the more isolation on it has.

• At summer solstice, the Southern Hemisphere receives longer hours of daylight, and the Northern Hemisphere receives shorter hours. The Arctic Circle has 24 hours of daylight and the Antarctic Circle has none.
• At winter solstice, the Northern Hemisphere receives longer hours of daylight, and the Southern Hemisphere receives shorter hours. The Antarctic Circle has 24 hours of daylight and the Arctic Circle has none.
• The equator receives around 12 hours throughout the year.

Time of the Day

Since the earth rotates every day, different longitudes are exposed to the sunlight at different times of the day.

• The time when an area begins to be exposed to solar radiation is called sunrise.
• At noon, the amount of solar radiation is at its peak. This is the midday sun.
• After sunset, the area is no longer exposed to sunlight.

Atmosphere

Atmosphere X Latitude

Let's look again at the diagram we saw above:

Radiation at higher latitudes need to go through a longer distance in the atmosphere before striking the ground. Thus they lose more energy in the transfer process.

Cloud Cover

Areas with dense cloud cover have more reflection and scattering by clouds. Thus they have less insolation.

Aerosols

Aerosols are small particles in the atmosphere that reflect and scatter insolation. Areas with aerosols, which can be released by human activities or volcanic eruptions.