Insolation

The idea

Insolation, the Sun's radiation reaching the Earth's surface, warms the surface and air; because gases, clouds and dust absorb and scatter some sunlight, it is at most about 1 kW m⁻² under clear skies, below the solar constant.

Losing sunlight on the way down

At the top of the atmosphere sunlight is about 1.4 kW m⁻². On a clear day, we stand at the bottom of the air.

Will the ground receive more, equal or less than the top of the air?

• Less
• More
• Exactly the same

On the way down

Press Next step.

Insolation is smaller

Insolation is the radiation that actually arrives at the ground. It warms the surface and the air. Some sunlight was absorbed or scattered on the way, so insolation is less than the solar constant.

Insolation is the Sun's radiation that reaches the Earth's surface; it warms the surface and the air. Because gases, clouds and dust absorb and scatter some sunlight, it is at most about 1 kW m⁻² under clear skies, below the solar constant.

Notes

Insolation is sunlight reaching the surface. Gases, clouds and dust absorb and scatter some, so it is at most about 1 kW m⁻² on a clear day, below the solar constant.

Check yourself

What is insolation?

Why is the maximum insolation lower than the solar constant?

Which value is larger?

The solar constant is about 1.4 kW m⁻² and the maximum clear-sky insolation is about 1 kW m⁻². About how many kW m⁻² less is the insolation?

Answer: 0.4 kW m⁻²

• The Sun's radiation that reaches the Earth's surface — correct. Yes!
• The Sun's energy at the top of the atmosphere. That is the solar constant. Insolation is the radiation that reaches the surface.
• The heat that greenhouse gases trap. Insolation is incoming radiation from the Sun that reaches the surface.
• The solar constant counts sunlight after it is reflected back to space. The solar constant is the energy before any absorption, scattering or reflection.
• Gases, clouds and dust absorb and scatter some sunlight — correct. Yes!
• The surface is cooler than the atmosphere. The chapter gives absorption and scattering by gases, clouds and dust as the reason.
• Maximum clear-sky insolation, about 1 kW m⁻². Maximum insolation is about 1 kW m⁻², which is less than the solar constant.
• They are equal. They are not equal. Some sunlight is absorbed or scattered before it reaches the surface.
• The solar constant, about 1.4 kW m⁻² — correct. Yes!