The solar constant
How much solar energy arrives at the top of the air.
Counting the Sun's energy
Scientists want one number for the energy the Sun sends to the Earth, measured before the air changes it.
Where do you think this number is measured?
What this lesson covers
The idea
The solar constant, about 1.4 kW m⁻² (1400 J s⁻¹ m⁻²), is the average solar energy received per unit time per unit area perpendicular to the Sun's rays at the top of the atmosphere, before any absorption, scattering or reflection.
Counting the Sun's energy
Scientists want one number for the energy the Sun sends to the Earth, measured before the air changes it.
Where do you think this number is measured?
- At the top of the atmosphere
- On the ground in a city
- Under the ocean
Measure at the top
Press Next step.
A number before the air
The solar constant tells us how much energy the Sun gives to a square metre facing it, at the top of the atmosphere. The air has not yet absorbed, scattered or reflected any of it.
The solar constant, about 1.4 kW m⁻² (1400 J s⁻¹ m⁻²), is the average solar energy received per unit time per unit area perpendicular to the Sun's rays at the top of the atmosphere, before any absorption, scattering or reflection.
Notes
The solar constant is about 1.4 kW m⁻² (1400 J s⁻¹ m⁻²): solar energy per unit time per unit area perpendicular to the rays, at the top of the atmosphere.
Check yourself
Where is the solar constant defined?
1.4 kW m⁻² is the same as how many J s⁻¹ m⁻²?
Answer: 1400 J s⁻¹ m⁻²
Taking the solar constant as 1400 J s⁻¹ m⁻², how many joules reach 2 m² facing the Sun's rays in one second at the top of the atmosphere?
Answer: 2800 J
The area used for the solar constant is …
- On the ground on a clear day. The solar constant is the energy before the atmosphere absorbs, scatters or reflects any of it.
- At the top of the Earth's atmosphere — correct. Yes!
- At the bottom of the ocean. It is defined at the top of the atmosphere, before the air has changed the sunlight.
- Parallel to the Sun's rays. The solar constant is measured on an area perpendicular to the rays, facing the Sun straight on.
- Tilted so that the rays slide along it. The solar constant uses an area perpendicular to the Sun's rays.
- Perpendicular to the Sun's rays — correct. Yes!