The energy that reaches Earth from the sun does so in the form of radiation — more specifically, electromagnetic radiation.
For many people, the word radiation immediately brings to mind nuclear accidents, weapons or dangerous invisible rays that cause illness.
In reality, radiation comes in many different forms. Some types are harmful to organic life, while others are completely harmless and, in fact, essential for life on Earth.
To understand this difference, we need to look at the electromagnetic spectrum.
The electromagnetic spectrum includes several familiar forms of energy.
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At the low-energy end are radio and television waves.
Near the middle lies visible light — the energy that allows us to see the world around us.
At the high-energy end of the spectrum are more dangerous forms of radiation, such as ultraviolet radiation, X-rays and gamma rays.
All of these forms of radiation are simply waves of energy, and the amount of energy they carry depends on the length of their wavelength.
Long wavelengths, such as radio waves, often measure around a metre in length and carry relatively little energy.
As wavelengths shorten, energy levels increase, moving through infrared and into the visible portion of the spectrum.
Visible light waves are extremely small, measuring roughly 400 to 700 billionths of a metre.
Travelling light
Given how hot the sun is, it might seem reasonable to assume that most of its energy would be emitted as high-energy radiation such as ultraviolet, X-rays or even gamma rays.
While the sun does emit energy across the entire electromagnetic spectrum, the majority of the radiation that reaches Earth arrives in the form of visible light.
This turns out to be critically important because visible light can pass through the atmosphere relatively easily and be absorbed at Earth’s surface.
One of the most remarkable properties of electromagnetic radiation is its ability to travel through the vacuum of space and reach Earth.
Once it arrives, that energy is either reflected back into space or absorbed by the surface and atmosphere, where it is converted into heat.
When we take Earth’s distance from the sun into account and calculate how much of the sun’s total energy actually reaches our planet, the result is surprisingly small, only about one two-billionth (1/2,000,000,000) of the sun’s total energy output. Despite this, the amount of energy added to Earth’s system is enormous.
On average, Earth receives approximately 1.7 × 10¹ watts every second, or 6.2 × 10² watts every hour, which is roughly equivalent to the total amount of energy used by all of humanity in an entire year.
This provides a natural jumping-off point for the next instalment of weather school, where we will explore insolation, the incoming solar radiation received by Earth, and introduce the concept known as the solar constant.
