The earth receives most of its heat from the earth, not from the sun. In this lesson of the Windy.app Meteorological Textbook (WMT) and newsletter for better weather forecasting you will learn more about what exactly causes the atmosphere to heat up from the ground and how exactly this happens.
In our lessons, starting from this one on air temperature, we often repeat that 'the air is heated by the earth's surface heated by the sun'. But why do we put so much emphasis on the earth's surface, isn't the sun supposed to heat the air directly? Let's get to the bottom of this. Really, this is one of our most difficult letters. We'll try, as always, to explain everything in simple terms, but... anyway, write back to us in response to this letter — did we succeed in explaining it clearly?
First, briefly: The sun's rays heat both the earth and the atmosphere. But since the earth heats more, as a warmer body, it shares heat with the air.
Now, more details...
First, we will need to recall two things: what is light and what is heat.
Everything we see is made up of unimaginably small particles with different properties. Light is no exception. And particles of light behave like waves.
'Particles of light behave like waves,' what does that mean?
It means that when a particle moves, it oscillates, not jumping from one position to another, but as if it were simultaneously in all the places where the oscillation occurs. The distance between the extreme positions within which the particle-wave oscillates is called the wavelength.
This, of course, is a very rough simplification of complex quantum phenomena, but for now this explanation is enough for us.
Light particles with different energy (= different frequency of oscillation) have different wavelengths. The wavelength determines whether we can see this light and what color it has. We wrote about this in detail in the article 'Why the sky is blue'.
By the way, these particles of light are called photons. When a photon hits something, often that body can absorb the photon — and turn it into heat, that is, heat up. This is how we get heat from sunlight: photons reach us and are absorbed.
Everything around us, including ourselves, is made up of molecules. Heat is simply a measure of how fast the molecules within a substance move or oscillate. Heat is always transferred from a heated body to a cooler body, as a fast molecule accelerates a slower molecule with its energy when it collides with one.
Any body with a temperature above absolute zero (minus 273 degrees Celsius) emits photons, which are mostly invisible to the human eye, but become visible if the object reaches very high temperatures — like a red-hot metal, for example.
Air molecules are no exception. When heated, they also emit invisible photons that the surrounding objects can absorb — and thus become warmer.
The Sun emits light at very different wavelengths — all colors at the same time, and in such ranges that the human eye can not even see.
And different photons behave differently in the atmosphere. It's also due to the fact that the Earth's atmosphere consists of different particles: mainly atoms of nitrogen, oxygen, and ozone, but also water vapor and other atomized particles — aerosols.
Remember that photons vibrate? The particles that make up air also vibrate a bit, and in different ways (depending on the substance they are made of).
So. Air particles vibrate. Photons vibrate. The frequency of vibration determines how they will behave when they collide.
A photon flies away from the Sun. An atom, for instance, ozone, gets in its way. If the photon vibrates at the same or close to the frequency at which ozone vibrates, two things can happen. The first is absorption: the photon gets 'stuck' in the particle, and it absorbs its energy — in other words, it heats up. In general, about 23% of all solar radiation received by the Earth is absorbed by the atmosphere.
Ozone, for example, is the best in the atmosphere to absorb the most dangerous — ultraviolet — radiation from the Sun, and such photons almost do not reach the surface. This is why the importance of preserving the ozone layer is so often talked about.
The second thing that can happen is scattering: the vibrations of the photon are in resonance with the vibrations of the air particle. Then the atom itself starts to emit light in all directions around itself, namely light of the wavelength it has absorbed.
It is because of scattering that our sky is blue (because air particles scatter blue photons most noticeably). Overall, about 26% of all solar radiation is scattered.
But even this is not the end of the earth's surface loss in received photons: about 29% of all incoming sunlight is simply reflected back into space — and not only by the atmosphere but also by the planet's surface itself. This happens when the light and the particle it encounters vibrate at very different frequencies, and the structure of the material prevents the light from simply flying through it like through glass. We will not go into more detail here, this is a topic for a separate letter about the properties of mirrors.
Fresh snow reflects light best. Clouds reflect a little more than half of the light falling on them, dry sand — up to 40%, the world ocean — 5% to 20%. Soil, depending on its type, reflects from 5% to 30% of light.
But despite all these losses, including absorption, scattering and reflection, the earth still receives and absorbs up to 48% of all incoming sunlight.
Most of the light scattered in the atmosphere still falls on the earth — it helps.
It turns out that the sun manages to heat both the air and the earth, but the earth receives more solar heat and gets hotter than the atmosphere, as the surface of the earth receives more photons. And as a warmer body, the earth transfers its heat to a cooler one — to air particles. That's why we say that the air is heated by the surface of the earth.
Cover photo: Johan Mouchet / Unsplash