What is it that actually heats the Earth?

A hundred and fifty million kilometres of vacuum

That is the distance between the Sun and the Earth, and it contains no air, no matter and no contact. Neither conduction nor convection can cross a vacuum. Yet the heat arrives, which proves the existence of a third mode of transfer.

Radiation, the only mode that needs nothing

Conduction requires contact, convection requires a fluid. Radiation travels with no medium, like light, and turns into heat at the moment it meets a surface that absorbs it.

What that changes about how we think of heating

Heating a room does not necessarily mean heating its air. Radiation directed at people and surfaces produces an immediate sensation of warmth, without having to set the whole volume in motion.

The demonstration everybody has already made

On a winter day, in the sun, the skin receives clear warmth while the surrounding air stays cold. Stepping into the shade removes the sensation in a second, without the air temperature having moved a tenth of a degree.

Every warm body radiates

Not only the Sun. A radiator, a wall, a ceiling, a human body all emit infrared radiation continuously. Perceived comfort therefore depends on the balance between what the skin receives and what it emits towards the surfaces.

Why a cold surface makes you cold

Because the body sends it more radiation than it receives back. That is the exact mechanism of discomfort near a window or under a poorly insulated ceiling, and it is not corrected by raising the air temperature.

The application to buildings is developed on the inspired by the sun page, and the principle on the passive heating page.

What is it that actually heats the Earth?

Most of us would answer: the heat of the sun.

• And yet the temperature at its surface does not exceed 6000 °C.
• It also sits 150 million kilometres away, across a vacuum, and a vacuum is an excellent insulator.

So the explanation has to lie somewhere else.

You will have noticed it in summer: with the sun overhead on a cloudless day, the sand is twice as hot as the air, even though the sun delivers the same energy to both.

How is that possible?

The sun's rays carry a form of light we call infrared. Light passes through the air around us, but it is stopped by matter, by sand or by skin, which absorbs it entirely.

These packets of energy interact very little with air. They excite the molecules and atoms that make up matter, and that agitation is what we feel as heat.

For most people, thermal comfort is a question of air temperature and of how that air is circulated.

Taking the sun as the model, two things follow:

1. heating matter uses half the energy of heating air,
2. and emitting infrared light from the ceiling warms the skin directly, with nothing in the way.

A radiant ceiling is the form of direct heating that comes closest to reproducing what the sun does.