Heat on the move:radiation.

When you toast bread under a grill, some heat travels downwards, from the grill to the toast.

This heat can't be carried by convection. Convection carries hot air upwards.

The heat can't have been conducted to the bread either. The air between the bread and the grill is a poor conductor.

The heat is travelling by-radiation.

Earth-warming radiation.

The Earth is warming by heat radiation. This Comes from the Sun. (The sun also gives off other kinds of radiation, including light.)

Only a small fraction of the sun’s heat radiation reaches the earth. To get there, it travels through space. Since space has very few particles in it, this shows that heat radiation is not carried by moving particles.

When the heat radiation reaches the Earth, some of it is taken in, or absorbed. Some is bounced back, or reflected. It's the absorbed heat which warms up the Earth. The more heat absorbed, the hotter it gets.

Clouds can cut down the amount of heat radiation reaching the Earth. Cloudy days are usually cool days! That's because heat radiation travels in straight lines. The heat can't bend round the edges of the cloud to reach the Earth underneath.