Sunlight at the top of the atmosphere creates the special gases that bend radio signals.

Ultra-violet light from the sun breaks up air molecules into particles called 'ions'.

It is these ions that can bend some radio signals.

The part of the atmosphere with the ions is called the 'ionosphere'.

The number of ions in the ionosphere is always changing. It depends on the amount of sunlight.

There are more ions during the day than at night. Also, there are more ions in summer than in the winter.

That is why radio signals can travel a greater distance in the daytime. They also travel more in summer than in winter.

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