‘You’re really going to fly around the Earth from North to South?’ said the young man to the aviator. ‘I’ll bet you’ll need your thermal underwear when you pass over the Poles!’

‘Actually,’ the intrepid airman replied, ‘the Poles are the least of my worries: I shall have to pass twice over a much colder area than that.’

What could it be?

Answer: The air above the Equator is much colder than the air over the Poles (because at that point there is a greater height of air and therefore the temperature can fall lower).