We’re taught hot air rises and we can see this in practice when a hot-air balloon climbs into the sky. So why does the air temperature plummet at greater altitudes? There are a number of variables that affect atmospheric temperature and the best known is solar radiation. This doesn’t heat the air directly though. Lapse rate describes the general decrease in atmospheric temperature with height, which occurs because the atmosphere is heated by conduction with the Earth’s surface. The farther you move from the surface, the less dense the air is and the more it struggles to retain heat. But the temperature doesn’t follow a unidirectional gradient. For example, while at 80 kilometres (50 miles) it can be -100 degrees Celsius (-148 degrees Fahrenheit), the air is much warmer at 115 kilometres (70 miles) due to ionising radiation.