At the poles, the troposphere reaches a height of around 8 km, with the temperature of the tropopause around 40 C in summer and 60 C in winter. Therefore, despite the higher surface temperatures, the tropical tropopause is much cooler than at the poles. In contrast to the troposphere, temperatures in the stratosphere rise with increasing altitude. This is greatest around 50 km, which is where the stratopause occurs. As well as a noticeable change in temperature, the move from the troposphere into the stratosphere is also marked by an abrupt change in the concentrations of the variable trace constituents.
Water vapour decreases sharply, whilst ozone concentrations increase.
Despite the dryness of the stratosphere, some clouds have developed in winter months over high latitudes at altitudes between 17 and 30 km, stretching into the stratosphere. They generally display iridescence and are known as nacreous clouds. 2.3 Mesosphere Temperatures in the mesosphere decrease with height from the stratopause up to the mesopause, at around 85 km. Temperatures at the mesopause vary from as low as 120 C at high latitudes in summer to 50 C in winter. The cold summer temperatures and the warm winter temperatures are therefore a reverse of what happens at the stratopause.