Little moisture enters the stratosphere, so clouds are rare. Even though the stratosphere has complex wind systems, violent storms don't occur there. Because the air temperature in the stratosphere slowly increases with altitude, it does not cause convection and has a stabilizing effect on atmospheric conditions in the region. Stability generally limits vertical extensions of cloud and leads to the lateral spreading of high cumulonimbus cloud with characteristic anvil heads. This means that weather (in the form of clouds) is almost entirely confined to the troposphere below. That's why airline pilots prefer to fly in the stratosphere.
Temperature in the lower stratosphere is isothermal and averages around -60°C, but increases markedly in the upper part, to reach a maximum of about 0°C at the stratopause. Temperature increases with ozone concentration. Solar energy is converted to kinetic energy when ozone molecules concentrated at 15-30 km absorb ultraviolet radiation, resulting in heating of the stratosphere. Due to the very low air density, even the small amount of ozone concentrated in the upper stratosphere is extremely effective in absorbing radiation, thus giving the 'high' temperatures at 50 km.
