The weather forecast on the radio this morning said that there would be cumulus cloud forming in the cloudless sky once the temperature reached 18 degrees Celsius. I have not been able to chase up that reference. However we were walking on the Cumbria Coastal Path from St Bees to Seascale and noticed this cloud over the Isle of Man. It shows up better through a polarising filter. It will be because the land gets hotter than the sea (lower specific heat capacity - blogged about it before...) The air over the hot land expands, becomes less dense and rises. As it goes up, pressure is reduced so the air expands further for a different reason. The expanding air does work and so loses energy. The particles in the air have less kinetic energy and thus the air is at a lower temperature. This is the same reason why an aerosol deodorant feels cold. The process is called adiabatic because heat energy does not flow into or out of the air. The temperature is solely related to the kinetic energy of the molecules. Colder air holds less water vapour so it condenses out to form the cloud.
