Originally Posted by FlyingForFun
Whatever the dewpoint is, if you take a parcel of air from the surface, and reduce the temperature of that air to the dewpoint, it will become fully saturated and start forming clouds. So what I have done (and it seems to have worked for me in the past) is reversed this, and said that, at the base of the clouds, the temperature and the dewpoint must be equal - or, to put it another way, the temperature at the base of the clouds is equal to the dewpoint as given on the ATIS. As I say, this has worked in the past (but maybe it's not actually correct?)
Whilst it is the case that, at the base of the clouds, the temperature will be the same as the dewpoint, the point you're missing is that dewpoint, like temperature, normally falls with height (in the case of dp, this is the 'hydrolapse rate'). So, normally, the temperature at the cloud base will be lower than the surface dewpoint. Of course, this is weather, so in practice anything's actually possible.
