When the swallows fly high - the weather will be dry? Among all the weather folklore used to predict the weather high-flying swallows announcing fair weather appears to be correct most of the time, true?
Well, generally, high-flying swallows are more likely a sign for fair weather in a current, rather than in a forecasting sense. That is, swallows fly higher when the weather is already fine rather than before the fine weather arrives.
However, there is some biological and meteorological background to this weather lore. Swallows are predators and follow their prey: insects and particularly flies. On warm summer days, thermal activity can carry bubbles of air many thousands of feet above the ground, without inducing the shower clouds that may mean a change to poorer weather. Such thermal activity or convection might be one of the main reasons for sweeping insects high into the sky on fine days and even more so near tall buildings having one site exposed to the sun. Updraft currents might be pretty strong along such exposed and heated surfaces, even earlier in the year. Consequently, the swallows fly progressively higher to feed on the insects.
If bad weather arrives and conditions are turning cooler and windy, insects will rather seek the protection of trees and structures on the ground and are consequently flying lower. Eventually the swallows will follow their food, flying nearer to the ground as well.