"AH, it's too bright to last!" is an exclamation not unfrequently heard on a fine morning. Ill-based as are many common beliefs about the weather, a few are well-based, and this is one of them: little as those who utter it understand why.
A specially fine morning is nearly always the end of a fine night, that is, a night throughout all or most of which the sky has been free from clouds. During such a night the Earth's surface radiates its heat into space without impediment. There is no canopy of opaque vapour floating above, which radiates back to the Earth much of the heat which it receives from it. Hence, during the early part of the following day, before the sun is high, a low temperature is reached, alike by the exposed parts of the ground and by parts clothed with vegetation, as is shown by the large deposits of dew. The chilled surface is now a good condenser, and if the air is well charged with water, as commonly it is when the wind is westerly, and especially southwesterly, precipitation results: clouds begin to form and presently there