But perhaps the upward rush of warm, moist air has been so great in the morning that the afternoon cooling cannot dispose of it all without spilling. Then occurs a little shower,--the April sort. Often in our mountainous districts it showers every day for this reason. The great thunderstorms come for greater reasons: they are yoked to a low pressure area and represent the summer's brother to the winter's three-day storm. Cumulus clouds are called fair weather clouds until their bellies swell and blacken and they begin to form a combination in restraint of sunlight. Even then it will not rain so much out of the blackness as out of the grayness behind it, and if there is no grayness chances are that you will escape a wetting. One can almost always measure the amount of rain that is imminent by the density of the curtain being let down from the rear of the cloud. If you can see the other clouds through it or the landscape the shower will be slight. If a gray curtain obscures everything behind it you had better pull your canoe out of the water and hide under it if time is less valuable than a dry skin. Such showers may be successive but rarely continuous. Rain clouds have been observed within 230 yards of the ground. Very often it can be seen to rain from lofty clouds and the fringe of moisture apparently fail to reach the earth, because the condensation was licked up and totally absorbed on entering a stratum of warmer air. The reverse of this occurs on rare occasions;--condensation takes place so rapidly that a cloud does not have time to form, and rain comes from an apparently clear sky. This phenomenon has been witnessed oftenest in dry regions and never for very long or in great amounts, although a half hour of this sort of disembodied storm is on record. If the cumulus clouds of the summer's afternoon do not decrease in size as evening approaches showers may be looked for during the night. And if the morning sky is full of these puffy little clouds the day's evaporation on adding to them will probably cause rain. A trained eye will distinguish between a stale and fresh appearance in cloud formation, the light, newly made, fresh clouds, like fresh bread, contain more moisture. If the clouds have much white about them they need not be feared as rain-bearers. Clouds are much higher in summer than in winter and the raindrops of warm air are larger than those of cool.