=105. General relations.= As the factor which exerts the most important control upon water-content and humidity, rainfall must be carefully considered by the ecologist. It is such an obvious factor, and is usually spoken of in such general terms that the need of following it accurately is not evident at once. When it is recognized that the fluctuations of water-content are directly traceable to it, it becomes clear that its determination is as important as that of any indirect factor. This does not mean, however, that the amount of yearly rainfall is to be taken from the records of the nearest weather station, and the factor dismissed. Like other instruments, the rain gauge must be kept at the base station of the area under study, and when this is extensive or diverse, additional instruments should be put into commission. While the different parts of the same general climatic region may receive practically the same amount of precipitation during the year, it is not necessarily true that the rainfall of any particular storm is equally distributed, especially in the mountains. Nothing less than an exact knowledge of the amount of rain that falls in the different areas will make it possible to tell how much of the water-content found at any particular time in these represents merely the chance differences of precipitation. The forms of precipitation are rain, dew, hail, snow, and frost. Of these, hail is too infrequent to be taken into account, while frost usually occurs only at the extremes of the growing season, and in its effect is rather to be reckoned with temperature. Snow rarely falls except during the period of rest, and, while it plays an important part as cover, it is merely one of several factors that determine the water-content of the soil at the beginning of spring. The influence of dew is not clearly understood. It is almost always too slight in amount and too fleeting to affect the water-content of the soil. It seems probable that it may serve by its own evaporation to decrease in some degree the water loss from the soil, and from bedewed plants. If, however, the dew is largely formed by the water of the soil and of the plant, as is thought by some, then it is negligible as a reinforcement of water-content. From the above, it is evident that rainfall alone exerts a profound effect upon the habitat, and it is with its measurement that the ecologist is chiefly concerned. [Illustration: Fig. 22. Rain gauge showing construction.]