When excessive rainfall extends beyond a duration of two hours the measurements are recorded at fifty-minute intervals.
The amount of rainfall necessary to insure a specific crop varies with locality and with the character of the crop. More especially it depends on the distribution of the rainfall over the growing season. Roughly, rain must fall during a period which covers three-quarters or more of the growing season for the particular crop. The growing season for wheat is over, in most localities, by the middle of July—in some localities by the middle of June. The growing season for corn extends into September. A rainfall of 12 inches, fortunately distributed, may be all that is required for a specific crop. Unfortunately distributed, a fall two or three times as great may not suffice.
From the nature of the case, the knowledge which concerns crop safety must be gathered locally. Through its Climatological Service, the Weather Bureau is gathering knowledge of this sort, but additional information is very desirable. The observer, whether official or volunteer, can aid in gathering useful information along the following lines:
The length of the growing season—that is, the number of days between late spring frosts and early fall frosts.
The months during which rain is necessary for each specific crop.
The duration of droughts—that is, the number of days during which no rain or only a trace of rain falls—that are hurtful or destructive to specific crops.
The character of soil with respect to rainfall necessary to crop growth.
As a rule the precipitation records of the nearest Weather Bureau station—regular or cooperative—will furnish the necessary information concerning the amount of precipitation. The specific locality sometimes requires its own rain measurements. A rain gauge of the Weather Bureau pattern is useful, but a metal container with straight sides will answer fairly well, and an inch rule will answer the purpose of a measuring stick. The volunteer observer who studies the rainfall of a locality may thus gain the essential information required; namely, the minimum amount of rain, and also the optimum rainfall both as to amount and distribution, for a specific crop.
