The very process that made the tumbler of ice-water sweat on the hot day causes dew. And the formation of frost is analogous to that of snow. Frost is not frozen dew, but the formation of moisture crystals at the temperature of 32° or below. Frost or dew form only on still, cloudless nights. Even if no clouds are visible, neither will form if a stratum of humid air has prevented radiation. Hence either dew or frost is a fairly good sign of clear weather. Three white frosts on successive mornings are followed by a rain. This saying holds water not because there is any virtue in frost to cause rain, but because a storm is normally due once a week. The frosts did not form when the anticyclonic winds were blowing and usually not more than three mornings elapse between the time that the anticyclone has lost its influence and the time for the next cyclone to appear. Frost indicates a considerable amount of moisture in the atmosphere, also, which tends to increase as the cyclone approaches. The heaviest dews come in late summer and the heaviest frosts in mid-autumn because the change in temperature is greatest then and there is a greater chance that there will be a calm at sunrise. The greatest frost damage occurs in the spring because the tenderer crops are growing then. Summer frosts used to occur in the northern parts of Minnesota and along the southern boundaries of the inland Canadian provinces before the forests were cleared off. The march of civilization has actually pushed back the frost line some distance. Frost may occur when the amount of humidity in the air is low and the barometer rising at any temperature under 50 degrees at nightfall, the clear skies permitting radiation enough under those circumstances to produce the necessary cooling. An evening temperature of 40 degrees with the clear skies and faint west breeze will almost surely produce a frost, provided the wind drops. In such circumstances the only hope for the farmer is that there is enough humidity in the air to cause a fog before the frost-point is reached. A temperature touching 34 degrees would not bring frost, however, if the sky was at all overcast. Frost is difficult to predict because a night shift in the wind, cloudiness that forms after midnight, or even a wind arising before the coolest period at dawn will prevent its formation. On the other hand, clouds may disperse, the wind may fall or radiation may be so rapid before sunrise as to cause a killing frost unawares. The farmer who lives in areas disputed by winter and spring may never be quite sure, but precautions should be taken on the still, clear, dry nights with the thermometer at fifty or below.