Although, from immemorial usage, we still speak conventionally of the “falling of the dew,” it has now been known for more than a century--especially since the publication of Wells’s “Essay of Dew” in 1814--that dew does not fall. The cooling of air below the dew point of its water vapor by contact with any cold object results in a deposit of visible moisture, which is liquid or frozen, according to whether the temperature is above or below the freezing point, respectively. This process is not exclusively nocturnal. It is observed by day in the familiar “sweating” of ice pitchers and also in the appearance of moisture on pavements, stone walls, and the like, in places shaded from the sun. At night the rapid cooling of the earth by radiation, especially (but not, as often stated, exclusively) under a clear sky and in still weather, favors this condensation of moisture, in the liquid form, as dew, or in the frozen form, as hoarfrost. The deposit occurs most copiously on objects that lose heat rapidly by radiation and gain it but slowly by conduction. Water vapor exhaled from the tissues of plants and from the soil undoubtedly contributes its quota to the moisture available for condensation, but this hardly seems to be a reason for asserting, as some writers have done, that dew comes mainly from the earth rather than the air. Hoarfrost is often described as “frozen dew.” This expression is misleading, for, although dew-drops are sometimes frozen into little globules of ice, hoarfrost is more often condensed directly from atmospheric water vapor in the shape of ice crystals. “Glaze” and “rime”--to use the latest official designations of the two kinds of ice coating formed from water in the atmosphere--differ greatly in appearance, as a rule, though transition forms are sometimes found. Glaze is produced by the falling of rain on surfaces whose temperature is below freezing, and is typically smooth and transparent. Rime is a rough deposit formed from fog, the drops of which are “undercooled”--i. e., are below the freezing point--and turn to ice on coming in contact with solid objects. The most remarkable examples of rime are seen on mountains and in the polar regions. It occurs on the branches and leaves of trees, and on the corners and edges of upright objects, rather than on horizontal surfaces. In drifting fog it grows most rapidly if not entirely on the windward side of objects--i. e., it builds up against the wind. On Ben Nevis it has been observed to grow at a rate of more than an inch an hour. Trees, posts, telegraph poles, and the like are thus eventually changed to shapeless masses of rough or feathery ice. CHAPTER VIII WINDS AND STORMS