1384. Dew is the moisture insensibly deposited from the atmosphere on the surface of the earth. This moisture is precipitated by the cold of the body on which it appears, and will be more or less abundant, not in proportion to the coldness of that body, but in proportion to the existing state of the air in regard to moisture. It is commonly supposed that the formation of dew produces cold; but, like every other precipitation of water from the atmosphere, it must eventually produce heat. Aristotle justly remarked, that dew appears only on calm and clear nights. Dr. Wells shows, that very little is ever deposited in opposite circumstances; and that little only when the clouds are very high. It is never seen on nights both cloudy and windy; and if, in the course of the night, the weather, from being serene, should become dark and stormy, dew, which has been deposited, will disappear. In calm weather, if the sky be partially covered with clouds, more dew will appear than if it were entirely uncovered. Dew probably begins, in the country, to appear upon grass in places shaded from the sun, during clear and calm weather, soon after the heat of the atmosphere has declined, and continues to be deposited through the whole night, and for a little after sunrise. Its quantity will depend, in some measure, on the proportion of moisture in the atmosphere; and is, consequently, greater after rain than after a long tract of dry weather; and, in Europe, with southerly and westerly winds, than with those which blow from the north and the cast. The direction of the sea determines this relation of the winds to dew; for, in Egypt, dew is scarcely ever observed, except while the northerly or Etesian winds prevail. Hence, also, dew is generally more abundant in spring and autumn than in summer. It is always very copious on those clear nights which are followed by misty mornings, which show the air to be loaded with moisture; and a clear morning, following a cloudy night, determines a plentiful deposition of the retained vapour. When warmth of atmosphere is compatible with clearness, as is the case in southern latitudes, though seldom in our country, the dew becomes much more copious, because the air then contains more moisture. Dew continues to form with increased copiousness as the night advances, from the increased refrigeration of the ground.
