Protection of Vegetation from Cold.

By William Thomson

Paper read before the Royal Society of Edinburgh, April 4, 1864.
From reprint in Popular Lectures and Addresses, Vol. ii, p. 1.

The effect of dew in protecting vegetation every clear still night of summer was long ago pointed out by Dr. Wells; the correctness and acuteness of whose views on this subject have been generally recognised. The hypothesis recently put forth by Dr. Tyndall, that absorption of radiant heat by aqueous vapour in the atmosphere is an effective defence against destructive degrees of cold, and the ready acceptance yielded to it by some of our highest authorities in the popular promulgation of the truths of science, seems to render it necessary to recall attention to Dr. Wells's admirable work. In the first place, when Dr. Tyndall announces, as a result of his experiments on radiant heat, that "It is perfectly certain that more than ten per cent. of the terrestrial radiation from the soil of England is stopped within 10 feet of the surface of the soil," by the absorption it suffers from aqueous vapour; it must be remarked that this absorption cannot go on at the same rate through any great thickness of air. For at the same rate half the radiant heat would be absorbed in 70 feet; 3/4 in 140 feet; 7/8 in 210 feet, and so on, which is inconsistent with known facts; as, for instance, the influence of clouds on terrestrial radiation. Hence the quality of rays which passes through the lowest 10 feet of air suffers less than ten per cent. of absorption in the next 10 feet; and it is quite certain that after passing through several times 10 feet of air, the radiant heat must, by having been deprived of the part of it specially liable to absorption by aqueous vapour, be in a condition in which not one per cent. is absorbed from it in its passage through 10 feet of clear air. If true vapour of water really does exercise any influence in checking, by its absorption, the loss of heat by radiation from the earth's surface, it is, even in the most humid conditions of optically clear atmosphere, insufficient to prevent heavy dews by radiation into space of the latent heat of the vapour from which they are condensed. The quantity of heat thus radiated into space through the clear moist air close to the ground is so great that if instead of being taken from the vapour it were taken from the blades of grass, or other finer parts of plants, it would leave them destroyed by frost.

In point of fact heat actually is radiated away into very high terrestrial atmosphere and distant interstellar air or æther, from the upper and finer parts of living plants, in so great amount every clear night of summer, that destruction by frost could not be delayed for many hours after sunset without a compensating supply of heat from some extraneous source. This source, on windy nights, is the thermal capacity of the air whirled about, up and down, and among the stems and leaves of the plants. On still nights it is the latent heat of the vapour condensed into dew. This vapour is taken chiefly from the air engaged among the stems and leaves, which, in the case at least of fine grass, is all nearly at the same temperature as the leaves; the temperature of the surface of these being of course rigorously the same as that of the air in contact with them. Thus the temperature of the leaves can never go below the dew-point of the air touching them, and any cooling which they experience after dew begins to deposit upon them is only equal to the lowering of the dew-point, occasioned by the amount of drying experienced by the air in consequence of the condensation of vapour out of it.

Clouds, as remarked first by Prévost, being practically opaque, prevent the surface of the earth from tending by radiation to a lower temperature that their own, which, unless they are very high, is generally not much colder than the dew-point of the lower air, but is at all events in general sufficiently warm to prevent the finest blades of grass from acquiring any very sensible dew, or to allow the general temperature of grass and the air engaged among it, even on the stillest night, to sink as low as the dew-point. Thus either clouds, by their counter radiation, or wind, by mixing a comparatively thick stratum of air with that next the earth, keep the grass and delicate parts of other plants from sinking to the dew-point. When there is not enough clouds and wind to afford this degree of protection, dew begins to form, and by preventing the temperature of any leaf or flower from sinking below the dew-point, saves them all from destruction, unless, as when hoar-frost appears, the dew-point itself is below the freezing-point.

[Added December 15, 1892.]—Thus when neither clouds, nor wind blowing among the plants, suffice to protect them from sinking to the dew-point, the temperature to which leaves, flowers and grass sink is lower the dryer is the air, not because dry clear air is more diathermanous than moist clear air, [1] but because the dew-point is lower the dryer is the air.