viii Atmospheric Phenomena 113 



the temperature of the land surface falls to the point at 

 which the water-vapour present becomes saturated, moisture 

 is deposited on all exposed objects in the form of drops of 

 dew or as small crystals of ice, called hoar-frost. The tem- 

 perature of saturation of water-vapour is hence called the 

 dew-point. The deposition of dew, or of hoar-frost, liberates 

 heat ( 159), and so diminishes the subsequent fall of tem- 

 perature. In last century, Dr. Wells made a number of 

 experiments on the cause of dew. He showed that it was 

 only deposited when the sky was clear, and on objects 

 which had become greatly cooled by radiation, and he 

 proved that these in turn chilled the air below the dew- 

 point, and so condensed the water-vapour on their surfaces. 

 On a cloudy night radiation is checked, the water spherules 

 of the clouds retaining and radiating back the heat lost by 

 the Earth, so that dew is not formed. Mr. John Aitken 

 has recently shown that though the chilling by radiation 

 of exposed objects is certainly the cause of dew, only a 

 small part of the moisture is extracted from the air. Indeed, 

 on a still night when there is no wind the air resting over 

 a cabbage, for example, could never have contained the 

 quantity of water found on the leaves in the morning. 

 This is really condensed from the water- vapour always 

 being breathed out by plants. On a gravelled road also, 

 the under side of the gravel and not the upper, is often wet 

 with dew, the stones chilled by radiation condensing the 

 water-vapour which is always rising from the ground. 1 



1 66. Condensation and Dust. It is remarkable that 

 water -vapour never condenses except .upon a solid 

 substance. In air quite free from dust, water- vapour 

 has been cooled far below the dew-point without condensa- 

 tion ; but the instant a puff of common dust-laden air is 

 admitted, each dust-mote becomes a nucleus, and a globule 

 of water is formed upon it. All condensation of water- 

 vapour in the air, whether it appears as rain, mist, fog, 

 cloud, or snow, takes place on a nucleus of dust. 



167. Fog and Mist. When dust-motes are very 

 numerous, and the temperature of air falls suddenly below 

 the dew-point, each mote can receive only a small coating 



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