258 PHYSIOGRAPHY 



land. If moisture condenses and the particles remain suspended in 

 the air above the top of a mountain, there is, to the observer on 

 the plain or in the valley below, a cloud about the mountain; but 

 if the observer were to climb up into the cloud, it would then appear 

 as fog. Fogs are often formed when the air over a lake in autumn 

 is blown over the cooler land, or when the air over warmer water 

 from one part of the ocean (e. g., a warm ocean current) blows over 

 colder water. 



Fogs often form in valleys at night (Fig. 1, PL XLV, p. 257), 

 especially in autumn, when the night temperatures are much lower 

 than those of the day. The cooler air settles in the valleys, which 

 are therefore more likely to have fogs than the uplands are. 



Fogs occasionally lead to shipwreck on sea, and interrupt busi- 

 ness on land. A dense fog in London, which lasted from Decem- 

 ber 10 to 17, 1905, was estimated to have cost the city $1,750,000 

 per day, in one way and another, largely through suspension of 

 business. Such estimates are, however, to be taken with reserve, 

 since much of the suspended business is transacted later. Heavy 

 fogs may t>e of service to one party in war, by allowing an army 

 to approach or retreat unseen. Thus a fog helped Washington in 

 his retreat to New York, after the battle of Long Island. 



The droplets of water in clouds and fogs must be very small to 

 remain suspended in the air. It has been estimated that many of 

 them are about ^fa of an inch in diameter, but there is doubtless 

 great variation. 



Clouds affect temperature by hindering radiation. A cloudy 

 night is not generally so cold as a clear one. In general, cloudiness 

 lowers the summer temperatures of middle latitudes, and raises their 

 winter temperatures. 



Forms of clouds. Clouds take on many forms. Among the 

 more common are the cumulus, the stratus, the nimbus, and the 

 cirrus clouds. Between these more distinct forms there are many 

 gradations, giving rise to the names cirro-cumulus, cirro-stratus, 

 cumulo-stratus, etc. 



Cumulus clouds are thick, and their upper surfaces are some- 

 what dome-shaped, with irregular and fleecy projections. Their 

 bases are nearly horizontal (Fig. 2, PL XLIV, and Fig. 2, PL XLV). 



