114 Water 



mean annual precipitation of 50 cm would be characterized as humid 

 if the mean annual temperature were — 7°C or less. On the other 

 hand, another locality with the same rainfall would be regarded as 

 having a semiarid climate if the annual temperature were 21 °C or 

 more. The contrast in climate between the Canadian prairie and 

 the Mexican desert, both with about 50 cm of rain but with very 

 different temperature conditions, is an excellent illustration of this 

 principle. 



The geographical variation in relative humidity is very great. Rela- 

 tive humidities of 80 to 100 per cent characterize the tropical rain 

 forest. Regions reporting values of less than 50 per cent are regarded 

 as having dry climates, and those with values of less than 20 per cent 

 are extremely arid. It is of interest to note in passing that in cold 

 winter weather the relative humidity inside our houses is rarely higher 

 than 35 per cent. 



At any one locality the relative humidity may remain relatively con- 

 stant for long periods of time or may vary widely. On many oceanic 

 islands the humidity is very nearly the same throughout the year. 

 In other localities, characterized by wet and dry seasons, the humidity 

 fluctuates widely from one part of the year to another. In certain 

 situations as on the plains and in desert regions considerable changes 

 in moisture content may occur during the course of each day. Records 

 made in a short-grass prairie in the United States showed a variation 

 from a relative humidity of less than 30 per cent in the early after- 

 noon to more than 95 per cent in the middle of the night (Fig. 4.11). 

 Obviously animals and plants living in such habitats must be equipped 

 to withstand these rapid and extensive changes in the water factor, 

 and their lives must be attuned to them. In deserts where daytime 

 humidities are extremely low and evaporation is excessive, the greater 

 relative humidity at night helps to relieve the critical moisture con- 

 dition. Many desert animals take advantage of this situation by 

 going abroad only during hours of darkness; in some desert plants the 

 stomata open only at night when transpiration loss is at a minimum. 



The foregoing has been a brief description of the amount of moisture 

 in the air in terms of its relative humidity. As already implied, the 

 chief ecological significance of the relative humidity is its effect on 

 the rate of water loss. Terrestrial animals and plants lose water 

 directly to the air by evaporation and transpiration. The water 

 supply in their substratum is also reduced by direct evaporation from 

 the soil and indirectly by the transpiration of the vegetation. Evap- 

 oration takes place very rapidly from soil because of the great surface 

 area presented by the fine particles and causes the drying of the upper 



