226 J. HOPKINSON THE CLIMATE OF WATFORD. 



In March the nights are colder than in February, and in. 

 July colder than in August. 



The mean daily range of temperatui'e is least in December 

 and January, and greatest in May and June. The increase and 

 decrease is as follows : — 



Thus the mean daily range increases in the early part of the 

 year (Jan. to May) twice as rapidly as it decreases in the rest 

 of the year (May to Jan.). The greatest increase (Feb. to March) 

 is mostly due to the cold nights of March. 



Temperatures below freezing-point have occurred in every 

 month but the three months of summer — June, July, and 

 August ; only once, however, in September. The average number 

 of frosty nights has been as follows*: — 



Jan. 15 April 4 July Oct. 3 



Feb. 10 May 2 August Nov. 8 



March 12 June Sept. Dec. 13 



The minimum temperature of the year has occurred four times 

 in January, three times in December, twice in March, and once 

 in April ; the maximum temperature has occurred four times in 

 July, three times in August, twice in June, and once in Sep- 

 tember. 



Temperature^ Humidity, and Cloud at 9 a.m. (Tables VII-IX, 

 p. 227). — The temperature of the air in these tables is that 

 shown by the dry-bulb thermometer, the temperature of evapo- 

 ration that shown by the wet-bulb. From these values the 

 temperature of the dew-point, or that at which dew would be 

 deposited, is calculated. The thermometric dryness is the differ- 

 ence between the temperature of the air and that of the dew- 

 point ; the relative humidity is the percentage of moisture in 

 the air to its complete saturation, represented as 100. 



The mean temperature at 9 a.m. is on the average 0°*1 higher 

 than the mean of the minimum and maximum temperatures ; in 

 no year has the difference exceeded 0°'4. 



From November to March the temperature at 9 a.m. is lower 

 than the mean of the minimum and maximum ; from April to 

 August it is higher ; and in September and October it is about 

 the same as this mean. The mean of the 9 a.m., the mini- 

 mum, and the maximum temperatures, probably gives a truer 

 mean for the day than does the mean of the minimum and 



* This of course does not include ground-frosts. The number of nights when 

 the temperature of the surface of the ground, or of other objects cooled by 

 radiation to a temperature below that of the air, has been below freezing, would 

 be considerably greater. Ground-frosts have occui*red in June, and frequently in 

 September. 



