Frosts in New York 



509 



over the city. Comparison of country and city temperatures at many other 

 places gives essentially the same results. 



The higher night temperatures of cities are attributed mainly to the 

 heat given off from buildings and pavements, and to the effect of smoke 

 from the many city fires, which collects over cities on quiet nights and, 

 by retarding the escape of heat from the surface, tends to hold the air at 



Fig. 135. — Maximum and minimum thermometers {Weather Bureau, pattern). 



Price, mounted as shown, about $8 



a higher temperature than would obtain otherwise. For this reason, 

 records of frost made in large cities cannot be regarded as a reliable guide 

 to the occurrence of frost in the open country; and, while the records of 

 frosts made in the cities of the State have been retained in the tables, 

 they have been disregarded, for the most part, in making up the accom- 

 panying charts. 



FROST IN THE OPEN COUNTRY 



The observations of frost and the temperatures recorded by cooperative 

 observers of the Weather Bureau — who, in the main, are located in the 

 open country or in small villages — may be considered as representing 

 very closely the conditions that obtain on the farm. 



The instruments used by cooperative observers are carefully stand- 

 ardized and their exposure is as nearly uniform as possible. The ther- 

 mometers — a maximum registering the highest temperature in each 

 twenty-four hours, and a minimum registering the lowest — are exposed 

 in shelters of standard construction so that the results are strictly com- 

 parable. Generally, the thermometers are placed about four feet above 

 ground, and their readings indicate the temperature of the air at that 

 elevation. 



The practice of recording a killing frost when the air temperature at 

 a height of four feet above ground falls during the night to 32 

 has been objected to on the ground that it does not represent the tem- 

 perature of the air in contact with plants at the surface. This is true, 

 but it may be pointed out also that the temperature of the air in contact 

 with the plants does not represent the temperature within the inter- 



