Report on Temperature and Rainfall. 



79 



As being closely connected with the range of temperature, a 

 table of the mean hvnnidity of the air at West Norwood is 

 appended, which is sufficiently near to, and will serve for, all the 

 stations in the district. It will be noticed that the mean 

 temperatures at Greenwich and at West Norwood are the 

 highest in the list. At the latter place this is probably 

 attributable to its closer proximity to London. The excess is 

 0°*42 above the average of the Croydon stations, and varies from 

 0°'66 in June to 0°'15 in November. It does not depend upon 

 difference of elevation, as the average elevation of the five 

 Croydon stations is 182 feet, and of West Norwood 185 feet, a 

 difference of only 3 feet. 



At the Eoyal Observatory, Greenwich, another important and 

 more potent factor comes in. There it seems that the recorded 

 maximum temperatures are vitiated by the character of the 

 thermometer stand, to which reference has been already made, 

 whereby the readings of the thermometers exposed on it are 

 raised above the true temperature of the air by reflected heat, 

 when the sun is shining. The excess above the Croydon 

 stations is small from October to March, 0°-20, and depends to a 

 slight extent on the lower elevation of Greenwich above sea- 

 level, but with the sun north of the equator it is accentuated, 

 being 1°*29 for the six months, April to September, and in July 

 it reaches 1°"67. In fact, in bright weather in the height of 

 summer the maximum temperature, as recorded at the Eoyal 



The upper dotted line shows the mean maximum daily temperature, the 

 lower dotted line the mean minimum, and the continuous line the mean 

 temperature from all the temperature observations, 1881 — 85. 



