

METEOROLOGY. 487 



" (3) The precipitation of moisture, as rain, snow, bail, dew, frost*- 

 Under this head the following are desirable : (a) The monthly and an- 

 nual sums of total precipitations of rain and melted snow ; (b) the max- 

 imum precipitation per day and per hour ; (c) the number of days having 

 one-half millimeter or more of precipitation, including dew or frost ; (d) 

 the percentage of rainy days in each month or the probability of a rainy 

 day ; the number of rainy days is a better indication of a moist or dry 

 climate than the total rainfall ; (e) the number of days with snow, the 

 depth of the snow covering, the duration of the snow covering, the dates 

 of frost, and last snowfall ; (/) similar data for the days with hail ; {g\ 

 similar data for the days with thunder-storms. 



tl (4) The precipitation of moisture as clouds. Under this head are 

 important, (a) the amount of cloudy sky expressed in tenths or hun- 

 dredths of the whole celestial hemisphere (the number of clear, cloudy, 

 and threatening days does not seem sufficiently precise) ; (b) the per- 

 centage of cloudiness is an ordinary substitute for the direct measure" 

 of the intensity of solar radiation, and as it varies very regularly in 

 diurnal and annual periods, it is desirable to give monthly means for 

 three or more separate hours of observation ; (c) the thickness of the 

 cloud layer is recorded by Campbell's sunshine recorder by the effect 

 of a burning lens in charring prepared paper — this apparatus gives us 

 the amount of clear sunshine whence we deduce the ratio of observed 

 amount of strong sunshine to the greatest amount possible at the re- 

 spective seasons and stations; (d) the number of foggy days, or the 

 total number of hours of fog, and the times of forming and dissipating; 

 (e) the number of nights or the quantity of dew. 



"VI. The wind affects humanity and equally the fauna and flora in 

 innumerable particulars, but the comparison of observations, especially 

 of force or velocity, is greatly hindered by the irregularities of methods- 

 of observing. It is, however, possible to draw some comparative con- 

 clusions from the following data: 



"(1) The monthly means (or totals) of wind velocity or estimated 

 wind force. 



" (2) The frequency of winds from different directions (to eight prin- 

 cipal compass points) ; this is most conveniently expressed in percent- 

 ages of the total number of observations, or else the number of obser- 

 vations of each wind is divided by the number of observations made 

 each day, so that we receive as quotient the number of days that each 

 wind has blown. 



" (3) The frequency of the winds for each hour of observation thus 

 showing the diurnal changes in wind direction, this may generally be 

 given for the annual sums instead of the individual mouths. 



"(4) The meteorological peculiarities of each wind direction as to/ 

 («) temperature, (b) moisture, (e) cloudiness, (d) probability of rainfall, 

 are shown by the respective wind-roses (thermic, atmic, nephic, aque- 

 ous) ; one such series of wind roses suffices to show the climatic char- 

 acteristics of a large section of country. 



