204 THE WEATHER 



1. The thermometer has been read every hour, day and 

 night; the 24 hourly readings have then been averaged. 

 This method gives a correct daily average as well as the maxi- 

 mum and minimum, but is impossible for any observer long 

 to continue. Men have thus taken hourly readings long 

 enough to discover sets of hours that give a fairly correct 

 average for the entire day in almost any kind of weather. 



2. The average of the 7 a.m. reading and the 7 p.m. reading 

 is convenient and gives a reasonably correct average for the 

 day. 



3. The reading at 7 a.m. plus the reading at 2 p.m. plus 

 two times the reading at 9 p.m., and the sum divided by 4, is 

 practically correct for the daily average. Moreover, usually 

 the lowest temperature is near 7 a.m. and the highest near 

 2 p.m.; this method, then, tells us something about the ex- 

 tremes for the day. 



4. The cooperating observers of the United States Weather 

 Bureau now use self-recording maximum and minimum 

 thermometers. These are set but once each day, near sunset 

 being the best hour. Their record gives the extremes for the 

 day and the average of these extremes gives a fairly correct 

 average for the day. Comparing the maximum, or the 

 minimum, or the average, with the same item for the preceding 

 or following days shows the changeableness, or variability, 

 of the temperature. 



Exercise 61. To Determine the Average Temperature of a Day or Week 



Obtain a thermograph sheet with a week's record of temperature. 1 

 Rule a sheet of paper with lines for each day and columns for each 

 hour as in the preceding table. Write in the proper spaces the tem- 

 perature at the end of each hour during the week, as in the example 

 just worked out. Find the average temperature of each day by each 

 of the four methods, writing results in the proper spaces at the right. 

 Average each of the 24 hourly columns, writing the average at the 

 foot. 



i If necessary, ask the nearest Weather Bureau station for a copy of a week's 

 thermograph record. 



