igOl.] FAIRCHILD — CYCLONIC STORMS. 313 



land is cold in winter, and a change in the wind, which may come in a twin- 

 kling, changes domination from'one area to the other. 



"The records of the minimum thermometer for December of many years 

 will probably be representative of Rochester weather, and will show with what 

 moderation we are favored. The temperature changes in cycles, averaging 4.2 

 days in length. It falls slowly 2.4 days, then rises rapidly 1.8 days. The fall 

 for each day averages 5.6 degrees, and the rise 6.7 degrees. The greatest fall 

 in one spell was 41 degrees in five days in 1884, and the greatest rise was 50 

 degrees in one day in 1875. This shows a climate not of abrupt changes but 

 one remarkably equable. 



" That the temperature at Rochester is continually on the weave up and 

 down is undeniable, but the ups and downs are, as a rule, almost too small to 

 be dignified by the name of changes. Stationary temperature (that is a range 

 of less than 6 degrees during 24 hours) may be predicted at Rochester all the 

 time and the prognostications will exceed 60 per cent, correct. To find greater 

 steadiness than this on^ will have to retreat to the ocean waters far from land. 

 Rochester has not in any proper sense of the term remarkably changeable 

 weather, but quite the opposite."* 



BAROMETRIC PRESSURE. 



Although we are quite unconscious of the v^arying weight of the 

 atmosphere it is a very important factor in weather conditions, and 

 seriously affects life. The normal pressure at sea level of 15 pounds 

 to the square inch varies within a range of about four inches, or from 

 a pressure of 27.5 inches to 31.5. This change is equal to that which 

 would be experienced in passing from a depth of 1,378 feet below 

 ground to 2,133 ^eet above, or through a vertical distance of 3,500 

 feet. A change of two inches of mercury is not infrequent in the 

 passage of the more violent cyclones. The tremendous effects of such 

 changes of pressure may be realized when we find that this difference 

 of two inches of mercury, or a pound to the square inch of surface 

 amounts on a square mile to over 2,000,000 tons. 



INFLUENCE OF LAKE ONTARIO. 



The positive factors in the temperature element of this region are 

 in summer the warm waves and in winter the cold waves. Neither of 

 them ever touch Rochester with great severity. The cold waves are 

 due to the indraught of cold air from the northern regions by the 

 cyclonic movements. The exemption of Rochester from the extreme 

 low temperatures is due to the protection of Lake Ontario. In this 

 respect the city stands quite alone, for while other cities feel some of 



♦This quotation is from a letter to the writer by Mr. Orin Parker. 



