3.^ Precipi tation 



Precipitation near and within the Long Island Sound Region 

 results from a variety of causes including tropical storms, air mass 

 thunderstorms and extra-tropical low pressure areas. The amount and 

 areal distribution of precipitation for water years 19^1-70 has been compiled 

 for the entire Long Island Sound drainage area by the National Weather Ser- 

 vice. Over the study region itself the average annual precipitation ranges 

 from 36 to 51 inches. About 5 to 20 percent of the annual total precipitation 

 is snowfall with maximum amounts in northern sections. The average annual 

 precipitation is greatest near the slightly elevated part of Long Island and 

 away from the immediate shore in Connecticut. Two factors affect the geo- 

 graphic distribution of precipitation: distance from the stabilizing influence 

 of water bodies and differences in altitude. 



Besides the amount and areal distribution of precipitation, infor- 

 mation on the intensity and duration of precipitation is required for many 

 purposes. For example, the extreme 24-hour precipitation event at New Haven 

 was 8.73 inches on August 8-9, 187^. This knowledge is useful, but short- 

 period excessive rainfalls are sometime even more important. Rainfalls of 

 1.38 inches in 10 minutes and 2.3^ inches in 30 minutes at New Haven on July 

 2k, 1928 are examples of precipitation events which caused serious stresses 

 on engineering works. Listed in Appendix C are (1) a table based on Hersh- 

 field ( 13 ) which gives maximum point-precipitation amounts for selected dura- 

 tions from 5 minutes to 2k hours and return periods from 2 to 100 years and 

 (2) a table listing the monthly extreme 2A^-hour precipitation and snowfall 

 together with their years of occurrence and (3) a listing of the mean monthly 

 24-hour extremes of the data and their standard deviations. 



Long-term precipitation records for New Haven, Connecticut, New 

 London, Connecticut, and Setauket, Long Island are shown in 6 graphs (see 

 Appendix C) . Three of the graphs permit calculation of the frequency with 

 which a given amount of precipitation can be expected at each location. For 

 example, the chances are that 80 percent of the time water-year precipitation 

 at Setauket will be equal to or greater than 39 inches but only 1 in 100 that 

 it will be as little as 32 inches. 



Tables which give the least and greatest recorded monthly and 

 annual precipitation amounts, together with their years of occurrence, are 

 listed in Appendix C. These tables show, for example, that annual calendar 

 year precipitation has varied locally from 25.35 inches at New London, Con- 

 necticut, in 1863 to 78.37 inches at Lake Konomoc near Waterford, Connecticut 

 in 1972. Three additional tables list the mean, greatest and least monthly 

 and annual number of days with precipitation equal to or greater than 0.01, 

 0.10, 0.50 and 1 .00 inches. 



Mean and greatest monthly and mean seasonal snowfall for the 

 1951-70 period are listed in a table (see Appendix C) . Average seasonal 

 snowfall on the north shore of Long Island ranges from about 23 to 28 inches. 

 Along the mainland coast, the amounts vary from 26 to 33 inches with the 

 larger totals between New Haven, Connecticut and Westbrook, Connecticut. Snow- 

 fall increases rapidly away from the coast and in the more hilly portions of 



