between coastal and inland areas are discernable, in addition to the 

 temperature decline from south to north. 



Monthly mean precipitation . Monthly precipitation means at Portland and 

 Eastport show that no dry or wet season exists (figures 2-8 and 2-9). The 

 most prominent feature of the distributions is a relative maximum that occurs 

 in late fall, preceded by a relative minimum in midsummer. Monthly means are 

 between 3 and 4 inches (8 to 10 cm) . 



This precipitation pattern is general to the coastal zone. Distributions at 

 other stations are given by region in figure 2-11. Minor variations in 

 pattern and magnitude are not systematic and appear to be the result of either 

 local effects or variations in the length of record. Distributions of the 

 number of days per month with precipitation 0.1 inch are virtually constant 

 from station to station. Significant precipitation occurs during an average 

 of 6 to 8 days/month, with weak maxima in early spring and late fall. 



While the average precipitation distribution favors particular months, it 

 should be emphasized that the wettest month in any particular year could be 

 any one of several. Variations in total annual precipitation seldom exceed 

 30% of the average and severe drought is very rare. Nonetheless, short dry 

 spells are common, usually occurring in the warmer summer months. 



Cloud occurrence . Cloudiness is classified as either "cloudy" (80% to 

 100% cloud cover), "partly cloudy" (40% to 70% cloud cover), or "clear" (0% to 

 30% cloud cover). Cloudiness is measured during daylight hours only. In 

 Maine, cloudiness is nearly evenly divided among the three categories, with 

 cloudy conditions more prevalent at both Eastport and Portland (figures 2-8 

 and 2-9). Maine has no true dry or wet season, because of its location within 

 the midlatitude storm track. The clearest skies occur in midsummer and the 

 cloudiest in late fall. At Portland an additional cloud maximum occurs in 

 early spring. A cloud maximum of lesser magnitude occurs at this time at 

 Eastport. These data are consistent with the monthly precipitation records. 

 Eastport has fewer hours of cloudy skies and more hours of partly cloudy skies 

 in spring than Portland and, therefore, it appears that Eastport lies on the 

 northern fringe of many spring storms that pass through Portland. 



Evaporation and relative humidity . Evaporation data are collected only 

 during May through October. As previously stated, the data shown in figures 

 2-8 and 2-9 are for New Gloucester (U.S. Department of Commerce 1963 to 1976) 

 and therefore are more representative of conditions at Portland than those at 

 Eastport. The data indicate that the months of greatest evaporation are in 

 midsummer, as might be expected. Total evaporation over the 6-month period in 

 the 14-year record averages about 30 inches (76 cm), or approximately two- 

 thirds of the total annual precipitation in the coastal region. Average 

 precipitation near Portland over the same 6 months totals about 18.5 inches 

 (47 cm) in the 30-year record. Periods with high temperatures, high 

 evaporation rates, and low rainfall sometimes necessitate agricultural 

 irrigation. The risk of fires in fields and forests also rises at this time, 

 because of the increased frequency of thunderstorms. 



2-14 



