Winters are cold and damp, and prevailing wind patterns that blow offshore minimize the moderating effects of the Atlantic Ocean; yet the Atlantic and the partial shielding from colder air by the Appalachians keep the city warmer in the winter than inland North American cities at similar or lesser latitudes such as Pittsburgh, Cincinnati, and Indianapolis. The daily mean temperature in January, the area's coldest month, is 32.6 °F (0.3 °C); however, temperatures usually drop to 10 °F (−12 °C) several times per winter, and reach 50 °F (10 °C) several days each winter month. Spring and autumn are unpredictable and can range from chilly to warm, although they are usually mild with low humidity. Summers are typically warm to hot and humid, with a daily mean temperature of 76.5 °F (24.7 °C) in July and an average humidity level of 72%. Nighttime conditions are often exacerbated by the urban heat island phenomenon, while daytime temperatures exceed 90 °F (32 °C) on average of 17 days each summer and in some years exceed 100 °F (38 °C). In the warmer months, the dew point, a measure of atmospheric moisture, ranges from 57.3 °F (14.1 °C) in June to 62.0 °F (16.7 °C) in August. Extreme temperatures have ranged from −15 °F (−26 °C), recorded on February 9, 1934, up to 106 °F (41 °C) on July 9, 1936.