Under the Köppen climate classification, Islip has a humid continental climate ("Dfa"), with some maritime influence, or under the threshold a humid subtropical climate ("Cfa"); it is part of USDA hardiness zone 7a. The normal average monthly temperature ranges from in January to in July; on average, there are 18 afternoons where the temperature remains at or below freezing and 7.5 afternoons with a high at or above annually; the last year to not reach the latter mark was 2014. Temperatures below or above are rare, and were last seen respectively on January 7, 2018 at and July 22, 2011 at . The record low is , set on February 13, 1967, while the record high is , set on July 3, 1966.