Richmond has a humid subtropical climate (Köppen Cfa), with hot and humid summers and generally cool winters. The mountains to the west act as a partial barrier to outbreaks of cold, continental air in winter; Arctic air is delayed long enough to be modified, then further warmed as it subsides in its approach to Richmond. The open waters of the Chesapeake Bay and Atlantic Ocean contribute to the humid summers and mild winters. The coldest weather normally occurs from late December to early February, and the January daily mean temperature is 37.9 °F (3.3 °C), with an average of 6.0 days with highs at or below the freezing mark. Downtown areas straddle the border between USDA Hardiness zones 7B and 8A, and temperatures seldom lower to 0 °F (−18 °C), with the most recent subzero (°F) reading occurring on January 28, 2000, when the temperature reached −1 °F (−18 °C). The July daily mean temperature is 79.3 °F (26.3 °C), and high temperatures reach or exceed 90 °F (32 °C) approximately 43 days out of the year; while 100 °F (38 °C) temperatures are not uncommon, they do not occur every year. Extremes in temperature have ranged from −12 °F (−24 °C) on January 19, 1940 up to 107 °F (42 °C) on August 6, 1918.[a]