summer is by far the sunniest season. The warmest month of the year in Pittsburgh is July, with a 24-hour average of . Conditions are often humid, and combined with highs reaching on an average 9.5 days a year, a considerable heat index arises. The coolest month is January, when the 24-hour average is , and lows of or below can be expected on an average 2.6 nights per year. Officially, record temperatures range from , on January 19, 1994 to , which occurred three times, most recently on July 16, 1988; the record cold daily maximum is , which occurred three times, most recently the day of the all-time record low, while, conversely, the record warm daily minimum is on July 1, 1901. Due to elevation and location on the windward side of the Appalachian Mountains, + readings are very rare, and were last seen on July 15, 1995. Average annual precipitation is and precipitation is greatest in May while least in October; annual precipitation has historically ranged from in 1930 to in 2018. On average, December and January have the greatest number of precipitation days. Snowfall averages per season, but has historically ranged from in 1918–19 to in 1950–51. There is an average of 59 clear days and 103 partly cloudy days per year, while 203 days are cloudy. In terms of annual percent-average possible sunshine received, Pittsburgh (45%) is similar to Seattle (49%). Air quality In 2019, the "State of the Air" report from the American Lung Association (ALA) found that air quality in the Pittsburgh-New Castle-Weirton, PA-OH-WV metro area worsened, not only for ozone (smog), but also for the second year in a row for both the daily and long-term measures of fine particle pollution. Outside of California, Allegheny County is the only county in the United States that recorded failing grades for all three. In a 2013 ranking of 277 metropolitan areas in the United States, the American Lung Association ranked only six U.S. metro areas as having higher amounts of short-term particle pollution, and only seven U.S. metro areas having higher amounts of year-round particle pollution than Pittsburgh. For ozone (smog) pollution, Pittsburgh was ranked 24th among U.S. metro areas. The area has improved its air quality with every annual survey. The ALA's rankings have been disputed by the Allegheny County Health Department (ACHD), since data from only the worst of the region's 20