year, though spring and fall are the wettest seasons. Sunshine is common during all seasons, with 2,650 hours, or 60.3% of the possible total, of bright sunshine per year. Summers in Austin are very hot, with average July and August highs frequently reaching the high-90s (34–36 °C) or above. Highs reach on 123 days per year, of which 29 days reach ; all years in the 1991-2020 period recorded at least 1 day of the latter. The average daytime high is or warmer between March 1 and November 21, rising to or warmer between April 14 and October 24, and reaching or warmer between May 30 and September 18. The highest ever recorded temperature was occurring on September 5, 2000, and August 28, 2011. An uncommon characteristic of Austin's climate is its highly variable humidity, which fluctuates frequently depending on the shifting patterns of air flow and wind direction. It is common for a lengthy series of warm, dry, low-humidity days to be occasionally interrupted by very warm and humid days, and vice versa. Humidity rises with winds from the east or southeast, when the air drifts inland from the Gulf of Mexico, but decreases significantly with winds from the west or southwest, bringing air flowing from Chihuahuan Desert areas of West Texas or northern Mexico. Winters in Austin are mild, although occasional short-lived bursts of cold weather known as "Blue Northers" can occur. January is the coolest month with an average daytime high of . The overnight low drops to or below freezing 12 times per year, and sinks below during 76 evenings per year, mostly between mid-December and mid-February. The average first and last dates for a freeze are December 1 and February 15, giving Austin an average growing season of 288 days, and the coldest temperature of the year is normally about under the 1991-2020 climate normals, putting Austin in USDA zone 9a. Conversely, winter months also produce warm days on a regular basis. On average, 10 days in January reach or exceed and 1 day reaches ; during the 1991-2020 period, all Januarys had at least 1 day with a high of or more, and most (60%) had at least 1 day with a high of or more. The lowest ever recorded temperature in the city was on January 31, 1949. Roughly every two years Austin experiences an ice storm that freezes roads over and cripples