Located on the Gulf Coast about 2.49° north of the Tropic of Cancer, Brownsville possesses a humid subtropical climate : winters are mild, and summers are hot and humid. Due to its proximity to the deserts of Chihuahua and Gulf Coastal Plains, Brownsville’s geographic location lies near the boundary of a hot semiarid climate. Snow is a very rare event in Brownsville. Its wet season is concentrated during the late summer and early fall, peaking in September, when the threat from tropical cyclones is greatest. In most years, November through April is the dry season. As such, Brownsville receives modest annual rainfall, averaging about 27.44 in (697 mm) annually based on records between 1981 and 2010.
The monthly daily average temperature ranges from 61.1 °F (16.2 °C) in January to 85 °F (29.4 °C) in August. However, the city is subject to episodes of heat waves during the summer, with 141 days of high temperatures over 90 °F (32.2 °C) and fewer than five days of temperatures above 100 °F (37.8 °C). The city is located along the boundary of USDA hardiness zones 9b and 10a. The hottest temperature on record in Brownsville occurred on March 27, 1984, when the city reached 106 °F (41 °C). On the other extreme, freezing temperatures occur once or twice a year typically. On December 25, 2004, Brownsville recorded its first instance of measurable snow in 109 years with 1.5 in (3.8 cm), and the first recorded White Christmas. Brownsville’s lowest temperature on record occurred on February 13, 1899, when the city reached 12 °F (−11 °C). Based on 30-year averages obtained from the National Oceanic and Atmospheric Administration’s National Climatic Data Center weather records, 24/7 Wall St. ranked Brownsville the fifth-hottest city in America in 2016.
In 2011, Brownsville became one of the first cities in the United States to require stores to charge a fee for single-use plastic shopping bags. The ordinance was enacted to reduce pollution and litter around the city. The city repealed the ordinance in 2018 after it was ruled illegal by the Supreme Court of Texas. Forbes identified Brownsville as one of 12 metropolitan areas in the United States with the cleanest air. In 2018, the Brownsville–Harlingen area was among the “Cleanest U.S. Cities for Ozone Air Pollution” in the American Lung Association’s “State of the Air” in 2018.