Halloween and October Nights

This fall's weather has been downright scary, and it’s not over yet. September tied for the 2nd hottest on record (both nationally and globally), and the southeastern United States experienced a flash drought while areas in Montana saw record snowfall. With wildfires and power outages in the western U.S., 30-inch downpours in Texas, and coastal flooding in the East, some might say that the often wild and changeable character of fall weather has taken on a new meaning.

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The effects of climate change are most often seen in extreme events, as seemingly small increases in average temperature can disproportionately lead to more frequent and intense extreme weather. In recent analyses we found that average fall temperatures in the United States have increased by 2.5°F over the past half century. While this increase has contributed to many daytime record high temperatures recently, it is actually overnight low temperatures that are warming fastest.

This week, we examine this trend by updating our analysis of warming fall nights, this time focusing just on the month of October. Of the 242 cities analyzed, 78% (188) have warmed by more than 1°F in the past half-century, while only 3% (7) have cooled more than 1oF. The West and Gulf Coast have seen the most warming—Reno, Nev., topped the list with 12.3°F of warming, followed by Las Vegas (9.3°F), El Paso, Texas (8.9°F), Panama City, Fla. (7.6°F), and New Orleans (7.1°F). 

Fall low temperatures act as an environmental cue for plants to prepare for the harshness of winter. As these low temperatures increase, they encourage plants to use their resources for continued growth instead of storing them —a notable example being the delay of fall foliage emergence. Such an extension of the growing season also means that ragweed pollen creeps further into the fall season—scary news for the nearly 20 million American adults who suffer from pollen allergies.

METHODOLOGY: Individual city temperature trends and records are calculated using data from theApplied Climate Information System. Temperature trends are since 1970; records include the entire period of record. Displayed trend lines are based on a mathematical linear regression. 

Climate Central's local analyses include 244 stations. However, for data summaries based on linear trends, only 242 stations are included due to large data gaps in St. Johnsbury, Vermont and Wheeling, West Virginia.