Fall is the transition season between summer and winter, so it should come as no surprise that October can be a very changeable month.
Here are seven weather changes to expect during the month of October.
1. Decreasing daylight
The amount of daylight quickly grows shorter as October progresses, with the fastest decrease at the beginning of the month. Many cities lose 2 to 3 minutes of daylight each day, leading to later sunrises and earlier sunsets.
New York City loses 1 hour and 17 minutes of daylight between the first and last day of the month, while Seattle finishes October with 1 hour and 38 minutes less daylight than when the month began.
For some, the last 7 p.m. sunset of the year occurs in October, ushering in a five-month period in which the sun goes down before 7 p.m.
2. Decreasing temperatures
Average temperatures quickly decrease during October because of the shorter days and longer nights.
In Philadelphia, the average high drops from 73 degrees on Oct. 1 to 62 degrees by Halloween. Chicago's average high falls from 69 degrees to 56 degrees between the first and last day of the month.
3. First freeze of the season
Along with October's lower temperatures come the first frosts and freezes.
The higher elevations of the West and areas along the Canadian border often have their first freeze of the season before October begins, but the season's first 32-degree temperature typically occurs in October for a large portion of the United States.
This includes much of the Northeast, Midwest, mid-South and the Northern and Central Plains. October is also when the first freeze usually occurs in much of Oklahoma and the Texas Panhandle, New Mexico and northeastern Arizona.
4. First snow of the season
The first snowfall can also accompany October's lower temperatures.
While the highest elevations of the Rockies can see snow in September or even August, many areas of the U.S. receive their first snowfall of the season during October in an average year.
The season's first snow usually falls in October in parts of the Four Corners, Central Plains, upper Midwest, Great Lakes and interior Northeast.
5. Late-season tropical threats along the Gulf and East coasts
Tropical activity gradually starts to wane in October, but it can still be an active month for tropical storms and hurricanes, especially at the beginning of the month.
Most areas of the Atlantic Basin remain open for tropical development, but the Main Development Region between Africa and the Lesser Antilles becomes less favorable, so a lot of the activity tends to shift westward when compared to September.
The western Caribbean Sea, the eastern Gulf of Mexico and the western and central Atlantic Ocean are the areas most likely to have a tropical storm or hurricane in October. This places parts of the Gulf and East coasts at risk for a landfalling storm.
South Florida has been struck by more hurricanes in October than any other month. Through 2020, 26 October hurricanes had passed within 100 nautical miles of Miami since recordkeeping began in 1851, according to NOAA's Historical Hurricane Tracks.
6. Peak of fall foliage
While the higher elevations of the northern Rockies are usually already past their peak in October, most other areas of the country will see their most vibrant fall foliage of the year.
In early October, peak or near-peak color is expected in other parts of the Rockies and Great Basin, as well as the upper Midwest, northern Great Lakes, interior Northeast and parts of the central Appalachians.
By mid- to late October, many other areas are expected to reach peak or near-peak conditions, including the Pacific Northwest, Northern Plains, Ozarks, Midwest, southern Great Lakes, mid-Atlantic, the rest of the Northeast and much of the central and southern Appalachians.
Peak fall foliage usually spreads into the South by early November.
7. Santa Ana winds return to California, increasing wildfire concerns
Normally, winds along the California coastline blow inland from the ocean. This is what gives that region such pleasant climates most of the year.
But during a Santa Ana wind event, the wind pattern shifts offshore and brings in hot, dry air from the east side of mountain ranges that border the coast.
Fall is when these Santa Ana winds typically become more common in California. This is the time of year a strong high-pressure system frequently parks itself over the Great Basin, while a complimentary low-pressure system sets up off the California coast.
Since the atmosphere prefers to be balanced, the air rushes quickly from the area of high pressure to the area of low pressure, resulting in the offshore Santa Ana winds over California.
"They typically commence in, you know, late September, October, right, and then they go throughout the winter," said John Abatzoglou, an associate professor of climatology at the University of California Merced.
While there is no direct connection between Santa Ana winds and wildfires, they can increase the risk of wind-driven fires in parts of California.
Following the summer dry season, soils are generally driest in the early fall, providing plenty of dry fuels for potential wildfires. If Santa Ana winds come at the right time, they can spread an existing wildfire out of control or turn a newly developed small brush fire into an inferno.
"When we move into September, October, November, that's when we begin to worry about the wind-driven fire season in these coastal environments," Abatzoglou said.