NOAA data shows US cities hit 32°F months apart, from September in Fargo to January in Phoenix

Fargo, North Dakota, gets hit by its first freeze in late September; Phoenix has to wait until January, while LA and Miami rarely get to 32°F at all. NOAA’s 30-year normals express the date of a first freeze as a median value at 50% probability—half the time the freeze will come before it, and half the time after. But these dates have been shifting. According to the EPA (US Environmental Protection Agency), the first fall freeze now comes later than the long-term average, and the growing season is over two weeks longer than in the early 1900s.

The first fall freeze doesn’t spread in an even wave going North to South. Putting the data on a map clearly shows clusters of early freezes in the North and at high elevations. October is the most common month—much of the high-elevation West to the Midwest and Northeast has to prepare for sub-32°F temperatures by the time Halloween comes around, including Denver, CO, Billings, MT, Indianapolis, IN, and Manchester, NH.

Looking at the map, we can notice that even the latitude doesn’t determine the date of the first fall freeze—Boston and Detroit are both at 42° latitude, but the latter experiences 32°F for the first time in October, while the former in November.

The date of the first freeze depends on elevation and proximity to the coast and to the Gulf of Mexico. For example, the weather station at Denver International Airport is located at an altitude of 5,434 feet—the city can expect its first freeze in early October. Wilmington, DE, situated at a similar latitude, is on the coast and at sea level, which keeps it insulated till November.

So, the first freeze happens at 32°F; but one day, you wake up to a crispy lawn while the local news station tells you the overnight low was 35°F. This is because the official temperatures aren’t measured on the ground. To get standardized readings, meteorologists position their measuring boxes about five feet above the ground.

When a professional thermometer set at five feet detects 35°F, the temperature down on the ground might be 31°F. On clear nights, the ground cools faster because the heat radiates into the sky, because it’s not blocked and sent back by clouds, and the dense cold air remains close to the ground.

Freeze dates are shifting. According to the EPA, since 1980, the first fall freeze has come about three days later than the 1895–2023 average. Three days might not seem like much, but it’s important for agriculture—this shift is connected to a broader phenomenon that has lengthened the growing season by over two weeks since the early 1900s. This means that we can grow different crops or harvest multiple times. But the longer growing season also has some downsides: invasive species and weeds might start developing, some crops cannot be grown, and there might arise a need to irrigate more.

If you’re curious to find out when your area gets hit by its first fall freeze, look at the table below.