Methodology

Data: NOAA National Centers for Environmental Information, Global Historical Climatology Network–Daily (GHCN-Daily), station files for U.S. Weather Service/FAA first-order stations with long records. U.S. government data, public domain. Elements used: daily snowfall (SNOW), snow depth (SNWD), and minimum temperature (TMIN). Values with a quality-control flag are discarded.

Period: the last 30 completed years: snow seasons July 1 – June 30 from 1995-96 through 2024-25; freeze years 1995–2024; white Christmas years 1995–2024. (NOAA’s official Climate Normals use 1991–2020, so figures can differ slightly from NOAA’s published ones. For example NOAA lists Chicago’s white Christmas chance at about 40% and we compute 39%.)

First snow: for each season, the first calendar day with snowfall of at least 3 mm (0.1 in.), 25 mm (1 in.), or 76 mm (3 in.). A season counts only if at least 85% of days from Oct 1 to Apr 30 have a snowfall record. The “median date” is the middle of the seasons that had such a day; “typical range” is the 10th to 90th percentile. Seasons are compared on a leap-year-neutral calendar.

Freezes: first fall freeze = first day on or after Aug 1 with a daily low at or below 32°F (0°C), or 28°F (−2.2°C) for a hard freeze; last spring freeze = last such day on or before Jul 31. A year counts only if at least 90% of days in the window have a temperature record. Freeze-free days = first fall freeze minus last spring freeze in the same year, median across years.

White Christmas: share of years with at least 1 inch (also 3 and 6) of snow on the ground on the morning of Dec 25, among years with a depth recorded that day (at least 24 of 30 required).

Limits: one station per place, typically an airport; your neighborhood can differ, especially in hills or near lakes and cities. Averages say nothing certain about any particular year. Station records can have gaps or measurement changes. These pages are not weather forecasts. For forecasts use the National Weather Service.

Refresh: the “this season so far” sections are rebuilt from NOAA data about weekly.