How to read climate data: Highs and lows, precipitation, snow, frost probabilities and records
Every PlainClimate page reads the same way: the place, the NOAA station behind it, the year in one sentence, then the months, frost dates, extremes, records and recent years. Take Denver: 14.5 inches of precipitation in a normal year at Denver International Airport, where NOAA publishes no snowfall normal, so the page says so instead of guessing.
First published · revised
Highs, lows and the mean
The normal high is the average afternoon maximum; the normal low the average overnight minimum; the mean is their midpoint. A month's high and low are averages of 30 years of days, so about half of the days in a normal month are warmer than its normal high.
Precipitation and snow
Precipitation is rain plus the water in melted snow, in inches; snowfall is the depth of new snow before it melts. A rough rule is 10 inches of snow to one inch of water, but it varies from 5 for wet snow to 20 or more for powder, so the two columns are not interchangeable. "Wet days" counts days with 0.01 inch or more.
Frost probabilities
In Denver the last spring frost comes after May 5 in half of years and after May 19 in one year in ten. The half-of-years date is the median; the one-in-ten date is the cautious one.
Records and recent years
Records come from the station's own history, or for large cities from the National Weather Service's official city record, which joins the stations a city has used since the 1800s. Recent years show how each year since 2021 compared with the normal: the normal is the yardstick, not the expectation for any single year.
Source: NOAA NCEI 1991-2020 normals.