Climate normal

NOAA's National Centers for Environmental Information (NCEI) calls the 1991 to 2020 U.S. Climate Normals, in the release's documentation, “conventional 30-year normals of many weather and climate variables,” and its product page defines one normal as the 30-year average of a particular variable's measurements over a uniform time period. Every month in a normals table is averaged over the same thirty years.

NCEI also says what normals are for, and it gives them two jobs: they act “as a ruler” to compare today's weather and tomorrow's forecast, and “as a predictor” of conditions in the near future. The Naples pages on this site use a narrower shorthand of their own, a thirty-year average and not a forecast for any single year. That wording is the site's rather than NOAA's, and it describes the ruler job: a particular year is measured against its normal, not read off it.

The set the site's Naples pages cite is the 1991 to 2020 monthly normals for Naples Municipal Airport, station USW00012897, printed side by side in the Naples year, month by month; what the summer normals say, and do not say, about a hot day is covered at whether a roof can be cleaned in the summer heat.

A normal does not need thirty years of observations behind it. NCEI's documentation says normals are calculated “with 30 years if the station has at least 10 years of valid observations,” a station it considers representative, “with the remaining years estimated using surrounding stations with more complete values.” That class is the R flag.

At Naples Municipal Airport the monthly rain and temperature normals carry R, each resting on between 16 and 19 years of the station's own record, while the station's counts of days with measurable precipitation carry P, for Provisional, the class whose missing months “cannot be filled due to lack of surrounding stations.”

The word provisional travels with those day counts wherever they are printed, and a count of days with measurable precipitation is not a count of dry ones; whether any normal counts the dry days a roof wash waits for is settled in whether a roof can be cleaned in the rainy season.

A normal is an average, and NCEI publishes a second view of the same rain beside it: the first, second and third quartiles of monthly nonzero precipitation totals, in its documentation's words. The second quartile is the median.

At Naples Municipal Airport the median sits below the normal in September, October and November. September's median of nonzero totals is 7.57 inches against a normal of 8.60, with its first and third quartiles at 6.08 and 10.76; October's median is 3.24 inches against 3.93, and November's is 1.08 against 1.39. Each figure is an NCEI statistic for 1991 to 2020 at that station, flagged R.

An average sitting above its median is what a few very wet months do to a thirty-year mean, pulling the average up while the middle of the record stays lower; that is general statistical reasoning, not an NCEI sentence. How a dry month and a dry day differ for a sealer is worked out at the right month for exterior cleaning in Naples, and the wet half of the year carries the name rainy season.