Data Sources and Methods

Where the data comes from

VSWeather is built on historical reanalysis data from Open-Meteo, which combines decades of observations into a consistent gridded record of daily weather reaching back to 1940. From it we compute climate normals for the years 1991 through 2020, the standard 30 year reference period that meteorologists use.

We also use station normals published by the United States National Centers for Environmental Information (NCEI), part of NOAA, to check and calibrate our figures. NOAA data is not shown directly on the site. It is the reference we measure our numbers against.

Required credits

VSWeather displays and builds on the following sources, with the credits their licenses require:

• Weather data by Open-Meteo.com, used under the Creative Commons Attribution 4.0 license (CC BY 4.0).
• This product contains modified Copernicus Climate Change Service information. The underlying reanalysis, ERA5, is produced by the European Centre for Medium Range Weather Forecasts for the Copernicus Climate Change Service. Neither the Service nor the Centre is responsible for any use of this information.
• Station normals from NOAA NCEI, which are in the public domain.

How we compute the numbers

For every city we reduce 30 years of daily values into typical yearly and monthly figures:

• Temperatures are the average daily high and low for each month across the period.
• Rainfall and snowfall are annual and monthly totals, averaged across the period.
• A rainy day and a snowy day use day count thresholds chosen to match the conventions NOAA uses, adjusted so the gridded data lines up with what gauges on the ground record.
• Sunny days per year counts days whose average cloud cover falls below a threshold we tuned against NOAA's published sky cover values, since sunshine is not part of standard climate normals and has to be derived.

Where a location sits in terrain that the weather grid represents poorly, we anchor its precipitation to nearby measuring stations rather than publish a figure we know is off. Snowfall is corrected against station records in cold and lake effect climates, where the raw model reads low. These adjustments, and the places they cover, are recorded with each city's data.

Units

Climate values are stored internally in metric units and shown in Fahrenheit and inches for our launch audience in the United States. Other unit options are planned.

A note on accuracy

Reanalysis describes a grid cell of roughly ten kilometers, not one exact spot, so a value can differ from a specific backyard or official station, especially in mountains and along coastlines. VSWeather is a guide to how places compare over the long run, not a substitute for official records. See our Terms of Use for the full disclaimer.