Methodology
How we collect, process, and present NOAA climate data for every US county.
Data Source
All climate data on WeatherByCounty comes from the NOAA National Centers for Environmental Information (NCEI), the United States government's authoritative source for climate information.
Specifically, we use the U.S. Climate Normals (1991–2020). Climate normals are 30-year averages of weather variables computed by NOAA, representing the baseline of typical conditions for a location. They are updated once per decade and are the standard reference for “normal” weather in the United States.
NOAA NCEI data is in the public domain and freely available for any use without restriction.
Weather and Climate Metrics
We extract and present the following metrics for each county:
| Metric | Description | Unit |
|---|---|---|
| Average Annual Temperature | Mean temperature averaged across all 12 months | °F |
| Average January Temperature | Coldest-month baseline for winter climate | °F |
| Average July Temperature | Warmest-month baseline for summer climate | °F |
| Summer / Winter Averages | Seasonal averages (Jun–Aug and Dec–Feb) | °F |
| Annual Precipitation | Total rainfall and liquid-equivalent snowfall | inches |
| Annual Snowfall | Total snow accumulation per year | inches |
| Extreme Heat Days | Average number of days per year exceeding 90°F | days/year |
| Monthly Normals | Temperature, precipitation, and snowfall for each of the 12 months | °F / inches |
How Weather Scores Are Calculated
Weather scores on WeatherByCounty use a percentile-rank methodology. Each county is ranked relative to all other US counties across multiple climate dimensions. The result is a 0–100 score where higher values indicate more moderate, temperate climates.
The scoring factors include:
- Temperature moderation: Counties with mild annual temperatures (closer to the national median) score higher. Extreme heat and extreme cold both reduce scores.
- Seasonal range: Counties with smaller differences between summer and winter temperatures score higher, reflecting more stable year-round climates.
- Extreme heat days: Fewer days above 90°F increases the score. Counties with many extreme heat days are penalized.
- Precipitation balance: Counties with moderate annual precipitation score higher than those with very low (drought risk) or very high (flood risk) rainfall.
A weather score of 75 means the county has a more moderate climate than 75% of all US counties. Scores are used on the CountyScore composite, where weather is one of several dimensions in the overall county ranking.
Geographic Coverage
WeatherByCounty covers approximately 89% of all 3,144 US counties. Coverage depends on the availability of NOAA weather stations within or near each county boundary.
Approximately 355 counties — primarily in sparsely populated areas of the western United States and Alaska — lack sufficient weather station data for reliable climate normals. These counties still have pages on the site but display a notice that weather data is not available.
Station-level data is aggregated to the county level using spatial averaging of all weather stations within or near each county boundary. Counties with multiple stations receive averaged values; counties with a single station use that station's data directly.
Data Freshness
Climate normals are, by definition, 30-year averages. The current dataset (1991–2020) was released by NOAA in 2021 and represents the most recent official normals available. NOAA typically updates climate normals once per decade; the next update (2001–2030) is expected around 2031.
Because normals represent long-term averages, they do not reflect short-term weather events or year-to-year variability. They are best used to understand the typical climate of a county rather than to predict specific weather conditions.
AI-Generated Content Disclosure
Some descriptive text on WeatherByCounty — including state climate overviews and county narrative summaries — is generated with the assistance of AI language models (Claude by Anthropic). All AI-generated narratives are based on the underlying NOAA data and are reviewed for accuracy.
All numerical data, tables, scores, and rankings are computed directly from NOAA source data and are not AI-generated. The AI is used solely to produce readable prose summaries of the data.
Disclaimer
Climate data on this site is informational only and represents historical 30-year averages. It is not a substitute for official National Weather Service forecasts, severe weather warnings, or emergency management guidance. Always consult official sources during weather emergencies. Data may differ from other published analyses due to differences in aggregation methodology.