Understanding Climate Normals

What Are Climate Normals?

A climate normal is a 30-year average of a specific weather measurement, temperature, precipitation, snowfall, wind speed, humidity, computed for a specific location and time period. NOAA publishes these averages for thousands of weather stations across the United States, organized by month and by year. When someone says a city "averages 45 inches of rain per year," they are almost certainly quoting a climate normal.

Climate normals are not predictions. They describe historical patterns. A city with a normal January high of 38°F will not necessarily have a 38°F high on any given January day, it might be 15°F or 55°F. The normal tells you what the average of many January days looks like across 30 years. This distinction matters because it sets realistic expectations without being falsely precise.

Explore state-level climate summaries or browse individual city pages on PlainClimate to see how normals vary across the country.

Climate vs. Weather: A Critical Distinction

The most common confusion in discussing climate data is treating weather and climate as interchangeable. They are not. Weather describes atmospheric conditions at a specific moment, right now, today, this week. Climate describes the long-term patterns that make those conditions more or less likely.

A useful way to think about it: weather is what you wear today; climate is what's in your wardrobe. Phoenix, Arizona has a hot desert climate, meaning you will generally need summer clothes most of the year, but an unusual cold snap might have you reaching for a jacket in February. One cold week does not change the climate record, but it is genuinely unusual weather.

Climate normals capture this distinction precisely. They filter out the noise of individual years and seasons to reveal the underlying signal of what a place is climatically like. Year-to-year variability remains real, climate normals do not eliminate it, but they tell you what you should plan for, not just what happened last year.

How NOAA Calculates the 1991-2020 Normals

The NOAA U.S. Climate Normals 1991-2020 dataset is the most comprehensive update in the program's history. NOAA collected daily observations from weather stations across the country, airport automated stations, cooperative observer networks, and remote monitoring sites, and processed them through a rigorous quality-control pipeline before computing averages.

Not every station contributed to every variable. A station that only recorded temperature and precipitation could not produce normals for humidity, wind speed, or cloud cover. NOAA sets minimum data thresholds, a station needs at least 10 years of valid data in the 30-year period to produce a normal for most variables, and higher thresholds for some specialized metrics. This is why some cities on PlainClimate have richer data profiles than others.

The dataset includes several distinct normal types: simple averages (the arithmetic mean over 30 years), standard deviations (measuring variability), and derived statistics like growing degree days, frost dates, and heating/cooling degree days. PlainClimate surfaces the most relevant of these for each city.

What the 1991-2020 Baseline Captures

The shift from the previous 1981-2010 normals to the current 1991-2020 baseline has measurable effects on the numbers. Because the US (and the world) warmed during the 2010s, the new 30-year window includes those warmer years and excludes the cooler 1980s. For most US cities, annual average temperatures in the new normals are 0.1°F to 0.5°F warmer than the previous baseline.

This matters practically: if you are sizing an HVAC system, estimating heating fuel needs, or planning a garden around frost dates, using the 1991-2020 normals rather than an older baseline gives you a more accurate picture of current conditions. Agricultural extension offices, building code standards, and real estate professionals all rely on the current NOAA normals for precisely this reason.

For relocation research, the 1991-2020 normals are the right tool. Browse the climate rankings to compare cities by temperature, precipitation, comfort, and more, all based on the 1991-2020 baseline.

What Climate Normals Don't Tell You

Climate normals describe central tendencies, not extremes or future conditions. They do not tell you the worst-case winter storm you might face, the probability of a 100-year flood, or how climate might change over the next 30 years. For extreme event planning, NOAA publishes separate products like return-period precipitation estimates and storm-frequency analyses.

Normals also smooth out micro-climatic variation within a city. Urban heat islands, elevation differences, and proximity to water bodies can cause temperatures to vary significantly within just a few miles, more than the difference between two cities in different states. A weather station at a regional airport may not perfectly represent the climate of a nearby neighborhood at higher elevation or closer to a river.

With those caveats, NOAA climate normals remain the most reliable publicly available benchmark for understanding US climate patterns across thousands of locations. PlainClimate makes this station record accessible location by location; search any of 6,861 station-mapped profiles using the homepage search.

Frequently Asked Questions