Understanding Climate Normals
What 30-year averages are, how NOAA calculates them, and why the 1991-2020 baseline is the authoritative benchmark for US climate research.
Key Takeaway
Climate normals are 30-year statistical averages that describe what weather is "normal" for a specific location. The current NOAA standard covers 1991-2020, based on data from 15,492 US weather stations. They are the standard reference for agriculture, engineering, urban planning, and anyone researching where to live. A single year's weather can be wildly unrepresentative — normals show the long-run pattern.
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 data accessible city by city — search for any of 6,915 cities using the homepage search.
Frequently Asked Questions
What exactly is a "climate normal"?
A climate normal is a 30-year average of a weather variable — temperature, precipitation, snowfall, wind — calculated for a specific location. NOAA computes normals for thousands of weather stations across the US. The goal is to establish what "typical" weather looks like for any given place and time of year, smoothing out the year-to-year variability that makes individual seasons misleading.
How often does NOAA update climate normals?
NOAA updates climate normals every 10 years, shifting the 30-year window forward by a decade. The current normals cover 1991-2020. The previous set covered 1981-2010. The next update will cover 2001-2030 and is expected around 2031. This decadal cadence is standard practice across international meteorological agencies.
What is the difference between climate and weather?
Weather is what is happening outside right now — today's temperature, whether it is raining, the wind speed. Climate is the long-term pattern of weather in a region. A simple way to remember it: climate is what you expect, weather is what you get. Climate normals quantify the "what you expect" part — they tell you what the typical July temperature range is for a city, not what any specific July will bring.
Why use a 30-year period specifically?
The World Meteorological Organization (WMO) established the 30-year standard because it is long enough to represent a stable climate signal while remaining short enough to reflect gradual climate changes. Shorter periods (say, 10 years) can be dominated by anomalous stretches of warm or cold years. Longer periods become less relevant for current planning. The 30-year window balances statistical reliability with practical relevance.
What does the 1991-2020 baseline mean for me?
The 1991-2020 baseline reflects the climate of the most recent three decades — capturing recent warming trends while excluding the exceptionally warm years of the 2020s. If you are researching a city's typical winter temperatures or comparing how rainy two cities are, the 1991-2020 normals give you the most current authoritative benchmark available. For most planning purposes — gardening, relocation research, HVAC sizing — this baseline is highly reliable.
How many stations contribute to NOAA climate normals?
The NOAA 1991-2020 Climate Normals dataset includes approximately 15,000 weather stations in the US, ranging from major airport stations with comprehensive observations to smaller cooperative observer stations that may track only temperature and precipitation. PlainClimate provides data for 6,915 cities matched to these stations, with the most data-rich cities benefiting from stations that also recorded humidity, wind, and cloud cover.
Explore PlainClimate Data
Related Guides
Sources
- NOAA National Centers for Environmental Information — U.S. Climate Normals 1991-2020 (v1.0.1)
- World Meteorological Organization — Guidelines on the Calculation of Climate Normals (WMO-No. 1203)
- NOAA NCEI — Climate Normals Technical Documentation
This content is for informational and educational purposes only. Climate normals describe historical averages and do not predict future weather conditions. Always consult NOAA and local meteorological services for current forecasts and planning guidance.
Understanding the Data
The information presented throughout this guide is informed by publicly available public records published by federal and state government agencies. Our database aggregates and standardizes these records to make them more accessible and easier to interpret for general audiences. When we reference specific statistics or trends, they are drawn directly from these authoritative sources unless explicitly noted otherwise.
It is important to understand the limitations of any large-scale data dataset. Records may contain errors from the original data collection process, some fields may be incomplete for older entries, and classification systems may have changed over time. Our analysis accounts for these factors by clearly labeling data vintage, flagging records with missing critical fields, and noting when temporal comparisons span methodology changes in the source data.
For readers who want to conduct their own research, we recommend going directly to the source whenever possible. federal and state government agencies provides detailed documentation on collection methodology, sampling frames, and known data quality issues. Our goal is not to replace primary sources but to make them more approachable and to highlight patterns that may not be immediately obvious when browsing raw records.
How We Analyze Data Records
Our analytical approach involves several steps designed to surface meaningful insights from large datasets. First, we clean and standardize the raw data, handling variations in naming conventions, date formats, and categorical labels. Then we compute summary statistics, distributions, and comparative benchmarks across relevant dimensions such as geography, time period, and category type.
Key metrics we examine include statistical records, geographic distributions, temporal trends. These indicators provide a multi-dimensional view of each entity in our database, allowing users to understand not just individual records but how they compare to peers, regional averages, and national benchmarks. We believe this contextual approach is far more valuable than presenting raw numbers in isolation.
