- Climate normals
- 30-year averages of monthly temperature and precipitation. NOAA publishes new normals every decade; this site uses the 1991-2020 normals.
- Average high (tmax)
- The average daily maximum temperature for a given month, measured in degrees Fahrenheit.
- Average low (tmin)
- The average daily minimum temperature for a given month, measured in degrees Fahrenheit.
- Average temperature (tavg)
- The mean of the daily high and low temperatures for a month — roughly what "typical" weather feels like.
- Precipitation
- Total rainfall plus the water equivalent of snow, reported in inches.
- Last spring frost
- The average calendar date of the final overnight low at or below 32°F in spring. Gardeners use this to time the planting of tender annuals and warm-season vegetables.
- First fall frost
- The average date of the first overnight low at or below 32°F in autumn — the traditional end of the growing season.
- Growing season
- The number of days between the average last spring frost and average first fall frost.
- p10 / p50 / p90
- Probability thresholds. For frost dates: p10 is the 10th percentile (early), p50 the median (average), p90 the 90th percentile (late). There's a 10% chance of frost after the p10 spring date, 50% after p50, and 90% after p90.
- GHCN / GHCN-Daily
- Global Historical Climatology Network — NOAA's primary source of daily temperature and precipitation observations worldwide.
- Station
- An official weather observation site, usually at an airport or dedicated meteorological facility, that feeds data into the GHCN.
- Weather vs climate
- Weather is what you see out the window today; climate is the long-term pattern. A single hot day in Phoenix is weather; the fact that Phoenix has 100°F+ summers every year is climate. This site is about climate, not weather forecasts.
- Köppen climate zone
- The most widely used climate classification, named after climatologist Wladimir Köppen. Uses two or three letters describing the broad climate group (tropical, arid, temperate, continental, polar), precipitation seasonality, and summer temperature. Browse cities by zone at /climate-zones/.
- USDA Plant Hardiness Zone
- A 1-13 scale (each subdivided "a"/"b") indicating how cold a place gets in an average winter. Used by gardeners to know whether perennial plants will survive year-to-year. Zone 5a is colder than 5b; Zone 6 is one band warmer than 5. Browse cities by zone →
- Heating Degree Days (HDD)
- A measure of how much heating energy a building needs over a year. Each day's heating need is calculated as max(65°F − daily avg temp, 0); HDD per year is the sum across all days. Minneapolis (HDD ≈ 7,500) needs roughly three times the heating energy of Seattle (HDD ≈ 4,200).
- Cooling Degree Days (CDD)
- The reverse of HDD — measures air-conditioning demand. Each day's cooling need is max(daily avg temp − 65°F, 0); CDD per year is the sum across all days. Miami (CDD ≈ 4,000) needs roughly 6× the cooling energy of San Francisco (CDD < 700).
- Growing Degree Days
- A measure of accumulated heat available for plant growth. Many crops require a specific GDD total to reach maturity. Often computed against a 50°F base for sweet corn, 32°F for wheat, etc. Higher GDD = faster crop development.
- Diurnal temperature range
- The difference between the day's high and the night's low. Desert climates have large diurnal ranges (often 30°F+); marine climates have small ones (sometimes < 15°F).
- Dew point
- The temperature at which water vapor in the air starts to condense. A higher dew point means more humid, "muggier" air. Dew points above 65°F feel humid; above 75°F feel oppressive. Not currently tracked on this site — coming with future data sources.
- Climate twin
- A US city whose 12-month climate pattern (monthly temperature + precipitation) most closely matches another city's, with a minimum geographic separation to avoid trivial near-neighbor matches. How we compute this →