Climate Design Data Workflow for Seattle, WA

Direct answer

NASA POWER's 2001–2020 climatology at the Seattle, WA reference point reports an annual mean 2 m temperature of 10.5 °C and relative humidity of 83.2%. The highest monthly maximum is 38.0 °C in July, while the lowest monthly minimum is -10.4 °C in December; these are screening statistics, not code design temperatures. The point response uses LST and the 20-year Meteorological and Solar Monthly & Annual Climatologies (January 2001 - December 2020); its annual corrected precipitation is 3.37 mm/day and annual 10 m wind speed is 2.04 m/s. Use the monthly series to screen source selection before applying project design conditions. NASA POWER is gridded climatology: verify elevation, microclimate, urban exposure, station or weather-file suitability, current code edition and the authority's required source before design use.

Content and cited inputs reviewed by Harth team · Published 2026-08-17 · Updated 2026-08-31.

Which climate inputs control the design?

1. Confirm the project jurisdiction and adopted energy-code edition.
2. Select the authoritative station and record why it applies.
3. Record heating dry-bulb percentiles and cooling dry-bulb with coincident moisture conditions.
4. Keep IP/SI units, edition, observation period and retrieval date with every value.
5. Separate climate-zone envelope requirements from HVAC sizing conditions.

Seattle, WA NASA POWER monthly climate screening table

Month-by-month climate interpretation

• January: mean temperature 4.38 °C is 6.13 °C below the annual mean; the monthly max–min span is 27.89 °C. Relative humidity is 90.36% (7.13 points above annual), while corrected precipitation at 5.60 mm/day is 2.23 mm/day wetter than the annual daily rate.
• February: mean temperature 4.58 °C is 5.93 °C below the annual mean; the monthly max–min span is 26.97 °C. Relative humidity is 87.28% (4.05 points above annual), while corrected precipitation at 3.83 mm/day is 0.46 mm/day wetter than the annual daily rate.
• March: mean temperature 6.66 °C is 3.85 °C below the annual mean; the monthly max–min span is 25.75 °C. Relative humidity is 85.29% (2.06 points above annual), while corrected precipitation at 4.46 mm/day is 1.09 mm/day wetter than the annual daily rate.
• April: mean temperature 9.02 °C is 1.49 °C below the annual mean; the monthly max–min span is 32.32 °C. Relative humidity is 81.91% (1.32 points below annual), while corrected precipitation at 3.18 mm/day is 0.19 mm/day drier than the annual daily rate.
• May: mean temperature 12.80 °C is 2.29 °C above the annual mean; the monthly max–min span is 32.84 °C. Relative humidity is 79.72% (3.51 points below annual), while corrected precipitation at 2.27 mm/day is 1.10 mm/day drier than the annual daily rate.
• June: mean temperature 15.20 °C is 4.69 °C above the annual mean; the monthly max–min span is 29.19 °C. Relative humidity is 79.96% (3.27 points below annual), while corrected precipitation at 1.71 mm/day is 1.66 mm/day drier than the annual daily rate.
• July: mean temperature 18.17 °C is 7.66 °C above the annual mean; the monthly max–min span is 30.09 °C. Relative humidity is 77.33% (5.90 points below annual), while corrected precipitation at 0.62 mm/day is 2.75 mm/day drier than the annual daily rate.
• August: mean temperature 18.27 °C is 7.76 °C above the annual mean; the monthly max–min span is 25.24 °C. Relative humidity is 75.99% (7.24 points below annual), while corrected precipitation at 0.95 mm/day is 2.42 mm/day drier than the annual daily rate.
• September: mean temperature 15.40 °C is 4.89 °C above the annual mean; the monthly max–min span is 28.98 °C. Relative humidity is 78.07% (5.16 points below annual), while corrected precipitation at 1.94 mm/day is 1.43 mm/day drier than the annual daily rate.
• October: mean temperature 10.76 °C is 0.25 °C above the annual mean; the monthly max–min span is 28.52 °C. Relative humidity is 83.89% (0.66 points above annual), while corrected precipitation at 4.03 mm/day is 0.66 mm/day wetter than the annual daily rate.
• November: mean temperature 6.66 °C is 3.85 °C below the annual mean; the monthly max–min span is 30.27 °C. Relative humidity is 88.95% (5.72 points above annual), while corrected precipitation at 6.27 mm/day is 2.90 mm/day wetter than the annual daily rate.
• December: mean temperature 3.83 °C is 6.68 °C below the annual mean; the monthly max–min span is 25.87 °C. Relative humidity is 90.27% (7.04 points above annual), while corrected precipitation at 5.65 mm/day is 2.28 mm/day wetter than the annual daily rate.

Harth retrieved this point climatology from NASA POWER API v2.9.7 for location record us:seattle-wa. It is a reproducible screening dataset, not an hourly weather file, forecast, code design condition or substitute for local professional review.

Where can you explore related guidance?

• Explore all climate data guidance
• ASHRAE / IECC Climate Zones Explained (Zones 1–8, A/B/C)
• Heating & Cooling Design Temperatures (99%, 1% / 0.4%)
• Climate Design Data Workflow for Portland, OR
• Climate Design Data Workflow for Boise, ID
• Sun Path & Solar Angles for Seattle, WA

Sources and method

Licensed ASHRAE Climatic Design Information / Standard 169 station data plus the locally adopted energy code. Coordinates 47.6062° N (America/Los_Angeles) are approximate routing references. Figures are illustrative — confirm the representative station and current edition. Climate screening data from the NASA POWER Climatology API documentation and exact point response, retrieved 2026-08-31; source period: 20-year Meteorological and Solar Monthly & Annual Climatologies (January 2001 - December 2020). NASA POWER values are gridded screening statistics, not code design conditions or guaranteed local observations.

Source links

• power.larc.nasa.gov

• power.larc.nasa.gov