# Climate Design Data Workflow for Seattle, WA

> A source-first workflow for retrieving and recording building-design climate conditions for Seattle, WA without mixing climate normals, code zones, and HVAC design percentiles.

- Canonical: https://harth.build/resources/climate-data/climate-design-data-workflow-seattle-wa/
- Last updated: 2026-08-31
- Data source: 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](https://power.larc.nasa.gov/docs/services/api/temporal/climatology/) and [exact point response](https://power.larc.nasa.gov/api/temporal/climatology/point?parameters=T2M%2CT2M_MAX%2CT2M_MIN%2CRH2M%2CPRECTOTCORR%2CWS10M&community=SB&longitude=-122.3321&latitude=47.6062&format=JSON), 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.

## Key facts

| Fact | Value |
| --- | --- |
| NASA POWER climatology | 20-year Meteorological and Solar Monthly & Annual Climatologies (January 2001 - December 2020) (API v2.9.7; LST) |
| Annual mean temperature | 10.5 °C (2 m gridded climatology; not a design percentile) |
| Highest monthly maximum | 38.0 °C (July) (monthly climatological statistic) |
| Lowest monthly minimum | -10.4 °C (December) (monthly climatological statistic) |
| Annual relative humidity | 83.2% (2 m gridded climatology) |
| Annual corrected precipitation | 3.37 mm/day (climatological daily rate) |
| Annual wind speed | 2.04 m/s (10 m gridded climatology; not a structural design wind speed) |

## 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](https://www.linkedin.com/company/harth-build) · 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.

| Field | Required record |
| --- | --- |
| Station | Name, ID, coordinates and elevation |
| Source | Publisher, edition and licensed/public status |
| Heating | 99.6% and 99% dry-bulb |
| Cooling | 0.4%, 1% and 2% dry-bulb plus coincident wet-bulb |
| Moisture | Dew-point or humidity design condition used |
| Code | Adopted edition, amendments and effective date |

## Seattle, WA NASA POWER monthly climate screening table

| Month | Mean °C | Monthly max °C | Monthly min °C | RH % | Precipitation mm/day | Wind at 10 m m/s |
| --- | ---: | ---: | ---: | ---: | ---: | ---: |
| January | 4.38 | 17.52 | -10.37 | 90.36 | 5.60 | 2.42 |
| February | 4.58 | 17.23 | -9.74 | 87.28 | 3.83 | 2.24 |
| March | 6.66 | 21.75 | -4.00 | 85.29 | 4.46 | 2.45 |
| April | 9.02 | 30.46 | -1.86 | 81.91 | 3.18 | 2.13 |
| May | 12.80 | 32.46 | -0.38 | 79.72 | 2.27 | 1.76 |
| June | 15.20 | 33.82 | 4.63 | 79.96 | 1.71 | 1.65 |
| July | 18.17 | 38.03 | 7.94 | 77.33 | 0.62 | 1.49 |
| August | 18.27 | 33.04 | 7.80 | 75.99 | 0.95 | 1.57 |
| September | 15.40 | 32.73 | 3.75 | 78.07 | 1.94 | 1.78 |
| October | 10.76 | 26.23 | -2.29 | 83.89 | 4.03 | 2.11 |
| November | 6.66 | 21.44 | -8.83 | 88.95 | 6.27 | 2.47 |
| December | 3.83 | 15.45 | -10.42 | 90.27 | 5.65 | 2.42 |

![Monthly mean, maximum and minimum air temperature for Climate Design Data Workflow for Seattle, WA](https://efwwel0vyt9vy5so.public.blob.vercel-storage.com/images/climate-data/climate-design-data-workflow-seattle-wa/chart-temperature-1344.webp)

*Monthly air temperature for Climate Design Data Workflow for Seattle, WA, plotted from the table above.*

![Mean daily precipitation by month for Climate Design Data Workflow for Seattle, WA](https://efwwel0vyt9vy5so.public.blob.vercel-storage.com/images/climate-data/climate-design-data-workflow-seattle-wa/chart-precipitation-1344.webp)

*Mean daily precipitation for Climate Design Data Workflow for Seattle, WA, plotted from the table above.*

## 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](https://harth.build/resources/climate-data.md)
- [ASHRAE / IECC Climate Zones Explained (Zones 1–8, A/B/C)](https://harth.build/resources/climate-data/ashrae-climate-zones-explained.md)
- [Heating & Cooling Design Temperatures (99%, 1% / 0.4%)](https://harth.build/resources/climate-data/heating-and-cooling-design-temperatures.md)
- [Climate Design Data Workflow for Portland, OR](https://harth.build/resources/climate-data/climate-design-data-workflow-portland-or.md)
- [Climate Design Data Workflow for Boise, ID](https://harth.build/resources/climate-data/climate-design-data-workflow-boise-id.md)
- [Sun Path & Solar Angles for Seattle, WA](https://harth.build/resources/solar-data/sun-path-and-solar-angles-seattle-wa.md)

## 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](https://power.larc.nasa.gov/docs/services/api/temporal/climatology/) and [exact point response](https://power.larc.nasa.gov/api/temporal/climatology/point?parameters=T2M%2CT2M_MAX%2CT2M_MIN%2CRH2M%2CPRECTOTCORR%2CWS10M&community=SB&longitude=-122.3321&latitude=47.6062&format=JSON), 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](https://power.larc.nasa.gov/docs/services/api/temporal/climatology/)

- [power.larc.nasa.gov](https://power.larc.nasa.gov/api/temporal/climatology/point?parameters=T2M%2CT2M_MAX%2CT2M_MIN%2CRH2M%2CPRECTOTCORR%2CWS10M&community=SB&longitude=-122.3321&latitude=47.6062&format=JSON)

## Frequently asked questions

### What does NASA POWER climatology show for Seattle, WA?

For the Seattle, WA reference point, the 2001–2020 NASA POWER climatology reports 10.5 °C annual mean temperature, 83.2% relative humidity and 3.37 mm/day corrected precipitation. The monthly maximum peaks at 38.0 °C in July.

### Can the Seattle, WA climatology replace project design temperatures or an hourly weather file?

No. The Seattle, WA response is gridded monthly climatology; its lowest monthly minimum of -10.4 °C in December is not a code percentile or observed site extreme. Verify elevation, local exposure, the required weather file or design-condition source and the current adopted code.

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