# London vs Seattle annual solar exposure

## Result

**Seattle receives more annual solar exposure by the matched sunshine-duration measure.**

For the common 1961–1990 WMO standard-normal period:

| City proxy | Annual sunshine | 1-sun equivalent* |
|---|---:|---:|
| London–Gatwick | 1574.2 h/year | 1574.2 kWh/m²/year |
| Seattle–Tacoma | 2169.7 h/year | 2169.7 kWh/m²/year |

Seattle exceeds London by **595.5 sunshine hours/year**, or **37.8%** (ratio **1.378×**).

## Method

Monthly mean sunshine-duration normals were transcribed from public WMO/NOAA climate-normal records for the same 1961–1990 period and summed in Python. The published annual totals were used as an independent check; both reconstructed sums agree to 0.1 hour.

For a common one-square-metre surface, the optional normalization is:

equivalent energy (kWh/m²) = sunshine duration (h) × 1 kW/m²

Thus the numeric total in hours equals the numeric 1-sun-equivalent kWh/m² total.

## Sources

- WMO Standard Normals via UNdata (London–Gatwick and global CLINO context): https://data.un.org/Data.aspx?d=CLINO&f=ElementCode%3A15%3BCountryCode%3AUK
- WMO CLINO sunshine dataset context: https://data.un.org/Data.aspx?d=CLINO&f=ElementCode%3A15
- NOAA/NWS Seattle climate portal (Seattle–Tacoma station context): https://www.weather.gov/wrh/Climate?wfo=sew
- NOAA-sourced Seattle 1961–1990 monthly sunshine table: https://en.wikipedia.org/wiki/Climate_of_Seattle

## Important limitation

Sunshine duration is **not measured irradiant energy**. A sunshine recorder counts periods meeting the bright-sun criterion; actual irradiance varies with solar angle, cloud optical depth, aerosols, and season. Therefore the 1-sun-equivalent values are a transparent normalization proxy, not annual GHI. The matched data robustly show that Seattle had more recorded bright-sun time in this normal period, but they do not quantify true joules received by a real horizontal or tilted surface. Both stations are airport-area proxies rather than city-centre sensors.
