WEATHER · CLIMATE NORMALS · 1991–2020
Boise Climate — Averages by Month
What the weather in Boise, USaverages across a standard 30-year period. Each month's figure is the 1991–2020 mean temperature and total rainfall from the ERA5 reanalysis — the long-run normal a place settles around, not a forecast for any particular day.
Month-by-month normals
| Month | Avg temperature | Avg rainfall |
|---|---|---|
| January | 0.4 °C · coldest | 49.5 mm |
| February | 3.0 °C | 34.6 mm |
| March | 7.4 °C | 39.9 mm |
| April | 10.9 °C | 34.5 mm |
| May | 15.9 °C | 33.4 mm |
| June | 20.4 °C | 17.9 mm |
| July | 25.9 °C · warmest | 3.1 mm |
| August | 25.0 °C | 3.8 mm |
| September | 19.4 °C | 11.1 mm |
| October | 11.9 °C | 23.3 mm |
| November | 4.9 °C | 37.4 mm |
| December | 0.6 °C | 53.8 mm · wettest |
How to read these numbers
These are reanalysis normals, not readings from a single weather station. They come from the ERA5 0.25° (~25 km) global reanalysis grid, sampled at the grid cell nearest Boise — centred at 43.50°N, 116.25°W. A reanalysis blends observations with a physics model onto an even grid, so a coastal or mountain city's cell can read a little cooler, warmer, wetter or drier than the city centre itself. The month-to-month shape — when it's hot, when it rains — is what these normals capture best.
“Normal” means the 1991–2020 average, the current World Meteorological Organization standard period. A normal is what a place tends toward over decades — any single day, month or year can and will depart from it.