Boulder weather by month
Boulder runs from about -5°C in January to 22°C in July, with April the wettest month at 72 mm.
warmest month
July
22°C
coolest month
January
-5°C
wettest month
April
72 mm
worst air month
July
10 µg/m³
The whole year, month by month.
| Month | Avg temp | Rain | Sun | PM2.5 |
|---|---|---|---|---|
| January | -5°C | 22 mm | 232 h | 7 |
| February | -1°C | 55 mm | 255 h | 7 |
| March | 2°C | 67 mm | 303 h | 6 |
| April | 8°C | 72 mm | 310 h | 5 |
| May | 12°C | 35 mm | 400 h | 7 |
| June | 22°C | 9 mm | 395 h | 8 |
| July | 22°C | 16 mm | 417 h | 10 |
| August | 22°C | 31 mm | 353 h | 10 |
| September | 19°C | 28 mm | 347 h | 8 |
| October | 13°C | 7 mm | 311 h | 5 |
| November | 2°C | 66 mm | 235 h | 6 |
| December | 2°C | 10 mm | 250 h | 7 |
About 3,808 hours of sun a year. Roughly 44 rainy days a year.
These are long-run climate normals, not a forecast. A given year can sit well outside them. PM2.5 is a global model: near the coast it counts sea salt as fine particulate and reads high against ground stations, sometimes by a factor of two. Use the shape of the year, not the absolute number.
where these numbers come from
- · Temperature, rainfall and sunshine: ECMWF ERA5 reanalysis climate normals via Open-Meteo (CC BY 4.0), fetched once and stored.
- · PM2.5: Copernicus CAMS reanalysis, monthly means. Modelled, not station readings; near coasts CAMS counts sea salt as PM2.5 and reads high.
Last reviewed 2026-05-23. Full method, licences and the list of sources we rejected are on the methodology page.