May is one of the warmer months of the year in Boulder, with average highs near 21.4°C and lows around 5.9°C (mean 13.7°C). It is a relatively wet time of year, averaging 68 mm of rainfall and 50% relative humidity across the period.
May is one of the warmer months of the year in Boulder. Across 45 years of records, the warmest May in Boulder was 2018 — when the month averaged 16.2°C — while the coldest was 1995 at 9.7°C. Typical May days reach 21.4°C with nights near 5.9°C. The long-term May temperature trend is broadly flat.
May is a relatively wet time of year in Boulder, averaging 68 mm of rainfall. The wettest May on record was 1995 with 149 mm, while the driest was 2006 at 26 mm.
Relative humidity in Boulder during May averages 50%. The most humid May was 2015 at 70%, and the least humid was 2002 at 38%.
May receives about 6 kWh/m²/day of solar energy in Boulder. The sunniest Mays were 1998, 2006 and 2020 at 6.7 kWh/m²/day, and the cloudiest was 2015 at 4.6.
Average wind speed in Boulder during May is 2.3 m/s. The windiest May was 1988 at 3.1 m/s, while the calmest were 1993 and 2015 at 2 m/s.
Ranked against the other 11 months using 1981–2025 averages.
| Metric | 1980s | 1990s | 2000s | 2010s | 2020s | Change |
|---|---|---|---|---|---|---|
| Avg Temp | 13.3°C | 13.7°C | 14.6°C | 12.7°C | 14.1°C | +0.8°C |
| Avg High | 20.9°C | 21.6°C | 22.7°C | 20.3°C | 21.9°C | +1°C |
| Avg Low | 5.7°C | 5.9°C | 6.6°C | 5.1°C | 6.3°C | +0.6°C |
| Avg Rain | 75 mm | 63 mm | 52 mm | 79 mm | 73 mm | -1 mm |
May temperatures in Boulder have risen by about 1°C between 1981–1985 and 2021–2025.
May has ranged from a record low of -8.3°C (2013) to a record high of 34.6°C (2000).
Rainfall in May is a relatively wet time of year, averaging 68 mm, with a wettest year of 149 mm.
A good time to visit Boulder
May is a good time to visit Boulder, with days averaging 21.4°C and nights around 5.9°C (mean 13.7°C, warmer than Boulder's yearly average) and moderate rainfall. August and July are marginally more temperate, but May still suits most travellers.
| Year | Avg High | Avg Temp | Avg Low | Hottest Day | Coldest Day | Rainfall | Humidity | Wind | Solar |
|---|---|---|---|---|---|---|---|---|---|
| 2025 | 21.8°C | 14.3°C | 6.9°C | 29.2°C | 1.6°C | 81 mm | 50% | 2.5 m/s | 5.9 |
| 2024 | 21.5°C | 13.5°C | 5.5°C | 30.7°C | -0.7°C | 27 mm | 46% | 2.5 m/s | 6.3 |
| 2023 | 22.1°C | 15.0°C | 7.9°C | 27.8°C | 4.0°C | 121 mm | 56% | 2.1 m/s | 5.6 |
| 2022 | 22.2°C | 13.7°C | 5.3°C | 32.3°C | -1.4°C | 78 mm | 43% | 2.5 m/s | 6.1 |
| 2021 | 20.1°C | 12.8°C | 5.5°C | 28.6°C | -2.0°C | 93 mm | 56% | 2.1 m/s | 5.6 |
| 2020 | 23.8°C | 15.1°C | 6.5°C | 33.5°C | -0.9°C | 41 mm | 46% | 2.4 m/s | 6.7 |
| 2019 | 17.7°C | 10.4°C | 3.2°C | 27.7°C | -3.1°C | 75 mm | 56% | 2.2 m/s | 5.3 |
| 2018 🔥 | 24.2°C | 16.2°C | 8.2°C | 33.4°C | 1.1°C | 66 mm | 48% | 2.2 m/s | 6 |
| 2017 | 20.3°C | 12.7°C | 5.1°C | 29.2°C | -5.5°C | 111 mm | 55% | 2.4 m/s | 5.8 |
| 2016 | 19.9°C | 12.2°C | 4.6°C | 28.8°C | -4.3°C | 61 mm | 55% | 2.1 m/s | 5.7 |
| 2015 | 16.8°C | 10.6°C | 4.4°C | 25.6°C | -3.8°C | 140 mm | 70% | 2 m/s | 4.6 |
| 2014 | 20.6°C | 13.1°C | 5.6°C | 29.6°C | -4.8°C | 90 mm | 54% | 2.5 m/s | 5.7 |
| 2013 | 20.8°C | 13.3°C | 5.9°C | 30.0°C | -8.3°C | 57 mm | 47% | 2.2 m/s | 6.1 |
| 2012 | 24.0°C | 15.5°C | 7.0°C | 33.1°C | 1.4°C | 39 mm | 39% | 2.3 m/s | 6.5 |
| 2011 | 18.5°C | 11.0°C | 3.5°C | 28.2°C | -2.5°C | 96 mm | 54% | 2.3 m/s | 5.5 |
| 2010 | 20.0°C | 11.7°C | 3.4°C | 32.7°C | -3.1°C | 58 mm | 52% | 2.6 m/s | 6.2 |
| 2009 | 21.9°C | 14.5°C | 7.2°C | 31.4°C | 2.3°C | 63 mm | 53% | 2.2 m/s | 6.1 |
| 2008 | 21.1°C | 13.0°C | 5.0°C | 30.3°C | -3.8°C | 58 mm | 46% | 2.6 m/s | 6.1 |
| 2007 | 21.8°C | 14.1°C | 6.5°C | 29.8°C | 0.5°C | 57 mm | 50% | 2.1 m/s | 5.8 |
| 2006 | 24.5°C | 16.0°C | 7.6°C | 33.2°C | -0.8°C | 26 mm | 39% | 2.4 m/s | 6.7 |
| 2005 | 22.3°C | 14.3°C | 6.3°C | 32.9°C | -5.2°C | 34 mm | 48% | 2.1 m/s | 6.3 |
| 2004 | 24.4°C | 15.7°C | 7.1°C | 31.5°C | -2.2°C | 42 mm | 40% | 2.4 m/s | 6.4 |
| 2003 | 21.9°C | 14.2°C | 6.6°C | 34.2°C | -2.1°C | 53 mm | 51% | 2.1 m/s | 6 |
| 2002 | 22.7°C | 14.0°C | 5.3°C | 34.2°C | -2.3°C | 48 mm | 38% | 2.2 m/s | 6.3 |
| 2001 | 21.5°C | 14.1°C | 6.6°C | 29.1°C | -1.8°C | 86 mm | 51% | 2.7 m/s | 5.9 |
| 2000 | 24.5°C | 16.0°C | 7.5°C | 34.6°C | -1.4°C | 52 mm | 41% | 2.4 m/s | 6.5 |
| 1999 | 19.6°C | 12.0°C | 4.3°C | 26.8°C | -2.9°C | 59 mm | 56% | 2.2 m/s | 6.3 |
| 1998 | 23.2°C | 14.8°C | 6.5°C | 31.0°C | -0.1°C | 47 mm | 47% | 2.2 m/s | 6.7 |
| 1997 | 21.3°C | 13.6°C | 5.9°C | 28.8°C | -2.8°C | 52 mm | 50% | 2.2 m/s | 6.6 |
| 1996 | 23.3°C | 14.9°C | 6.5°C | 32.7°C | -0.3°C | 75 mm | 49% | 2.1 m/s | 6.2 |
| 1995 ❄️ 🌧️ | 16.4°C | 9.7°C | 2.9°C | 26.5°C | -3.7°C | 149 mm | 65% | 2.4 m/s | 5.1 |
| 1994 | 23.9°C | 15.9°C | 7.9°C | 32.0°C | 1.5°C | 37 mm | 47% | 2.4 m/s | 6.2 |
| 1993 | 21.0°C | 13.7°C | 6.4°C | 27.5°C | -1.6°C | 38 mm | 54% | 2 m/s | 6.1 |
| 1992 | 22.7°C | 15.2°C | 7.7°C | 29.9°C | 2.7°C | 45 mm | 47% | 2.3 m/s | 5.6 |
| 1991 | 22.8°C | 14.6°C | 6.3°C | 29.6°C | -3.6°C | 78 mm | 47% | 2.7 m/s | 5.8 |
| 1990 | 21.4°C | 12.9°C | 4.4°C | 29.9°C | -2.6°C | 54 mm | 47% | 2.3 m/s | 5.8 |
| 1989 | 23.2°C | 14.7°C | 6.1°C | 31.7°C | -4.5°C | 58 mm | 43% | 2.5 m/s | 6.1 |
| 1988 | 21.8°C | 14.0°C | 6.1°C | 30.4°C | -2.0°C | 84 mm | 45% | 3.1 m/s | 6.3 |
| 1987 | 21.0°C | 14.2°C | 7.4°C | 26.9°C | 1.2°C | 97 mm | 56% | 2.3 m/s | 5.3 |
| 1986 | 20.9°C | 12.9°C | 4.9°C | 29.1°C | -1.0°C | 51 mm | 47% | 2.3 m/s | 6.3 |
| 1985 | 22.0°C | 14.6°C | 7.1°C | 28.9°C | 1.0°C | 44 mm | 46% | 2.4 m/s | 6 |
| 1984 | 23.4°C | 14.8°C | 6.2°C | 31.2°C | -1.7°C | 27 mm | 46% | 2.3 m/s | 6.6 |
| 1983 | 17.2°C | 10.3°C | 3.3°C | 26.8°C | -1.6°C | 95 mm | 58% | 2.5 m/s | — |
| 1982 | 19.2°C | 12.1°C | 4.9°C | 28.4°C | -2.8°C | 107 mm | 54% | 2.2 m/s | — |
| 1981 | 19.6°C | 12.5°C | 5.5°C | 26.6°C | 0.2°C | 109 mm | 55% | 2.4 m/s | — |
Actual recorded May data for each year, 1981–2025 — sourced from NASA POWER.
The average high in May is around 21.4°C with lows near 5.9°C (mean 13.7°C), based on 45 years of NASA POWER data.
May averages 68 mm of rainfall in Boulder, a relatively wet time of year. The wettest on record was 149 mm in 1995.
The hottest May reached 34.6°C in 2000. The coldest dipped to -8.3°C in 2013.
Comparing the earliest years on record (1981–1985) with the most recent (2021–2025), average May temperatures have risen by about 1°C. Daytime highs moved from 20.3°C to 21.5°C and overnight lows from 5.4°C to 6.2°C over the same span.
May is one of the warmer months of the year in Boulder and a relatively wet time of year, so plan for around 21.4°C daytime highs and 5.9°C at night.
Not quite — May is the 8th coolest month in Boulder, averaging 13.7°C.
For May in Boulder, pack warm layers and a jacket for chilly mornings, with typical highs of 21.4°C and lows of 5.9°C. Bring an umbrella or light rain jacket too.
Relative humidity in May averages 50% — relatively dry.
Average wind speed in May is 2.3 m/s. The windiest May on record averaged 3.1 m/s (1988).
The wettest May on record was 1995 with 149 mm. On average, May sees 68 mm.
All figures are derived from NASA POWER (Prediction Of Worldwide Energy Resources), based on MERRA-2 satellite reanalysis, covering 1981–2025.
Explore how the weather in Boulder behaves each May, based on 45 years of NASA POWER climate data from 1981 to 2025. This page breaks down average temperatures, rainfall, humidity, wind and solar radiation for May specifically, along with year-by-year records and how the month has changed over the decades.
