Boulder
United States · 21 May 2018
Average
12.8°C
low 3.3° · high 21°
Rainfall
0.9mm
dry day
Wind
2.1m/s
daily average
Humidity
66.4%
daily average
Cloud
49%
cover
Pressure
1012hPa
daily average
The temperature ranged from 3.3°C around 04:00 to 21°C around 14:00. Rain was recorded for one hour, around 15:00 (0.9mm total).
How unusual was this day?
Compared against the same 135 calendar days (±7 days) across the other 9 years on file.
High temperature
73th percentile
Low temperature
32th percentile
Rainfall
39th percentile
Wind
56th percentile
A percentile of 100 means this was the hottest/wettest/windiest of the comparable days on file; 50 is right in the middle. This window's average temperature was 10.8°C, versus 12.8°C on this specific day.
Sun & daylight
Sunrise
05:40
Sunset
20:15
Day length
14h 34m
Hour by hour
Temperature (line) and hourly rainfall (bars), local time — America/Denver.
Cumulative rainfall through the day.
| Local time | UTC | Temp °C | Feels °C | Rain mm | Wind m/s | Dir | Humidity % | Pressure hPa |
|---|---|---|---|---|---|---|---|---|
| 00:00 | 06:00 | 5.8 | 3.5 | 0 | 1.7 | W | 93 | 1015 |
| 01:00 | 07:00 | 6 | 3.5 | 0 | 1.7 | W | 90 | 1015 |
| 02:00 | 08:00 | 5.8 | 3.3 | 0 | 1.7 | W | 90 | 1014 |
| 03:00 | 09:00 | 4.5 | 1.7 | 0 | 1.9 | W | 91 | 1014 |
| 04:00 | 10:00 | 3.3 | -0.7 | 0 | 3.4 | W | 95 | 1016 |
| 05:00 | 11:00 | 3.3 | -0.7 | 0 | 3.5 | W | 94 | 1014 |
| 06:00 | 12:00 | 3.4 | -0.6 | 0 | 3.4 | W | 93 | 1013 |
| 07:00 | 13:00 | 4.8 | 1.6 | 0 | 2.9 | W | 99 | 1013 |
| 08:00 | 14:00 | 9.2 | 7.2 | 0 | 1.9 | W | 85 | 1013 |
| 09:00 | 15:00 | 13.7 | 12.3 | 0 | 0.7 | W | 61 | 1013 |
| 10:00 | 16:00 | 15.9 | 14.8 | 0 | 0.2 | S | 52 | 1012 |
| 11:00 | 17:00 | 19 | 17.5 | 0.01 | 0.8 | ENE | 42 | 1012 |
| 12:00 | 18:00 | 19.4 | 17.2 | 0 | 1.9 | ENE | 41 | 1011 |
| 13:00 | 19:00 | 20.3 | 17.8 | 0.01 | 2.7 | E | 43 | 1011 |
| 14:00 | 20:00 | 21 | 18.4 | 0.02 | 3 | ESE | 43 | 1010 |
| 15:00 | 21:00 | 20.6 | 19 | 0.79 | 1.2 | SW | 40 | 1011 |
| 16:00 | 22:00 | 18.3 | 15.1 | 0 | 2.7 | SSW | 38 | 1009 |
| 17:00 | 23:00 | 19.9 | 17.4 | 0.03 | 1.6 | SSE | 34 | 1009 |
| 18:00 | 00:00 | 19.8 | 16.7 | 0.01 | 2.4 | SSE | 34 | 1010 |
| 19:00 | 01:00 | 18.4 | 16.3 | 0 | 2.7 | SW | 55 | 1011 |
| 20:00 | 02:00 | 16.3 | 14.5 | 0 | 1.9 | WSW | 57 | 1011 |
| 21:00 | 03:00 | 13.6 | 12 | 0 | 1.7 | W | 70 | 1011 |
| 22:00 | 04:00 | 12.6 | 10.9 | 0 | 1.9 | W | 75 | 1011 |
| 23:00 | 05:00 | 11.7 | 9.9 | 0 | 1.8 | W | 76 | 1010 |
Nearby cities
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Data source & coverage — ERA5 reanalysis, 2016–2025 (10 years)details
- Source
- ERA5 reanalysis, accessed via the Copernicus Climate Change Service (C3S) Climate Data Store
- Data type
- Reanalysis — a physically consistent reconstruction combining a forecast model with historical observations, not a live station reading or a forecast
- Coverage
- 2016–2025 (10 years) for United States
- Location
- Boulder — 40.000°, -105.250°, ERA5 grid cell center, 2.4 km from requested point
- Time zone
- America/Denver
- What's shown here
- The hourly and daily record for 21 May 2018 specifically — not a monthly average. See the Boulder climate overview for typical conditions.
- Spatial resolution
- 0.25° (~25 km) distributed grid — ERA5's native model resolution is finer; this is the publicly distributed regridded product
- Version / refreshed
- Methodology v2026.07 · data last refreshed December 31, 2025
- Limitations
- A model reconstruction, not a station observation — hyper-local extremes (a single storm cell, an urban heat pocket) can differ from a nearby gauge. See the full methodology.
- Attribution
- Generated using Copernicus Climate Change Service information
Cite this data
PastClimate (2026-07-17). Historical climate data for Boulder, United States (40.00°N, 105.25°W), derived from ERA5 (Hersbach et al., 2023, Copernicus Climate Change Service). Retrieved from https://www.pastclimate.com/
PastClimate. (2026). Historical climate data for Boulder, United States [Data set]. Derived from ERA5 (Copernicus Climate Change Service). Retrieved July 17, 2026, from https://www.pastclimate.com/
"Historical Climate Data for Boulder, United States." PastClimate, derived from ERA5 (Copernicus Climate Change Service), July 17, 2026, www.pastclimate.com.
@misc{pastclimate,
author = {PastClimate},
title = {Historical climate data for Boulder, United States},
year = {2026},
note = {Derived from ERA5 (Hersbach et al., 2023), Copernicus Climate Change Service. Accessed 2026-07-17},
url = {https://www.pastclimate.com/}
}Notice something wrong with this data? Report a data issue.