Boulder weather by month: averages, rainfall & climate trends

Each city's history comes from one ERA5 grid cell, about 28 km across, shown by the dashed box. Near mountains or coasts, conditions can vary across the cell.

Historical weather for Boulder is sampled from the ECMWF ERA5 reanalysis at 40.01°N, 105.27°W, with daily records since 1940.

Coordinates

40.01°N, 105.27°W

Time zone

America/Denver

Period

1940–2026

Data source

ERA5 (ECMWF)

26 of the last 30 days were warmer than the historical average for that date. Average difference: +8.4°F.

Each bar is one day, from morning low to afternoon high. Warm-colored bars are days whose mean ran above average; cool bars ran below. The dot inside the bar is the daily mean. The shaded band is the typical 10–90% range expected for that date. Average = the day's mean temperature averaged across every year of record (1940–2026) for that calendar date.

One dot per year, showing the mean temperature on this calendar date. Dots are warmer or cooler than the long-term average (dashed line); the shaded band is the typical 10–90% range, and the highlighted dot is today's forecast. Based on ERA5 reanalysis: modelled estimates, not station readings.

July is the warmest month, January the coolest: a yearly swing of 43°F. Wettest month: May (~4.6 in). Whole year averages ~29.4 inches of rain.

Bars = average monthly rainfall (right axis). Lines = average daily high and low (left axis). Average = each month's value averaged across every year of record (1940–2026).

About 98 rainy days a year: most in May (~13), fewest in December (~4).

Bars = average number of days with ≥ 0.08 in per month. Hover for the monthly rainfall total.

Average daily peak wind at 10 m, by month.

Average daily incoming solar energy in megajoules per square metre: the record's own proxy for how sunny the month is.

Typical conditions for each month, averaged across the full record (since 1940). Daylight is the time from sunrise to sunset. Highest and Lowest are the most extreme values in the ERA5 dataset (modelled since 1940), so they can differ from official weather-station readings.

Annual mean shifted from 1940–1949 to 2016–2025 by +5.7°F.

Each vertical stripe is one year. Color encodes how much that year's annual mean differed from the long-term average. Average = each year's annual mean compared to the average of all years (1940–2026). cooler ← → warmer

Long-term trend: +0.51°F per decade.

One point per year, showing the temperature averaged across the whole year. The dashed line is the least-squares long-term trend. Based on ERA5 reanalysis: modelled estimates, not station readings.

Mar–May is warming fastest: +0.66°F per decade.

Each faint line is one three-month period's average per year; the bold dashed line is its long-term trend. Different parts of the year often warm at different rates. Based on ERA5 reanalysis: modelled estimates, not station readings.

Days ≥ 86°F per year: 15.2 early in the record → 54.6 recently. Frost days: 164 → 123.

Thin lines are raw yearly counts; thick lines are the smoothed trend that removes year-to-year noise. The hot-day threshold is auto-picked per city so the line actually moves. Average = a centered 5-year rolling average to smooth weather noise.

Hottest day since 1940 in 1954, coldest in 1963: 100.2°F / −32.9°F.

One point per year: the single hottest and coldest day recorded that year.

~29.4 in/year on average. Last decade: −8% vs that average. Long-term trend: −0.38 in per decade.

One bar per year of total rainfall. Dashed line is the long-term average. Average = the average annual rainfall across every year of record (1940–2026).

Each tiny square is one calendar day across the full record, about 30,000 days per city. Use the mode switch above the chart: Anomaly colors each day by how far it ran from the historical average for that date (red = warmer, blue = cooler), Daily mean temp shows the absolute mean temperature for the day (useful to see seasons and heatwaves), and Precipitation shows daily rainfall (useful to spot wet/dry seasons and droughts). Average = the long-term average for that calendar date (1940–2026).