Weather in Denver in September

Prime time. September is about as good as Denver gets. Highs average 28.1 °C, lows 12.1 °C, and rain falls on 6 days — nothing in the month is working against you.

September ranks 2 of 12 in Denver. Only October scores clearly higher, and the difference is small enough to be settled by price rather than weather.

Rain falls on only 6 days, so an itinerary with no wet-weather fallback is a reasonable risk. Pack for 28.1 °C days and 12.1 °C nights; the spread matters more here than any single number.

What the averages hide

Averages flatten a month, and this one hides a few things: 12 days top 30 °C — hot spells arrive, but they break; the day swings 16 °C from low to high, which means one bag has to cover both. Across the twenty years measured, the hottest September day reached 37.5 °C and the coldest night fell to -0.7 °C, and the wettest single day dropped 42 mm — roughly 117% of the month's rain in one day.

This is the volatile part. September rainfall has ranged from 4 mm to 148 mm across the twenty years measured — a 144 mm spread around an average of 36 mm. The average is real but almost no individual September looks like it, so treat the figure as the middle of a wide range rather than a forecast.

Wind is light at 8.1 km/h, so the temperature on the page is close to the temperature on your skin, and humidity is unremarkable at 44%.

How September sits in the year

The month is on the way down: 3.7 °C cooler than August, with October dropping another 7.5 °C. The first week and the last are not the same trip. Rain has already shifted: 7 days fewer than August had.

Daylight in September

12.5 hours of daylight, losing 1.3 hours across the month. That is between Denver's extremes of 15 and 9.4 hours, so days are long enough not to constrain the plan.

An inland climate

With no coast nearby, the year swings a fair way — 37 °C between the warmest afternoons and the coldest nights, wider than any coastal city at this latitude would manage. At 1609 m, altitude is doing as much work here as latitude: thinner air means fierce sun by day and heat that leaves the moment it sets. Within September itself the daily swing is 16 °C, so mornings and afternoons need different clothes.

How busy it gets

Peak season. Expect the highest prices of the year and to be booking well ahead.

Crowd levels are estimated from climate desirability and school-holiday calendars, not from visitor numbers.

September in Denver versus nearby

If Denver is one option among several, the nearby comparison is worth making. Aspen is close enough to be a coin toss — 7.1 °C cooler, 3 days more rain. Moab is the weaker option in September — 2 °C warmer, 1 day more rain.

So: September is effectively as good as it gets here — 5 points behind October, which is inside the margin where the weather stops being the deciding factor and price starts being it.

Every month in Denver

See all twelve months in Denver, or where else to go in September.