Weather in Aspen in September

Prime time. If you get to pick your dates, September is the one. Highs average 21 °C, lows 6.4 °C, and rain falls on 9 days — nothing in the month is working against you.

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

Rain on 9 days, 1 of them heavy. Enough to need a plan B, not enough to need a plan around it. Nothing specialised needed — this is the month you pack for the trip rather than for the weather.

What the averages hide

Averages flatten a month, and this one hides a few things: it never really gets hot: not one day in an average September reaches 30 °C. Across the twenty years measured, the hottest September day reached 30.9 °C and the coldest night fell to -4.9 °C.

September is moderately variable: 15 mm in the driest year against 95 mm in the wettest, around an average of 46 mm. Roughly two Septembers in three land near that average, and the third does something else entirely.

Wind is light at 1.6 km/h, so the temperature on the page is close to the temperature on your skin, and humidity is unremarkable at 56%.

How September sits in the year

The month is on the way down: 4.1 °C cooler than August, with October dropping another 8 °C. The first week and the last are not the same trip.

Daylight in September

12.5 hours of daylight, losing 1.3 hours across the month. That is between Aspen's extremes of 14.9 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 — 39.6 °C between the warmest afternoons and the coldest nights, wider than any coastal city at this latitude would manage. At 2405 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 14.6 °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 Aspen versus nearby

If Aspen is one option among several, the nearby comparison is worth making. Denver is close enough to be a coin toss — 7.1 °C warmer, 3 days less rain. Moab is the weaker option in September — 9.1 °C warmer, 2 days less rain.

So: September works, with June the better month by 8 points if the dates can move. If they cannot, nothing here should stop you booking.

Every month in Aspen

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