It’s another one of the many signs that winter is just around the corner: Denver’s average first frost usually comes around this time of the year, in late September or early October.
The city’s average date for the first low temperature of 36 degrees or below – the rough criteria for a possible frost, which we’ll explain shortly – is on September 27th. Denver’s average first freeze (32 degrees or below) falls on October 7th, and first frost and freeze dates are about the same in Colorado Springs, Fort Collins and Pueblo. While neither a frost or a freeze appear to be in the cards over the next week, now is the time to prepare for the end of growing season along the Front Range.
For starters, it’s worth knowing that when low temperatures are expected to drop between 32 and 36 degrees.
A freeze warning is issued when temperatures are expected to fall at or below 32, and a hard freeze warning is in place when temperatures are expected to fall below 28 degrees. Temperatures below 28 are usually associated with a widespread killing freeze that can kill perennial plants and crops. The covering up plants when the temperature is at frost level (32 to 36 degrees) and bringing plants indoors when the mercury falls below the freezing mark.
Frost can form even when temperatures are above 32 degrees. Because colder air is denser than warmer air, cold air sinks as close to the ground as possible. Temperatures – for example, the thermometer on your deck or patio – are usually measured a few feet above the ground. Think about walking barefoot on grass during a crisp fall morning; the air by your feet is typically a few degrees colder than the air by your head. That’s due to colder air’s higher density, and that allows patchy frost to sometimes form, even when your thermometer might say it’s above freezing.
In the near term, temperatures aren’t expected to drop that close to frost levels this week. Lows are expected to stay at or above 45 degrees through the weekend and well into next week. It’s quite possible that Denver will have a later first frost or freeze than usual, based on near-term weather trends.
Last year, Denver bottomed at 36 degrees (the threshold for possible patchy frost) for the first time on September 25th, and hit freezing for the first time on October 10th.
That said, there are indications that next weekend (October 5-7) may – finally – usher in more fall-like temperatures after a sizzling September to date. That may be the point at which you might prepare for the first frost or freeze of the fall season along the Front Range.
