When you see this dew-point number, it likely means storms are on the way

The old mariners’ weather saying goes like this: A red sky at morning, sailors take warning. Red sky at night, sailors’ delight.

In Denver, you could rewrite that to say: 60-degree dew point in the morning, bikers take warning.

While that second sentence might not have quite the poetic prose of the first, a 60-degree dew point has proven to be a number with solid predictive power this summer.

If you see a dew point of 60 or above in the morning, it could be a good signal that high-impact weather might be on the way later that day.

Through last Thursday, 10 days this year have featured a dew point of 60 or above in Denver.

Among those 10 days, nine have featured “thunder,” according to hourly weather observations at Denver International Airport — including two notable high-impact weather days in July. High-impact weather could include anything from large hail to damaging winds to flash flooding.

Denver averages about 14 days per year with a dew point of 60 degrees or higher, based on historical data from 1996 through 2018.

A dew point, in short, is a measure of the amount of available moisture.

Technically, it’s the temperature that the air needs to cool to in order for condensation to form. If, for example, the air temperature is 85 and there’s a 60degree dew point, it’s probably not raining, because the atmosphere can still hold a good chunk of the moisture as a gas. If there’s a 60-degree dew point and the air temperature is 60, however, it’s probably raining (or, like Friday morning, creating fog and/ or dew), because the atmosphere can’t hold any of that moisture as a gas, and it’s forced to condense into either dew or precipitation.

Don’t worry, that’s the hard part.

If you just moved to the area and know a thing or two about weather, you might know that a 60degree dew point would actually be considered to be pretty dry in, say, New York, Chicago or Atlanta, at least in the summer months. For the eastern two-thirds of the country, a 60-degree dew point in the middle of the summer is considered a relatively comfortable air mass. But in Denver, we only see a 60-degree dew point a handful of times per year, simply because Colorado is located in a far drier part of the country.

So why does all of this matter?

The limited number of times we’ve wound up with a 60 dew point or higher, it’s generally led to strong to severe storms for the metro area. In essence, it comes down to this: moisture is one of the main bits of fuel a thunderstorm needs to grow and sustain itself. There are other key factors as well, specifically wind shear, lift and upperlevel winds, but a high dew point means there’s lots of low-level moisture in place to develop strong storms.

“(A 60-degree dew point) is pretty wet for this area. With those higher dew points, that’s adding higher moisture to the atmosphere, that’s adding more energy,” said Lisa Kriederman, a meteorologist at the National Weather Service office in Boulder. “That’s a more buoyant air mass, and that’s due to the molecular weight of a wet air mass versus a dry one. There’s definitely a correlation.”

In case you’re unaware, the Front Range has a somewhat volatile climate. It’s hard to apply a single number or metric and apply it in a sure-fire way; there are and will be times where a 60-degree dew point will produce tranquil weather in the afternoon. Sometimes, the air can dry out in the afternoon, or the hit-or-miss nature of summer storms bypasses your backyard, or there’s not enough shear in place to sustain those storms. There are lots of factors that have to come together for severe thunderstorms to erupt, and sometimes they don’t all come together.

But as a general rule of thumb, if you see a 60degree dew point in the morning, that should be a cue that the afternoon could be particularly active. In the 10 days of 60 or above dew points in Denver so far this year, eight have featured measurable rainfall, including the deadly flash flood in Lakewood last month and the fireworks-dousing Fourth of July storms.

“Anytime you add moisture to the atmosphere, especially in the low in the mid levels, you’re going to get more juicy storms,” said Dr. Sam Ng, a professor of meteorology at Metro State University. “It doesn’t necessarily mean you’ll see more severe weather, but you’ll get more high-impact weather.”

So if you’re looking for a quick cheat code on your afternoon forecast, ditch your weather app and the computer-generated rain percentage it spits out. Take a look at the dew point in the morning — at least during the summer months — and if it’s 60 or above, that’ll give you a good hint that we’re probably in for some active weather in the afternoon.