The science behind how humidity can lead to severe thunderstorms

The hazy Boston skyline was seen from Kings Beach in Lynn during a hot day on June 19.David L. Ryan/ Globe Staff

Stepping outside and feeling the thick wall of humidity has been a constant occurrence through the beginning of summer.

The region has already seen two heat waves, and with the heat has come the mugginess. Loads of it. Boston is on pace for one of the most humid summers on record.

As climate change continues to warm the atmosphere, the air can now simply hold more moisture, and we’re feeling the impacts — more tropical and uncomfortable days, more frequent storms, and an increasing vulnerability to more extreme events. Just take this month’s dousing of 4 to 6 inches of rain that closed I-93 and led to stranded vehicles. We should get used to this because summers are becoming hotter and wetter for longer, with more rain and snow likely in the winter.

Remember, dew point is a measurement of how much moisture is in the air. It’s the gold standard in telling us how swampy it’s going to feel outside. The higher it goes, the more uncomfortable we become.

Dew point temperatures reflect how you will feel when outside. You'll notice the change in air above 60 degrees.NWS

I went into the records at Logan, where dew point data has been tracked since 1936, and what I plucked out was pretty alarming. Including the first 50 days of this summer, the past eight summers have seen above-average daily dew points across Greater Boston, and the trend has been steadily increasing for decades.

The increase in humidity means more than just uncomfortable days: There’s also a higher risk for heat-related illnesses as high dew points make it difficult for our bodies to sweat and cool. Increased dew points also amplify the intensity of severe thunderstorms and make much more water available to pour on us.

The average daily dew point in Boston since 1936 sits right at 60 degrees, which is coincidentally the threshold used to determine whether or not you’ll notice the moisture presence in the air. That’s where bodies start to work harder to sweat, and where it takes longer for sweat to evaporate, slowing the cooling process.

Take a look at the trend below. It doesn’t hold 2025 data because there’s still half of a summer to go. But this past June and the first part of July have run more than 2.7 degrees above average, pacing Boston to see a top-three muggiest summer on record should this year’s trend continue, and it likely will.

I also looked across all six New England states, and the trend is the same. The average daily dew point at many of the regional and major airports has been above average for the last five summers.

And we haven’t reached the height of dew point temperatures in a typical summer quite yet! Those usually peak mid-July through the first half of August. Take a look at how 2025 readings (blue dots) compared with the average (black line):

A typical range and average dew point temperature in Boston throughout the year, peaking in late July and early August.IEM

Boston used to see only about 20 days during summer when the dew point reached 70 degrees, but since the mid-’80s, the trend has been much closer to 40.

So, we’re looking at more muggy days to come this summer. Boston will continue feeling more like Washington, D.C., and Baltimore this season.

What do higher dew points mean?

Higher dew points are a result of a warmer atmosphere. With more moisture in the air, everything from an uptick in frequent and intense storms to increasing populations of insects like ticks and mosquitoes are a result.

Let me get into some real meteorology stuff here (my favorite!):

The combination of a more interior track of the jet stream has brought a more frequent southerly flow into the region, drawing moisture from the Southeast and the Gulf Stream. This influx of moisture contributes to the rise in humidity.

Particularly this summer, a strong Bermuda high, which always sits off the East Coast of the United States but is particularly forceful this year, supports and even amplifies these southerly flows.

So what does this all mean? We’re seeing a double whammy: More intensely hot days plus higher dew points keep nighttime temperatures elevated, not allowing for daytime heat to escape as easily, and making evenings more uncomfortable.

A strong Bermuda High has been present, fueling the region with moisture from the southwest often this summer.boston globe

With more moisture comes more energy. So when something like a cold front passes, higher dew points here mean there’s a bigger chance that moist air will condense and produce thunderstorms and/or downpours. This is how humidity can lead to more severe storms.

We still have at least half of summer to go, and it’ll be both interesting and concerning to see how things play out.