“It's not the heat, it's the humidity.”

“When will the humidity break?”

“Is it more humid than it used to be years ago?”

Statements and questions like this have become more common over the past couple of weeks as our weather went from one of the sunniest Mays on record to the cloudiest Junes since 2009. It's also been a cooler-than-average month, although rainfall has not been above average for most of us.

With regard to the humidity: It's not your imagination. There are more muggy days than there used to be.

A quick check at the chart below shows the number of days where the dew point is at or over 66 degrees. I chose that number as a rough marker of where humidity starts to feel more uncomfortable rather than just a routine part of summer.

This chart looks at the dew points between May and September, when they are at their highest. Folks who grew up in the 1960s '70s, and even '80s may remember there were, of course, summers when humidity was more prevalent, but it was interspersed with other years without as much humidity.

Now routinely, once the humidity starts, it tends to last longer is more intense. And we don't see the breaks of dry air, either.

An interesting side note to the chart below is that the multi-drought years of the 1960s show up as dry summers without as much humidity.

A bar graph of dew points from 1945 to 2022, showing an increase in humidity.
NOAA data

The dew point, which is a temperature variable measuring the amount of moisture in the air, is also a good representative of the potential low temperature at night. If the dew point is at 68 degrees, then the temperature can't go below 68 degrees.

This is because when the temperature and dew point are the same, the relative humidity is 100% and, here at the ground level, it doesn't go over that. Higher dew points mean warmer nights and it's more uncomfortable for sleeping.

Decades ago, stretches of high humidity didn't last as long. Air conditioning use was less frequent and there were even a significant number of people who didn't use it at all.

Now our nights are warmer and most people have an air conditioner at least in the bedroom. The idea that there's only a few nights that are uncomfortable doesn't hold anymore as our summers this decade are more like what people in the northern Mid-Atlantic states would have experienced 40 years ago.

A line graph of mean minimum temperatures from the 1870s to 2020 between May and September, showing a slow but steady increase in low temperatures.

Average minimum temperatures in Boston have increased significantly over the year. This chart shows average lows from May through September when it would typically be warmest, according to NOAA Data.

Climate change has increased the overall temperatures, and warmer air has the ability to hold more moisture.

A potential analogy would be that cold air is like a tiny sponge, and warm air is like a huge one. Both hold water, but the warmer air sponge contains a lot more of it. This also leads to heavier precipitation when it does occur. Over the past couple of weeks, a basically uninterrupted flow of southerly air has kept humidity higher than average. Until this pattern breaks, it will continue to feel quite tropical with the chance of showers in the afternoon on many days through the holiday weekend.

A map of the eastern United States showing low pressure system coming from the south.