July will be a memorable month for all the wrong reasons. We just couldn’t shake the humidity all month long and we sure went out with a bang today with dewpoints soaring into the mid and even some upper 70s! Gross! The entire month we only had four days that we would typically classify as comfortable days, however we acclimated so much to the tropical humidity that some of the days that we’d normally call “muggy” probably felt decent. We had 20 days this month see tropical humidity, we average nine in a given July.
So is this some kind of record for humidity? No, not really, but we’re in rare company. I looked back 50 years (to 1974) to see how often we’ve seen this kind of humidity in Boston and found the answer is not often. In fact since 1974 (last 50 years) this is only the third time that’s happened. We also had 20 days with tropical humidity in 1988 and 2013.
While the number of days with tropical humidity might not have been a record, there is a signal here that we can’t ignore. That signal is July is becoming more humid in Boston (and really all of New England, Boston is just being used as a proxy city to pull data for). If you add a trend line to the data above, you notice the trend is increasing — more days with 70° dewpoints. I mentioned above that our long term average is nine days in a given July, but if you look at the trend line, we go from about seven days in 1974 to the average being 12 in 2024. So in the last 50 years, we’ve added about five days to our average!
The trend line looks at how things are trending over the entire period. So you’ll notice that we still have the 20-day spikes like we did in 1988 but what’s more telling is the minimums. The first half of the bar graph has several years where the low end is below five days. The second half of the graph (more recently) all of the lower bars stay above five. So, again, the trend is for more humidity more often. Is this coincidence? No, it’s not. It’s a signal of climate change.
The science is this: warmer air can hold more water vapor. So as the average temperature increases over time, the amount of water that is held within the atmosphere increases over time. The amount of water in the atmosphere shows itself in two ways. The first, obviously based on this blog, is more humid days. The second his higher rainfall rates. Don’t confuse that with amount of rain — it’s the age old saying “when it rains it pours”. There’s some science to that. If there’s more water vapor in the atmosphere, when showers or thunderstorms develop they have more vapor (or water) at their disposal. So instead of lighter showers, they can produce downpours and thunderstorms. Heavy rainfall rates and tropical rains have plagued northern New England the last few years with major flooding events taking shape in Vermont last year and two already this summer. It’s not just about mountain runoff and steep terrain, St. Johnsbury, Vermont set two of their top four rainiest days on record just this year. Three of the top five happening since 2010. St. Johnsbury also just recorded its wettest month on record with six of those top 10 records occurring since 2005. Records, by the way, go back to 1894 there.
Ok so on to the forecast… does the humidity break? Well, kinda. We get a break for Thursday and Friday but of course it’s all relative. Dew points will fall from the 70s to the upper 60s. So referring to the calendar at the top, it’s one of those situations where any other summer we’d call it muggy but this summer we call it relief. But it’s not just the humidity, the heat returns too with temperatures Thursday and Friday returning to the 90s. Both days feature the chance of a pop up storm with the heat and humidity in place, but the chance is low — a 10% chance on Thursday and a 20% chance on Friday.
