Using 1981-2010 climatology, I went through each summer (June-July-August) back to 1875 to find the top 5 with the highest average dew point for that 3-month period. This does NOT take air temperature into account.
For example, if an observation had 91 with a dew point of 69 & in another year at the same time, the ob was 83 with a dew point of 75, then the 83 would be muggier than the 91 by 6 degrees.
You could also have an extremely muggy July, but if June & August have lower dew points, then it may show up as not so muggy.Also, the hottest summers on record rank as having some of the lowest average dew points due to Extreme to Exceptional Drought usually being in place. For example, I found that the 1934 summer in our area had dew points of around 7 degrees below normal (biggest negative anomaly on record).
The grid is not nearly as dense prior to 1950, but I was able to interpolate from various stations in Indiana. The Purdue data goes back to 1879. Logansport has data back to 1854. Rensselaer has data back to 1859 that I can convert the temperature & relative humidity to the dew point. I do have some data back to 1854 for the city of Lafayette & southeast of Lafayette.
The higher dew points are directly related to the amount of Precipitable Water present.
So, these are the top 10 summers with the highest average dew point back to 1875 below:
(Average dew point temperature for summer as a whole is 64-66 over the viewing area with an average of one day each summer that the dew point reaches 80 for the viewing area................Though it varies from 0.5 day in the northeast to 1.5 days in the southwestern parts of the viewing area & right at about 1 in Greater Lafayette).
1. +3.00 1886
2. +2.71 1877, 1942
3. +2.57 1898, 1906, 1943
4. +2.29 1944, 1995
5. +2.23 1945, 2010
The top 5 summers with the highest average dew point back to 1975 include:
1. +2.29 1995
2. +2.23 2010
3. +2.18 1977
4. +1.98 2002
5. +1.93 2018
What do the 1875-present muggiest summers on record have in common? Each year seems to have a slightly different mode to bring about the high dew point anomalies, but some commonalities were noted.
One singularity noted was very wet spring & partial summer, then rain shutting off with heat overspreading the area. Wet soils & lush vegetation tended to transpire tremendous amounts of water into the air.
This map of the years combined show that Precipitation Rates were unusually high, so when it could & did rain, it tended to rain very heavily.
The night-time lows were warmer to much warmer than normal over a massive area in the eastern U.S., including our entire viewing area.
The high temperatures were more normal on average, however. So, unusually warm nights, but more normal days.
Precipitable Water amounts were way above normal, so there was a lot of water to squeeze out in these summers overall if a sufficient mechanism is present.
It is interesting to note that precipitation ended up slightly below normal when averaged up with these summers, but it was well above normal southwest of the area.
Mean temperatures were above normal here.
There is a tendency for flash Abnormally Dry condition to Moderate Drought to eventually develop late in the season.
Again, I found that one commonality seems to be very wet at the onset, then the heat & drier weather build over wet soils, producing the high dew points. Also, there tends to be very wet soil & wet weather southwest of our area in the path of our dominant southerly winds.
Southwest winds need to be consistent to carry that moisture in.
Cooler summer in the West & hotter summer in the East likely resulted in strong low-level jet advection high amounts of water into the Midwest & Plains (& higher dew points).
Modern-day air conditioning would be needed greatly, not necessarily from such hot, hot days, but warm, warm, muggy nights.
Where the days are hotter & the nights are much warmer, air conditioning use would be maximized & way above normal east & northeast of our area.