How does the recent humidity stack up against prior Mays?

The dramatic shift in our weather that took place over the last week will likely be memorable for a number of reasons.

First, on Tuesday Madison reached 90 degrees for the first time this year, the fourth earliest first 90-degree day in Madison’s history.

The all-time earliest was on April 22, 1980, when the temperature reached 94. The other two earlier first 90-degree days were April 28, 1952 (90, followed by 90 on April 30 — making April 1952 the only one in at least the last 80-plus years in Madison with two 90-degree days) and May 5, 1949 (90).

As if that were not enough, we also became the second May ever with four days at or above 90 — the first time was May 26-29, 2018.

Tropical moisture on May 13th tracked by satellites. (Image credit: CIMSS Total Precipitable Water)

Along with such unusually warm temperatures, we also experienced extraordinary humidity with this episode. Along with a high temperature of 91 on Wednesday , the dew point soared to 73 degrees at the airport — and 79 degrees on the roof of our building on campus. Dewpoint is one of several measures of the water vapor content of the air — it measures the temperature to which air must be cooled (at constant pressure) to reach 100% relative humidity. The higher the dewpoint, the greater the water vapor content of the air.

This dramatic moisture surge set the stage for a low temperature of 73 degrees on Thursday — the highest Madison daytime minimum in May since May 29, 1874. At the time of that recording, the official measurement spot for Madison was Bascom Hall. In fact, that observing site was in use on the only other such instances as well — the record 74 degrees on May 28, 1871 (followed by 73 degrees on the 29th and again on the 31st).

The fact that the recent extreme humidity values were recorded more than two weeks before any other such events in Madison history — and as the regional leaf-out began — testifies to the purely tropical nature of the air.

Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at 11:45 a.m. the last Monday of each month. Send them your questions at or

Category: History, Meteorology, Seasons

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