Where's our summer humidity?

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Dewpoint temps midday Thursday showing incredibly dry air in place for this time of year

We’re now into the middle of June and it hardly feels like it.

But before you blame our temperatures, you may be interested to know that the first half of June has actually been above average. So, it’s not the temperatures, but the lack of humidity that has us feeling less like June and that humidity has been shockingly low.

Mid-way through our Thursday, dewpoints dropped to the upper 20s in the metro and even into the upper teens in Red Wing. The rest of the area experienced dewpoints in the 30s. That’s PHENOMENALLY dry air for this time of year.

The dewpoint represents the amount of moisture in the atmosphere because it’s the temperature at which dew forms. The lower the number, the drier the air. For some perspective, dewpoints in the 20s are more common in early spring (March) and late fall (November). This time of year though, these numbers are well into the 60s more often than not, and can easily get into the 70s, which is a more Florida style air mass--VERY humid.

So why haven’t we seen a lot of humidity yet? Well, it’s likely for two reasons.

The first needs little explanation: the weather pattern. The majority of the moisture in the Upper Midwest actually comes from the Gulf of Mexico, a large body of very warm water. With southerly winds, air and moisture from the Gulf can actually stream all the way into northern Canada on some occasions.

It's exceedingly common to get a large amount of Gulf moisture into Minnesota this way. However, our winds this spring have been primarily out of the north or the west, coming off a much cooler and drier place than the Gulf. This gives us the opportunity to have low humidity levels.

The other reason, though, is a bit more complex: farmers.

Yes, humans have an impact on our summer humidity levels. With our cool and very wet spring, many of the fields in Minnesota, and much of the Upper Midwest for that matter, were just too wet for farmers to plant on time. Therefore, much of the state is still in the infancy stages of the growing season. With most fields just dirt, there’s no vegetation around to sweat. That’s right, plants sweat. Well…sort of.

The act of plants “sweating” is called evapotranspiration. Yeah, BIG word. It’s the combination of evaporation, which is liquid water changing into water vapor, as well as transpiration, which is moisture transference from the leaves of vegetation into the air. It’s referred to as sweating because it’s very relatable to what happens when you get warm, you sweat to cool off your body. It’s that moisture transference that cools the body.

Plants don’t cool from the transpiration process, but with so much water in their vegetation (leaves are essentially 90 percent water) the sunshine and warm air basically whisks away some of the moisture. This moisture then ends up in the air, which can and does drastically increase humidity levels. That’s another reason why places like the desert southwest don’t have a lot of humidity… because there’s very little leafy vegetation.

Now, the overall impact to the environment from plant “sweat” is not entirely known, and is likely fairly small. But, there’s no denying that the vegetation around us has an effect on the local area. In fact, Iowa State University actually completed a study last year that shows the amount of plant transpiration and evapotranspiration per day based off the number of days after crops have been planted.

As you’d imagine, the first few weeks of growth sees very little overall transpiration because the plants are very small. But, by the fifth and sixth weeks, overall transpiration begins to skyrocket. In a typical year, that is right about the middle of June. But this year, that drastic increase in transpiration likely won’t happen until the first half of July. This gives Minnesota potentially another two to four weeks of drier than average humidity levels, which I think many of us will appreciate.