Why is it always so humid in July and August?

Weather

If you’re here, you either clicked on the last story or are wondering the very question in the title.

TL:DR
A few factors contribute to increasing humidity, but what specifically contributes more than usual in July and August is a process known as evapotranspiration. What occurs is water from the soil is absorbed by crops like corn, which is then evaporated off the leaves and into the air. This can then increase the relative humidity locally making things feel warmer.

Well, there are a few factors at play here. One of them is the movement of water vapor from the gulf. Where warm moist air further south travels north thanks to the winds and upper atmosphere carrying the characteristics of that air mass.

Another factor is the fact that warm air can “hold” more water vapor than cold air. This is why the summertime usually feels more humid. To break that down further, heat is a measure of energy. Meaning the more energy, the faster atoms and molecules move. Atoms and molecules move at a range of speeds at a given temperature, but it’s the average speed we care about. At room temperature, the average water molecule in its liquid state moves around at about 1300 mph! When the temperature increases the average speed of the atoms increases thus increasing the chances of those molecules going from a liquid state to a gaseous state. This is a process known as evaporation! Simply put it’s when liquid water changes to a gas as water vapor!

Ok, so what does that have to do with the added humidity in July and August and not May, June, or September? Well, here in the Midwest, and more specifically in Iowa, Illinois, and Nebraska, we have lots and lots of corn! In fact, according to the USDA, the United States plants around 90 million acres of corn across the country, with Iowa having 13.1 million acres dedicated to corn, that’s 36% of the state that is just corn! Illinois plants around 11.3 million acres of corn as well, which is about 29% of the state that is covered in cornfields!

Image courtesy of USDA

All that corn in one place plays role in adding more moisture and humidity into the area, especially during July and August. So how does that impact how muggy it is outside? The process that adds moisture to the air is known as evapotranspiration. A big word for sure! But when you break it down there are two processes wrapped into one, transpiration and evapo[ration].
Transpiration is the transporting of water from the soil into the plant and on the leaves. The water exits through tiny holes in its leaves known as stomata. The image below is a close-up of the stomata in the leaf. These same holes also allow the plant to “breathe” by controlling the intake of oxygen and exhaling CO2.

Image courtesy of NGEE-Tropics

The other is evaporation, the process of liquid water becoming a gas.
So, all the water that is pushed out of the stomata (transpiration) evaporates away into the air, which is evapotranspiration!

With all that in mind, an acre of corn can give off about 4,000 gallons of water each day! Adding the acres of corn between Iowa and Illinois we are looking at around 24 million acres of corn. That means that each day Iowa and Illinois contribute about 96 billion gallons of water! Ok, that’s a lot but keep in mind at any given instant the atmosphere has about 37.5 million billion gallons of water (that’s 37.5 with 14 zeros added or 37,500,000,000,000,000). Which is only 0.0000256% of the total water in the air.

So for the entire planet that’s rather insignificant. According to the USGS, evapotranspiration accounts for 10% of moisture. but locally evapotranspiration accounts for 10% of moisture in the atmosphere. Locally a 10% increase in the relative humidity is quite noticeable. That can be the difference of a heat index of 90ºF to 95ºF from just the corn alone!

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