Humidity is all about perception

- Humidity is something we as meteorologists talk a lot about in the warmer months.  Not only can it lead to rainfall and thunderstorms, but it can also turn an otherwise pleasant day into a soupy sweat filled mess.  This is because warmer air can “hold” more water than colder air.  This very simple act can have some pretty substantial consequences for humans.

Our bodies are designed to run at 98.6°F.  When our core temperature starts to rise, we start to sweat.  The perspiration on our skin evaporates into the air which cools our skin and allows our bodies to cool as well.  When the level of moisture in the atmosphere is low, our bodies cool more efficiently because the sweat on our skin can evaporate quickly because of all of the room in the air for moisture to go.  But if moisture levels are high, then it takes much longer for that sweat on our skin to evaporate and therefore our body temperature can’t cool as efficiently, so our body temperature starts to rise and we as humans start to feel pretty miserable.  This is the basic concept behind the heat index; combining the temperature and moisture levels to help us understand what it actually feels like outside.

Well, the science behind all of this heating and cooling may be constant, but when it comes to us humans, humidity is all about perception and what you are used to.

Take for example, Minnesotans.  The humidity levels drastically climb here in the summer.  Moisture streams in from the Gulf of Mexico and saturates our days and nights with plenty of moisture.  We measure this by using a constant index; the dewpoint.  The dewpoint is a literal term, meaning it’s the point (or temperature) at which dew forms.  The higher the number, the more moisture there is in the atmosphere.  BUT, I stated earlier that humidity is all about perception right?

Well, in Minnesota, we use this general index to describe comfort level:

It’s still fairly comfortable outside until dewpoints get into the 60s… then it starts to feel more humid.  Low 70s is considered tropical (just think about what Florida is like in the summer) and very uncomfortable and near 80 is considered to be downright ridiculous.  And yes, we have had 80 degree dewpoints in the state on several occasions over the last decade.  But if you look at dewpoint scales from other parts of the country, it’s not going to be the same.  Check out Boise Idaho’s index…

They consider any dewpoint over 60 degrees to be miserable! Nearly EVERY day from June to August would qualify as miserably humid if we were to use that scale in Minnesota.  But to their defense, they just aren’t used to it.  Idaho is more of a steppe climate; fairly dry and warm.  Their average summer dewpoint is in the low 40s so yes, I think they would think a 60°+ dewpoint is pretty miserable.  Now look at the other side of the coin and head east to Miami…

Their index is similar but has some differences.  They have qualifiers saying that any dewpoint under 65° is considered to be “wintry”.  That clearly isn’t the case in Minnesota when dewpoints are often below zero in the winter.  But the top of their scale is fairly similar with 78° or greater considered oppressive.  Why is the top of Miami’s scale similar?  While Miami has pretty consistent humidity all year round, it’s Minnesota that can often have higher dewpoints in the summer than much of Florida.  This is because of a host of factors with crop and vegetation transpiration, moisture pooling, advection of the Gulf of Mexico airmass, and many other components that I’m not going to get into in this post.  But hopefully this sheds some light on just one of the many aspects of meteorology that isn’t just science, but perception.


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