Humidity, “Relative” Humidity, and Dew Point

I know this has nothing to do with the typical subjects of my blogs. But, let’s see … this is about God’s creation!

Eleven years ago, Tennessee had a very hot summer while I was living in a school bus converted into an RV … with no air conditioning because I didn’t have enough electricity to run an air conditioner. At some point that summer I noticed that as the weather got warmer, the humidity percentage got lower.

For example, in the morning, when it would be maybe 75 degrees out, the relative humidity would be way up around 90%. But by mid-day it would drop to 50 or 60%, and if the temperature got over 100 degrees, then it would drop to maybe 44% or something like that.

When it’s 100 degrees outside, 44% humidity seems miserable, and I always wondered why.

Today I found out.

The Important But Unknown "Dew Point"

Last year sometime I heard a newscaster say that the humidity was a certain percentage because the dew point was 72 degrees.

Suddenly I understood why humidity would change as the temperature rose.

When the dew point is 72 degrees, then if the temperature is also 72 degrees, you have 100% humidity.

Warm the temperature up to 80 degrees, and the air can hold more water, so the same amount of water in the air is now less than 100% humidity.

Also, you’ll see dew on the ground in the morning only if the temperature drops below 72 degrees overnight.

So relative humidity changes with the temperature, even though the day is just as humid whether it’s 80 degrees outside or 90 degrees.

It’s dew point that’s constant.

As a side note, I remember reading about a Boston Marathon that was run in 90-90 weather, meaning 90o with 90% relative humidity.

That didn’t happen and doesn’t happen in the US.

It was probably 90% relative humidity in the morning when the temperature was in the 70’s, but it wasn’t anywhere near 90% when the temperature reached 90 degrees.

America just doesn’t get dew points in the high 80’s … ever

Judging Humidity by the Dew Point

Houston is one of the most humid cities in the nation because it’s dew point averages 74 degrees during the summer.

Selmer, Tennessee, where I am, is nearly as humid, often running over 70 degrees in the summer.

Corpus Christi, TX and the Florida Keys have the worst humidity in the US, averaging a dew point of 75 degrees in July. The Florida Keys are more humid in winter, though, averaging over a 60-degree dew point even then, making it the most humid place in America.

This is how dew points feel …

  • Under 60 degree dew point: comfortable
  • 60 to 60 degrees: starting to feel humid
  • 70 to 75 degrees: very humid for most people
  • Over 75 degrees: oppressive
  • Over 80 degrees: Basically never happens in US; More on the Persian gulf below
  • Under minus 20-degrees-fahrenheit dew point there’s real problems with skin drying out

I’m visiting in Auburn, CA right now. It almost never feels humid here. That’s because most of the time the dew point is around 55 degrees, and it drops into the 40 sometimes. Not much moisture in the air at all.

Now, I want to remind you that if you have a 75-degree dew point, the weather will feel very humid even though the relative humidity is only going to be around 50% at 93 degrees. It will be around 90% relative humidity on the same day if the temperature is only 80 degrees.

Specific Humidity Versus Relative Humidity

Relative Humidity is that percentage you always hear. As I’ve been pointing out, it’s a useless number. On the very same day, with the very same real ("specific") humidity, you’ll have one percentage at 85 degrees and a lower percentage at 90 degrees.

Real humidity is measured in grams of water per kilogram of air. It’s a constant, and it tells you how humid the air feels, but no one ever tells you that number. Instead, weather reports do give the dew point.

For example, weather.com tells me that the dew point is 73 degrees in Selmer right now. (Note: You have to click the "details" link under the current temperature to see dew point.) The temperature is also 73 degrees, so the relative humidity is 100%.

It’s midnight here in California as I write this. It’s 2 a.m. back home in Selmer, but I know that the grass is almost certainly wet already.

The specific (real/actual) humidity is some amount of grams of water vapor per kilogram of air, but no weather service tells me that. Instead, they give me the dew point.

At 85 degrees, the dew point and the specific humidity will be the same as they were at 73 degrees, but the relative humidity won’t be 100% anymore; something closer to 70% instead.

The formula for calculating relative humidity is pretty complicated. I won’t give it here, and I don’t understand it fully anyway.

By the way, if the temperature drops to 72 degrees in Selmer, the dew point will drop to 72 degrees as well.

Why?

Because 72-degree air can’t hold enough water to maintain a 73-degree dew point. Water condenses out of the air onto grass and cars, dropping the specific humidity and the dew point.

The dew point will continue to drop as the temperature drops, and the air loses more and more water.

Extreme Weather and Record Temperatures

That book I link above has some really cool statistics in it.

The highest humidity ever recorded was a 95-degree dew point in Dhahran, Saudi Arabia on July 8, 2003. Since the temperature was 108 degrees, the relative humidity would fake you out and say it was 90% or less, even though it was the highest real humidity ever recorded.

Do you wonder what the heat index was that day?

So do weather men. There’s no chart for temperatures and dew points that high. The guess is that it felt like 155 to 160 degrees that day.

Wow.

More Extreme Weather

Here’s more wow …

The town of Bender Qaasim in Somalia averages—AVERAGES—a high of 105 degrees and a dew point of 83 degrees. While this is only a relative humidity of 61%, we have learned that percentage is meaningless. An 83-degree dew point is extremely humid. Such humidity pretty much NEVER happens in the US, not anywhere, not even in Florida or anywhere along the coast of the Gulf of Mexico.

Thus, the AVERAGE summer heat index is Bender Qaasim is 145o.

145o HEAT INDEX!!!

ON AVERAGE

Because they’re close to the water, their temperature is fortunately very stable and doesn’t get above 113 degrees.

However, at 113o with their over 80o dew point, the heat index is—literally—off the charts. Weathermen have to guess that it’s between 150 and 160o.

Wow.

Hot Seas

The reason the Persian Gulf and Red Sea areas experience that kind of humidity is because the water in those seas gets up near 90o during the summer. That provides a lot of heat for evaporation, filling the air with water vapor.

The record for the Persian Gulf is 96o water. For the Red Sea it’s 98o.

Swim, anyone?

Bangkok, Thailand qualifies as possibly hotter than Persian Gulf countries because it stays hot year round, breaking 90 degrees pretty much every day. One day in April of …

Wait, wait …

I’m pulling too much from the Extreme Weather book above. You’ll have to get it yourself if you want to know the rest.

Dew point is 54o here in Auburn tonight. That’s comfortable no matter what the relative humidity is (55% right now at 71o). The grass will be dry in the morning because it won’t drop to 54 degrees overnight.

 

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5 Responses to Humidity, “Relative” Humidity, and Dew Point

  1. Nathan says:

    Thanks Shammah,

    I always enjoy your tid-bits about weather and stars. (I know this post didn’t mention stars)

    Now I can better understand how the dew point (humidity in the air) affects the heat index.

    Always new there was some reason I did not like Tennessee’s high humidity!

    • shammahbn says:

      Oh, that's great to hear!

      This dew point thing fascinated me. More than one weather web site, and that extreme weather book, said that a dew point of 80 was very rare in the US, but we've had three days that were over 80 the whole day since I wrote that blog.

      I've been checking Orlando, FL; Houston, TX; Biloxi, MS, and New England since I wrote this blog, and Selmer has been more humid than all of them almost every day the last month. That's only changed this last 3 or 4 days.

      Corpus Christi and Key West are usually the most humid places in the US, averaging a dew point of 75 for July. Selmer is usually closer to 70, but we've rarely dropped to 75 since I wrote this blog.

  2. shammahbn says:

    I'll write y'all a shorter version of this tonight.

  3. allison m says:

    Um, I didn't understand anything beyond the first 5 paragraphs. Maybe I'll try again tomorrow… But we miss you! Hope you're doing well. Happy belated birthday. šŸ™‚

  4. John Cullimore says:

    Been answering phones and dealing with customer issues all day. I had a down minute. "Hey," I thought, "It's been a few days since I looked at Shammah's blog. That's always encouraging."

    I get half way through and…

    ***pop***

    There goes my brain.

Comments are closed.