Neal’s #1 soapbox topic—humidity!
How often have you run into someone who gets up on their soapbox and talks and talks and talks? Unfortunately, it happens to all of us and when you are on an airplane, for example, there is unfortunately no escape.
When I think of the passion that propels these people into such fits of prolixity, I have to admit that I have many pet issues of my own. Things like loud music in stores and restaurants, endless phone menus, and constant requests to check out a company’s website when what I really need is help from a human are among the annoyances that wind me up—much to the chagrin of those around me when they have to hear me complain about them.
But have you ever had someone bend your ear about relative humidity? I imagine the answer is no! Yet relative humidity is a strong contender for being the chief among my pet peeves. Since you have never heard someone belly-ache about humidity, please allow me to plead my case.
We humans are sweaty beasts and we perspire without even being aware of it. When the air around us is dry, the moisture on our skin quickly and completely evaporates and so we don’t even notice it. Yet, when there is a lot of water in the air we feel uncomfortable and seem to be drowning in a sea of perspiration.
Unfortunately, when a weather reporter gives the relative humidity percentage we are led to believe that this figure alone indicates whether the air is dry and pleasant or tropical and sticky. That percentage does not tell the whole story and can even be misleading.
The good news is that there are more accurate predictors how comfortable you will be outside. In fact, I would argue that the dew point, the wet bulb temperature, and the heat index are all better than relative humidity in determining how pleasant you will perceive the air around you to be.
Unfortunately, and for reasons that seem to remain clouded in mystery, the dew point and wet bulb temperature are not as well known. I say that is unfortunate because just knowing the outside ambient temperature and the dew point is enough to flawlessly predict comfort levels. But before I explain what dew points and wet bulb temperatures are and how they work, let’s go back a few years in weather history for some background.
Around the late 1700s, as scientists explored the natural world, they quickly recognized that a deeper understanding of weather was both intellectually and economically worthwhile. Weather forecasts regarding farming, flooding, and fires, for example, were critically important not only to economic productivity, but also to the general safety and well-being of the populace.
As scientists learned more about the composition of the atmosphere, they developed methods to measure how much water vapor the air could absorb at different temperatures and pressures. They even came up with a term, psychrometry, to describe this study—a term by the way, that is still in use.
Today we know that air consists 21% Oxygen, 78% Nitrogen, and 1% other gases. This means that in 100 liters of air there are 21 liters of Oxygen, 78 liters of Nitrogen, and 1 liter of water vapor, carbon dioxide, Argon, and other trace gases.
For those of us living in a sub-tropical climate, waking up to windows like this is all too common with a 99% humidity level.
The atmosphere can, just like a sponge, absorb only so much water vapor before it is considered fully saturated. When the air around us is fully saturated fog, dew, and rain start to form as there is, quite literally, no place in the air that the water can go. Those early scientists came up with a term, relative humidity, to describe the exact ratio of water vapor in the atmosphere compared to how much it can actually hold. Again, think of the sponge. It might be 50% saturated, 60%, or 100% full.
For our atmosphere this ratio is dependent on the air temperature, Why? Well, hotter air can, because of expansion, carry significantly more water vapor than cooler air. 90% relative humidity on a very cold day can still be quite comfortable, while on a blazing hot summer day even 50% relative humidity might be miserable. That we automatically rely on this relative humidity percentage is, from my experience, a very poor way to determine comfort and discomfort.
Ok, time for a couple of definitions and I promise that this will make sense. Lets, start with the dew point.
The dew point is the temperature at which the air is completely saturated and can no longer uptake/accept any more water vapor. Now technically, for any weather aficionados out there, the dew point is that temperature to which the ambient air must, under constant pressure, be cooled before it is fully saturated. In other words, when the outside air temperature drops to the dew point temperature the sponge, so to speak, will be full. The key is to remember that the dew point is nothing more than an indicator the point at which the atmosphere can no longer accept water vapor–your sweat for example.
The wet bulb temperature is, on the other hand, the lowest temperature that can be reached under current ambient conditions by the evaporation of water only. That sounds like a mouthful, but it means that if the wet bulb temperature is high that evaporation will not be nearly as efficient and you cannot continue to be cooled as well by the sweat evaporating from your skin as the atmosphere just cannot wick away that energy very well. So just as the dew point indicates a saturation point, the wet bulb temperature indicates the “evaporative capacity” of the atmosphere and how well it can cool you.
Why is the atmospheric evaporative capacity important to you and me? Well, if evaporation is how humans stay cool and is absolutely critical to human comfort and safety in hot months, then we want that process to work efficiently. I concede that evaporation and wet bulb temperatures might not be what piques your interest, but your well-being certainly depends on it!
As an aside, I encourage you to please see Note 3 for an outstanding explanation by one of my airline colleagues, Captain Chris Dahler, as to exactly how evaporation works and how it keeps us cool and functioning. It is time well spent I can assure you.
A quick reminder that relative humidity is expressed as a percentage while the dew point and wet bulb temperatures are exact numbers such as 65F or 72F. An easy example: As I type this, the outside air temp is 79F/26C and the Dew point is 59F/15C. If that outside air were suddenly cooled to 59F/15C, the air would be fully saturated, the relative humidity would be 100%, and rain, fog, or dew would immediately form. The atmospheric sponge, in a manner of speaking, would start to drip! I am sure that after a cool night that you have noticed dew on the grass. That dew formed as the overnight temperature has dropped to the dew point. The atmosphere, was no longer able to accept/uptake any more water vapor and thus the dew forms.
As you might have guessed, the lower the better for both wet bulb temperatures and dew points. A low wet bulb temperature indicates that the atmosphere is dry and can perform a lot of evaporation and cooling for you while a low dew point indicates, in another way, just how saturated (sticky) air is. In fact, weather personnel use the wet bulb temperature to get the dew point so you can definitely think of them as being related but without being exactly the same.
Have you noticed that when you step out of the shower that you feel chilly? This is the effect of evaporation. The surrounding air is drier than your wet skin and, as the water evaporates, energy is used in converting it from a liquid to a gas. This leaves your skin feeling cooler
If we were to measure the wet bulb temperature in your bathroom, it would indicate how much evaporation can take place and just how much cooler your skin will feel.
Ok, you are saying, “but how do you come up with the Dew point and the wet bulb temperature?” “You are talking about these terms without explaining how we determine them.” “What causes the dew point to change?”
All these are fair points and questions so here we go. Although digital thermometers are more often now in use, the traditional method of determining the ambient/wet bulb temperatures and in turn the dew point was by using a device called a sling psychrometer. (Neat video: How a psychrometer works ) A weather observer would swing this device around in the same manner that a Japanese martial artist employs a pair of nunchakus.
There are two thermometers attached to the sling. One measures the ambient temperature while the other is encased in a wet cloth/wick from which the moisture evaporates as the sling is rotated through the air. How much evaporation takes place during these rotations yields the wet bulb temperature.
The difference between the ambient temperature and the wet bulb temperature is then plugged into a Dew Point Equation to calculate the dew point. Depending on whether the day is dry or wet, the ambient and wet bulb temperatures and the dew point can be far apart or very close. That difference is part of that complex equation to yield the dew point and the relative humidity. Again, the wet bulb temperature is subtracted from the ambient temperature. That result is part of a calculation that gives us the dew point and relative humidity percentage and so we know a couple of important things at all once–how well water can evaporate and cool us, and how saturated the atmosphere is.
| Dew Point Comfort Levels | ||
|---|---|---|
| Degrees Fahrenheit | Degrees Celsius | "Feels Like" |
| <60°F | <15°C | Dry and Comfortable |
| 60-65°F | 15-18°C | Uncomfortable |
| 65-70°F | 18-21°C | Sticky |
| 70-80°F | 21-27°C | Steamy |
| >80°F | >27°C | Dangerous |
As a side note, have you ever noticed those Feels Like or Heat Index figures given on weather reports and apps? Meteorologists generate these figures by combining ambient temperatures, dewpoints, the wet bulb temperature, and other factors into another complex equation (or read from a chart these days) that provides a better idea of what it really feels like outside. Obviously I have been emphasizing warm weather, but you can think of the Feels Like figures being comparable to the wind chill indexes that you see in the winter. Both strive to give you a more accurate description of what it feels like to be outdoors.
Before I move on, the answer the question of what makes the dew point go up and down is weather systems–from microclimates all the way to wide swaths of the earth. Weather systems above the Gulf or Mexico and the far eastern Atlantic Ocean, for example, in the summer pump huge amounts of moisture into states as far apart geographically as Texas and Maine. In the Western U.S. the Pacific ocean can fill places like San Francisco with wet air that produces the famous fog that the city is known for.
There is a lot of research going on at the moment as to determine how long the body can survive in high dewpoints and wet bulb temperatures. In recent years, areas of northwest India and Pakistan have suffered, over long periods of time, from extreme dewpoints and wet bulb temperatures. Such conditions unfortunately give, even at night, no opportunity for people without air conditioning, fans, or cooling stations an opportunity to cool down and to stay safe.
In fact, above dewpoints above 80F/27C and wet bulb temperatures in excess of 85F/29.5C the body struggles mightily to maintain its ability to regulate its internal temperature. Needless to say, countless victims in areas of hot wet air have died as a result. Experts are discovering that as more and more areas around the world are experiencing high dewpoints and wet bulb temperatures, more and more people are heat endangered.
There are a lot of fun facts and records regarding dewpoints and wet bulb temperatures. Where, for example, in the United States and around the world are they the highest? What are the all-time records?
In the United States the areas around the Gulf of Mexico have the highest consistent dewpoints with, unsurprisingly, the worst being near New Orleans and Houston. Interestingly though, the Midwest of the U.S. also experiences high dewpoints and wet bulb temperatures. The cornfields in states like Wisconsin, Iowa, Illinois, and Minnesota trap tons of moisture under the canopies of the growing corn, and on hot sunny days the dewpoints rocket upward. In fact, Wisconsin and Minnesota have logged some of the highest dewpoints ever measured in the U.S. at a whopping 90F/32C!
That, however, is practically a cool paradise compared to the, until recently, long-term record holder. That honor belonged to Dhahran, Saudi Arabia. Where, in July 2003, it notched a dewpoint of 95F/35C and a heat index of a staggering 178F/81C—truly a blast furnace and one in which a human can survive outdoors only for the very shortest of time.
Yet on the 27th of August 2024 the Dayrestan International Airport broke even that record by logging a mind-boggling 97F/82C Dew point with a 180F/82C heat index. That truly borders on the edges of Hades!
And let’s not forget the cold of winter—particularly at outside temps below 23F/-5C. At this point the air gets very dry and not only dries our skin and chaps our lips, but also causes rapid dehydration.
Wisconsin, Iowa, Illinois, and Minnesota trap tons of moisture under the canopies of the growing corn, and on hot sunny days the dewpoints rocket upward
A large split between the ambient temperature and the dew point can also indicate rough air and high winds. Once, while piloting a Boeing 757 from Washington to Denver, we noticed that the temperature near the Denver airport was 93F/34C and the dew point was 32F/0C. That is an astonishing gap and while the air was quite clear, it was an extremely rough ride due to the shifting air currents. It was one of those days that I really felt sorry for the passengers.
Dew points and wet bulb temperatures even play a role in firefighting. Wildfires in the Western United States are a danger every year and so firefighters and meteorologists carefully watch these figures to know when the air is the driest and thus which areas are vulnerable to the outbreak of widespread fires.
Naturally, sunlight, shade, and wind also play major roles in your comfort. The bright winter sun can seemingly shine, as any snow skier knows, just as intensely on a January day as in August.
Neal’s rule of thumb: Check the ambient temperature and the dew point (and if possible, the wet bulb temperature) instead of the relative humidity and you will be well on your way to determining what comfort level to expect. There is very little that is certain in this life, but the dew point is never wrong!
I know this all sounds rather geeky and perhaps it is, but I have convinced many people to look at the dew point and they have all told me how much they have enjoyed being aware of this little detail of the wonderful world around us. Try it. You just might find it a lot of fun. At the very least, ditch the reliance on relative humidity and go with dew point!
Ok, I’ll get off my soapbox now and let you enjoy the rest of your flight in peace and quiet. Thanks for listening.
Stay old!
Notes:
1. You will notice if you do much reading about the weather that there are lots of spellings for dew point. Sometime it is Dew point and I am not sure why the D is capitalized but the P is not–such are the vagaries of spelling in the English language spelling. You will often see dewpoint and in the plural you will most often encounter “dewpoints.”
2. In addition to the video link that I embedded in the article, I strongly recommend two videos from a chemical engineer who is creating content under the name Process with Pat. He does an outstanding job presenting the concepts of both Dew point and the wet bulb temperature. The first is Wet Bulb and Dew Point and the second: More on the wet bulb and dew point
3. Captain Chris Dahler’s superb explanation of evaporation:
Atoms and molecules within a liquid or a gas are in constant motion. In other words, they carry kinetic energy, no different from a billiard ball rolling along or a baseball that’s been thrown. The colder any substance gets, the less kinetic energy it has, and that is the basic definition of absolute zero: there is no remaining energy at all and all motion stops. The warmer something is, the higher its energy, so the faster the molecules will be whizzing around.
Getting to what’s really happening when these molecules interact with each other would require a multi-page discussion of quantum physics, but to understand evaporation, it’s sufficient to just think of atoms or even a water molecule as a simple ball whizzing around and knocking into other balls.
Water molecules are very small compared to other molecules like ethanol, so water is comparatively very dense. The molecules are electrostatically attracted to each other because each molecule has a slight positive charge on one from the hydrogen end and a slight negative charge from the oxygen on the other, so they kinda-sorta want to stick to each other like magnets. This cohesiveness is what makes water have the high surface tension it’s known for, how you can fill a glass a little above the rim. The molecules are still moving around, still knocking into each other, but it takes a pretty jarring amount of energy to knock a water molecule completely away from the rest of them because of this cohesiveness.
Ok, that out of the way, these molecules fly around and bang into each other, and every time they knock into another molecule, they transfer some of their kinetic energy to that other molecule, the same way a billiard ball transfers its energy to another ball and sends it flying off. Near the surface of a drop of water, occasionally one molecule will hit another at just the right angle and with just the right kinetic energy to basically just knock it free from the surface.
The freed molecule is now considered water vapor. Assuming this doesn’t take place in a vacuum, as soon as the molecule is free from the liquid, it starts banging into all the molecules that make up air. Sometimes the water molecule will get knocked right back into the water. But unless that happens, remember that this freed water molecule carried some kinetic energy away with it. Because it is no longer in the liquid, the total sum of the kinetic energy contained within the liquid goes down. That means the water cools by that tiniest little fraction.
At first, some of the water molecules will make it far enough away from the surface of the water that they remain banging around in the surrounding air instead of getting knocked back into the liquid. More molecules escape than get knocked back into it, so the water volume shrinks. But eventually you get so many water molecules in the air that this process reaches an equilibrium…just as many water molecules are escaping as are getting returned, and at that point the evaporation process stops. That’s where you reach 100% humidity.
Depending on the density of the air, you’ll get to this point either sooner (with very dense, cold air) or later (with warm, less dense air).
If you got your handy Acme Molecule Microscope out and looked closely, what you’d see would be a little expanding cloud of water molecules above the liquid. Down on the molecular level, there actually is no “surface” per se… the line between the liquid and gaseous phases is very fuzzy and undefined because these molecules are always flying away and also getting knocked back into the water.
Anyway, the sun helps this process along in several ways. First, it heats the surrounding air. That causes the air to become less dense, and between the sun and gravity, you get convection and air movement, so instead of this evaporation taking place in a sealed container, the air is moving the escaped water vapor away from the liquid, which all means more liquid can evaporate into the air. Also, the sun just simply heats the water up, driving up that kinetic energy in the liquid, meaning more water molecules get knocked out of the water more rapidly. But that also means the water cools rapidly because it quickly loses all that kinetic energy to the surrounding air as those molecules fly away faster.
The cooling effect you feel on your skin is just your skin trying to “keep up” with this energy loss. As the water on your skin evaporates and loses its kinetic energy (and thus temperature), heat from your own body is used to keep the drop heated up. It’s that energy loss from your own body that you feel as “cooling off.” Essentially, it’s the heat from the blood in tiny capillaries in your skin that is having heat drawn away by the evaporating water, so that cools the blood, and in turn cools your body.
And then…….my head exploded
Always interesting and I’m fascinated with that brain of yoursQ