# How Well Do You Understand Psychrometrics?

So you think you understand psychrometrics, eh?  OK then.  Let’s do a little quiz today and test your psychrometric knowledge.  These questions include some basic stuff and some more complicated stuff about the subject, but you don’t need to use math to answer them.  It’s not the same as playing Psychrometric Chart Simon Says (photo above), but it’s a good exercise.

#### Psychrometric fundamentals

Before we start, though, let me bring the uninitiated into the fold with some basics about psychrometrics.  First, let’s define the term:

Psychrometrics – the science that involves the properties of moist air (a mixture of dry air and water vapor) and the processes (meteorological, air conditioning, drying, humidification, dehumidification, agricultural soil evaporation) in which the temperature and/or the water vapor content of the mixture are changed.

That definition is from the bible of psychrometrics, Understanding Psychrometrics* by Donald Gatley.  It’s a great book that covers the whole subject, including history, terminology, and more.  It goes pretty deep and has a lot of math, so it’s not for everyone.  I wrote a little series on psychrometrics a while back, and that might be a good way to go deeper.

In the old days, engineers used to design HVAC systems by using a psychrometric chart (below).  It’s a visual representation of the relationships of the psychrometric variables.  Nowadays, the chart is more of an educational tool.  Engineers use computer programs or apps to do numerical work in psychrometrics.

You can use the chart shown above as an aid to answering the questions below.  Also, one of the questions below relies on a numbering scheme for the surfaces in a double pane window.  That system also applies to a single pane window with a storm window.  Here’s the scheme:

1. Outer surface of outer pane  (You can touch it from outside the house.)
2. Inner surface of outer pane  (between the two panes but on the outer pane)
3. Outer surface of inner pane  (between the two panes but on the inner pane)
4. Inner surface of inner pane  (You can touch it from inside the house.)

Ready to see how well you understand psychrometrics?  Here we go.

#### A psychrometric quiz

1. On a cold rainy day, outdoor air leaks into a house and warms up to room temperature.  What happens to the indoor relative humidity?

2. A double-pane window develops a leak that allows humid air to get into the space between the panes.  Which surface is most likely to have condensation on it in summer?  (Use the numbering scheme above.)

3. You walk outside and your glasses fog up.  What time of year is it?

4. Your car’s windshield fogs up on the inside in winter.  What’s the best way to clear it?

5. Can outdoor air in the southeastern US dry out a vented crawl space in the summer?

6. What happens to the dew point temperature when the dry bulb temperature increases while the water vapor concentration remains constant?

7. Can relative humidity go higher than 100%?

8. An air conditioner in Miami, Florida cools and dehumidifies the air.  What do you know about the relative humidity of the air right after it comes through the cooling coil?

9. What happens to the dew point temperature when the relative humidity stays constant while the dry bulb temperature increases?

10. Have you or anyone you know experienced conditions of 100 °F (38 °C) and 90% relative humidity?

You’re not looking ahead without putting forth a good effort to answer those questions, are you?  If so, go back and write down some answers.

I’ll wait.

Done?

Well, go do that last one.  Don’t you really want to understand psychrometrics?

I’m giving you plenty of opportunity to avoid accidentally seeing the answers here.

OK, here you go.

1. On a cold rainy day, outdoor air leaks into a house and warms up to room temperature.  What happens to the indoor relative humidity?

It drops.  For example, 32 °F (0 °C) outdoor air at 100% relative humidity will drop to ~20% when heated to 70 °F (21 °C).  See more in my article on cold air and humidity.

2. A double-pane window develops a leak that allows humid air to get into the space between the panes.  Which surface is most likely to have condensation on it in summer?  (Use the numbering scheme above.)

Surface 3.  If the indoors is air conditioned, it’s going to stay consistently cooler than the outer pane of glass.

3. You walk outside and your glasses fog up.  What time of year is it?  What kind of climate are you in?

Summer in a humid climate.  Humid outdoor air meets cool glasses.

4. Your car’s windshield fogs up on the inside in winter.  What’s the best way to clear it?

Blow heated air at the windshield with the air conditioner running and the system set to fresh air, not recirculate.  For even more dehumidification, open the car windows.  See my article on that topic for full details.

5. Will outdoor air in the southeastern US dry out a vented crawl space in the summer?

Nope.  It often leads to higher relative humidity.  Details here.

6. What happens to the dew point temperature when the dry bulb temperature increases while the water vapor concentration remains constant?

If the water vapor concentration doesn’t change, the dew point also doesn’t change.  (Well, I didn’t say anything about barometric pressure, so we’re assuming it’s constant.  If it changes, too, then the dew point temperature can change.)

7. Can relative humidity go higher than 100%?

Yep.  If there’s nothing for the water vapor to condense onto, you can get supersaturated air at higher than 100% relative humidity.

8. An air conditioner in Miami, Florida cools and dehumidifies the air.  What do you know about the relative humidity of the air right after it comes through the cooling coil?

It’s going to be very high, maybe even close to 100%.  But it dries out the indoor air because it has a lower dew point.  When it mixes with the rest of the indoor air and warms up to room temperature, the relative humidity drops.  See the answer for question 1.

9. What happens to the dew point temperature when the relative humidity stays constant while the dry bulb temperature increases?

To maintain constant relative humidity at higher temperatures, you have to have more water vapor in the air.  That causes the dew point to increase.

10. Have you or anyone you know experienced conditions of 100 °F (38 °C) and 90% relative humidity?

Not on Earth in any outdoor weather conditions ever recorded.  The dew point temperature for those conditions would be 96.5 °F (36 °C).  The highest recorded dew point is 95 °F (35 °C).  More on that topic here, with US dew point records, too.  Now, it’s possible you or your friends could experience a dew point temperature higher than the world record, but you’d be in a sauna.

There you have it.  If you understand psychrometrics, you should have gotten at least 7 out of 10 right.

Allison A. Bailes III, PhD is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia.  He has a doctorate in physics and is the author of a bestselling book on building science.  He also writes the Energy Vanguard Blog.  For more updates, you can subscribe to our newsletter and follow him on LinkedIn.

Related Articles

Make Dew Point Your Friend for Humidity

Psychrometrics – Impenetrable Chart or Path to Understanding?

Vented Crawl Spaces and the Psychrometric Chart Are Not Friends

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#### This Post Has 22 Comments

1. RoyC says:

What is denser: dry air or humid air? Assume that the dry-bulb temperature and atmospheric pressure are constant.

1. RoyC says:

Since no one was willing to take a stab at this question, I will give the answer. Humid air is less dense than dry air. The difference isn’t a lot, but most people I know think that humid air is “heavier”.

1. JayW says:

See you saying that air at the same temperature that is more humid is lighter?

1. RoyC says:

Yep

2. RoyC says:

When air passes through a furnace, the RH decreases. Does that mean the furnace is “drying” the air, i.e., removing water vapor from it?

3. I was correct on numbers 1, 2 5,8 and 10.

I am giving myself partial credit on 3 and 4: I did not read 3 completely…missed the “what climate are you in?” part of the question. On 4 – I said to just warm up the inside of the windshield, forgot the part of AC and setting the system to non-recirculating air.

I was wrong on 6 and 7.

4. Marty R says:

A good set of questions! I initially interpreted the first one to be about the resultant humidity in the indoor space, rather than being strictly about the infiltrating air’s relative humidity, since the latter cannot be separated from the other air in the house. If the outside air were relatively cold vs. the inside (say 55F) and near the dew point (as we sometimes experienced in the Atlanta area when we lived there…fog over the area), leakage into the house would bring in a significant amount of moisture, while on the other side of the house (the leeward side, or maybe the recessed light fixture cans) the leaving air would presumably be at a the 70F interior temperature, and much lower RH and dew point than the incoming leakage air. So wouldn’t the answer to this question, if considering the RH of the living space, and not just the intruding air, be that the moisture in the space (dew point) and thus the RH have risen (slightly), not fallen, since the indoor temperature stays essentially the same (assuming a normal ratio of intruding air to total house air volume)?

5. JayW says:

“ Yep. If there’s nothing for the water vapor to condense onto, you can get supersaturated air at higher than 100% relative humidity.”

What is an example of this? Fog? As I recall fog is water entrained in 100% RH air.

6. JayW says:

“ 1. On a cold rainy day, outdoor air leaks into a house and warms up to room temperature. What happens to the indoor relative humidity?”

On a similar note, if the HVAC system is bringing in a lot of OA during the winter on say a commercial application with an economizer and high internal load, the specific humidity inside will approach the specific humidity of the OA.

An exception would be if you have a high latent load inside the space such as a relatively large number of people.

7. Curt says:

I consider myself, a thoughtful HVAC contractor and local (northeast Florida) BS guru, pretty good on this psychrometric stuff. You got me fair and square on #7… I’ve seen similar behavior with liquid water heated above its boiling point without being stirred or agitated, and same with liquid water cooled below freezing. In both situations there needs to be some sort of physical starting point for vapor bubbles and ice crystal formation.

I agree that the temperature and humidity conditions in #10 are so extreme as to have yet to be recorded (Though my news feed recently reported 158*F heat index in Iran…not sure what the underlying drybulb and humidity conditions went into that 158*F heat index).

That said, I wonder if we in the southeast and southwest US do very occasionally experience an unrecorded micro-climate at or near 100*F drybulb and 90%RH…stay with me a minute or so:

Scenario – Hot sunny day with area weather stations reporting drybulb mid 90s or higher. Picture a typical dark asphalt shadeless parking lot outside a mall or Walmart supercenter – acres of asphalt with a surface temperature easily above 125*F…way too hot to touch. Air temp right above that parking lot is easily 5-10*F above local weather station figures. An afternoon thundershower passes nearby, close enough to drop, say, a tenth of an inch of rain on that asphalt…not enough to substantially cool the pavement, but definitely enough to be rapidly vaporized by the hot pavement and temporarily increase the humidity of the air just above the pavement – the air we walk through enroute to / from the store.

While feelings aren’t data, I feel like I’ve experienced 100*F DB 90% RH while walking across parking lots right after the above sequence of events…classic microclimate situation.

Thoughts?

8. Sam S says:

Like Marty R and JayW, I too answered #1 as “it increases”, because I was considering the air in the entire space, not just the air that leaked into the house. Is this thought process correct?

1. I also approached the question this way. The change in relative humidity within the space would depend on whether the dewpoint inside is higher or lower than the dewpoint outside. From a humidity control standpoint, a rainy day at 60F will result in a very muggy interior because the there is enough infiltration to meet the sensible load but the rh could be 70% or more. It all depends on what counts as “cold” for a rainy day.

9. JayW says:

You could ask what your thoughts are about plotting a point on the psychrometric chart using enthalpy and wet bulb as the two values.

Those two lines are almost parallel.

10. RoyC says:

On most psych charts, such as the one in this article, wet-bulb and enthalpy lines are shown as being the same, thus, you can’t plot a point by only knowing the wet-bulb and enthalpy. The ASHRAE psych charts show the wet-bulb and enthalpy lines separately, and they are not quite parallel. If you do try to define the state of moist air with wet-bulb and enthalpy as inputs, you better have very accurate values or you will be way off on determining dry-bulb, RH, humidity ratio, etc.

1. JayW says:

RoyC

Agreed that the two lines for Wet Bulb and Enthalpy are almost parallel and thus wouldn’t work for defining a point. If nothing else, a little inaccuracy in one or both would throw the point way off.

As I recall if you have any other two properties of a moist air condition you can use them to find the exact point on the psych. chart.

This would include specific humidity, enthalpy, DB Temp, WB Temp, RH, DP Temp.

Actually for the above besides enthalpy and WB, you also can’t use specific humidity and DP temp together. Specific humidity and DP Temp lies are actually parallel.

11. This 9-variable topic is broken, no new charts* since this horrible psychrometric one came out. This article about the confusion, proves the need for a new way to simplify & organize air-wetness information for use by consumers. Nobody needs to be familiar with the chart shown because they should only care about what goes on between the coolest space they control and the warmest space they control, or between say 60°F and 85°F.

This is the number one google result on Earth if you search “ Mold Chart”: https://energyhandyman.com/knowledge-library/mold-chart-for-temperature-and-humidity-monitors/

The purpose, is for use with standard temp and RH sensors, and to help people not to open their windows maybe. The phrase “Mold Chart for Temperature and Relative Humidity Sensors” or something similar is searched on Google millions of times a month by people who have already found the chart provided in this article.

Stay unconfused my friends.

1. JayW says:

That is an interesting chart in the link you shared. Do you know the source of this data?

Interesting argument for possibly not needing dehumidification in a well sealed crawl space. I wouldn’t want to get too close to the point where there is a possibility of a mold issue.

1. Hey Jay,
100% of the information is from DPCALC.org and there is an older version with more columns and all of those figures also come from the same place. Their formulas are really useful but not when you are presented with the whole spectrum versus the relavant sections only, and also “MY CHART” had to have some rounding out of necessity and as simple as it is people refuse to read the accompanying write-up. The main benefit of both of “MY CHARTS” is the limited temperature range, and the explanations at the bottom. Lots of people who store baseball cards, books, boxes, library systems ACTUALLY USE THIS CHART. You might never find the standard charts in use by anyone other than the author of an article about how tricky it is.

12. Christopher Wilkins says:

Great article and good questions although I do have a quarrel with a couple of them. On #7, the definition of relative humidity is based on the ratio of actual vapor pressure to vapor pressure at saturation. The actual vapor pressure cannot be >saturation. You can have a 2-phase situation, which is what you have when there is fog, but the vapor pressure cannot be > saturation so relative humidity cannot be >100%. There is an analogy with Steam flow. You can have 2-phase steam flow but the pressure is still saturation pressure. The presence of liquid does not increase the “vapor” pressure. The vapor is still at saturation. The saturated vapor just happens to be mixed with some liquid. I get the point you are trying to make, that you can have more moisture than saturation level, but the vapor itself will be saturated.

My other comment is a nit pick. The question was if I had experienced conditions greater than 100F/90%. My answer was yes, it was in a sauna in Finland, not an ambient condition, but I have experienced it.

1. Tyler Keniston says:

I believe you are incorrect, unless I’m misunderstanding your point. Air can indeed be ‘supersaturated’ in which vapor pressure is higher than saturation. RH greater than 100% is definitely possible.

Supersaturation is not prolonged, but for cloud formation, supersaturation must be greater than 0.