Answers to my previous question How is relative humidity determined from a wet and dry bulb readings? provide equations for converting the two temperature measurements to a relative humidity.

In comments under this question I have heard of a Whirling Hygrometer which seems to be the same basic beast (two thermometers, one with a wet bulb from a cotton sock and small reservoir) but like the name says, this one you have to "whirl" around for most of a minute; presumably the evaporation is more efficient in the blowing air.

But I don't understand if that leads to a lower cold temperature and therefore requires a different equation, nor if the speed that it is whirled makes a difference. And I'm not sure why whirling is used if the static non-whirled kind also can be used, without moving parts.

Question: How does a whirling hygrometer work, how does it differ from static dry/wet bulb thermometer, and how to obtain humidity from its readings?


wet/dry bulb humidity measurement click for larger

  • $\begingroup$ "and how to obtain humidity from its readings" I think is covered by your previous earthscience.stackexchange.com/questions/19151/… question? Not easy, and while I'm no expert on the topic, maybe I can offer some additional input. But hopefully that can help you limit this to one question? If you want some other measure of humidity, perhaps you could ask a separate question, though it's quite easy to get from RH to like dew point temperature or vapor pressure, and there's probably already questions on such. $\endgroup$ Commented Apr 30, 2021 at 18:09
  • $\begingroup$ @JeopardyTempest I'm not sure that's true yet. As I've just explained in this question the whirling affects the rate of evaporation. Isn't there a chance then that the temperature difference could be larger than for a static pair? If it's the same, then I'd like to see that in an answer, along with an explanation of why it isn't different. I've asked over 3,000 SE questions, and I have found that when there are questions so tightly coupled as this, the answerer to one will almost certainly be able to answer the other. I think this will be just fine! $\endgroup$
    – uhoh
    Commented Apr 30, 2021 at 18:13
  • $\begingroup$ @JeopardyTempest The wet bulb temperature is always read first because it will start to warm as soon as the hygrometer stops whirling. proves that it can't be the same equations. $\endgroup$
    – uhoh
    Commented Apr 30, 2021 at 18:18
  • 2
    $\begingroup$ I never used a static wet bulb, dry bulb thermometer. I always used a whirling hygrometer. The one catch about using a whirling hygrometer is the wet bulb temperature must be read first & it must be read immediately the hygrometer stops rotating. The reason for this is once the wet bulb thermometer stops rotating it will slowly start to warm up due to the warmer temperature of the atmosphere. $\endgroup$
    – Fred
    Commented Apr 30, 2021 at 18:23
  • 2
    $\begingroup$ The answer will be no to your question "isn't there then a chance that the temperature difference could be larger for a static pair", any properly done wet bulb measurement will result in the same value for the same air (and any improper measurement has error that won't be accountable by a simple equation). Give me some time and I'll update answers to both this and the other question from what I've found\know. $\endgroup$ Commented Apr 30, 2021 at 19:04

1 Answer 1


Instead of «whirling thermometer» you probably refer to «sling psychrometer» consisting of a dry-bulb thermometer and a wet-bulb thermometer. Yet, different to a static wet-bulb thermometer shown in the question, the constant rotation of the two thermometers permits evaporation from the wet cloth around the web bulb thermometer to «pristine parts» of surrounding air.

enter image description here


If the air around the wet-bulb thermometer is less than water saturated, the temperature of shown on the wet-bulb thermometer will be lower than the one read on the dry-bulb thermometer. The swirling motion aims to expose the wet-bulb thermometer to air which was not yet in contact with the former. Thus, both recordings allow you to report both (dry) temperature, dew temperature, and humidity of the air.

A beginner's introduction is equally given here, a computation chart relating the temperature difference read (one for the Celsius, an other for the Fahrenheit scale) to the relative humidity here.

Suggested by user @uhoh, it is worth to set the quality of the readings obtained with this instrument into a context. The 1993 edition of Conserve O Gram 3/1 (accessed from the series' program overview) mentions «These instruments are accurate to $\pm \pu{2\% RH}$ when properly maintained and operated by trained and practiced users.» Equally, the guide reminds to the Clausius-Clapeyron relation when stating:

«Psychrometers and psychrometric charts are intended for use within a certain range of atmospheric pressure. High altitude, above 900 m (approx. 3,000 ft), will directly affect the accuracy of the RH reading unless a pressure correction formula is applied or a psychrometric chart or slide rule for the appropriate pressure is used. For further information see Ann Hitchcock and Gordon C. Jacoby, “Measurement of Relative Humidity in Museums at High Altitude,” Studies in Conservation 25 (1980): pp. 78-86. (Copies are available to NPS sites from Curatorial Services Division, Harpers Ferry office.»

A study by Wang and Tang / Singapore's metrology centre published by 2007 mentions:

«... that although the psychrometer coefficient of a particular psychrometer can be quite consistent with long-term stability typically within the range of ± 1% rh equivalent, there are larger variations (up to 4% rh equivalent) among different psychrometers even of the same type.»

  • $\begingroup$ Excellent! There seems to be a small difference at least from the chart on my thermometer to the one in your link. It's not huge but it's in the right direction to match your explanation. Mine (I think, I can't read Chinese) says a 5 degree difference at dry=30 C indicates 65% relative humidity i.sstatic.net/gESbF.png whereas the conversion chart you link to says 67%. This suggests that a 5 degree difference would have to result from dryer air for mine than for yours, consistent with a bit higher rate of evaporation, $\endgroup$
    – uhoh
    Commented May 1, 2021 at 23:00
  • $\begingroup$ @uhoh Depending on the source consulted, at least for non-calibrated devices, one observes $\pm2\dots\pu{4\% RH}$ difference to the true value of RH. See equally the edit of the answer, too. $\endgroup$
    – Buttonwood
    Commented May 2, 2021 at 1:25
  • $\begingroup$ Sure, but I'm talking about a systematic difference, not a +/- random variation, and in tabulated values used for the RH determination, not in measurements. $\endgroup$
    – uhoh
    Commented May 2, 2021 at 1:55

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