# Relative humidity at different temperatures misconception

People in hot arid deserts often look at the weather report and find out the RH is 20%. Then they say it's 20% in a warm tropical country, so we're in just as humid a place as that!

Doesn't a RH of 20% in air of 45 Celsius ambient hold way lesser water than 32 Celsius ambient air with a RH of 20%? Also, does altitude above sea level matter?

Technically, the dew point is the more appropriate indicator of comfort than the relative humidity (RH). I will use the example in the dew point link for illustration. You could live in an area with a temperature and dew point of 30 degrees, which would mean the air is at 100% RH. In contrast, it could be 80 degrees out with a dew point of 60 degrees for a RH of only 50% but this hotter day would feel much more humid and muggy. The higher the dew point relative to the temperature on a warm-to-hot day, the worse it will feel outside.

An example I like to give is to compare Minnesota to Maryland. The RH in northern Minnesota is often quite high in the summer, i.e., >80% (yearly average is ~70-75% https://www.forbes.com/sites/brianbrettschneider/2018/08/23/oh-the-humidity-why-is-alaska-the-most-humid-state/?sh=7bf40524330c). The dew point, however, is typically much lower relative to the temperature than in Maryland, so a typical 90 F day in July can feel way worse in Maryland than in Minnesota. The lower latitude also makes the sunlight more direct.

• I find your examples puzzling. Are you sure of your claim that 100% RH (where the water litterally starts to condensate on you) could ever feel less humid than a 50% RH where the water gently stays in the air? (I don't question that the hotter day will feel more unpleasant) Also, your comparison of two typical 90F days would be equally clear using RH as dewpoint (RH or AH for that matter can't be tricky if you're at the same temperature). I don't reject your points because you sound like have more experience in this aspect, but I am also not yet convinced by the arguments as stated above. Jun 22, 2021 at 13:28
• @BarbaudJulien - If you follow the Forbes link you will see that Alaska has the highest annual average RH of any of the 50 US states. This is common in cold places because as you mention, it's easy to get near saturation when cold because the air cannot hold much water vapor. However, I very much doubt folks in Alaska would say it's very humid there especially in winter. This is also an issue in Antarctica. If you take the cold outside air with a high RH and heat it up in a habitat, the RH drops precipitously causing it to be extremely dry. This makes it difficult to stay hydrated. Jun 22, 2021 at 13:38
• The fact that we tend to change the temperature of the outside air in our habitat, thereby changing the RH is an interesting point that I didn't take into account :) I personnaly find this comment more convincing than the answer itself. It leads me to believe that RH is "theoretically" a good measure of humidity for comfort but that the outside/meteorological RH is not necessarily representative of the actual living conditions (in the habitations), thereby making the dewpoint more relevant (?) Jun 22, 2021 at 13:49
• @BarbaudJulien - I am trusting that the good folks at NOAA do have a good handle on the difference between dew point and RH. The dew point link above points their explanation, which is what I used as well. I also know this from the experience of living in northern MN for >10 years as well as MD for >10 years. The RH in MN is higher than MD, but MD heat feels way more oppressive because the dew point is higher. Jun 22, 2021 at 14:22
• @BarbaudJulien See also this chart of thermal comfort. For a sufficiently steep negative-slope line, one can move from a perception of humid to dry even as the relative humidity climbs to near 100%. When perspiration is minimal, the relative humidity is nearly irrelevant. Jun 22, 2021 at 15:17

The answer to your question is definitely no on 2 accounts. First, what you call "less" depends on the purposes of your analysis (more on that in a sec). Second, a hotter air can hold more water before saturation, not the contrary.

Absolute humidity (AH) is defined as the mass of water per unit volume of air. Relative humidity (RH) is defined as the ratio of the current AH, to the saturation AH (which is the maximum absolute humidity the air can hold before it starts to condensate into droplets).

The saturation humidity increases with temperature. So, in a sense, you are right (if we invert your claim) in saying that at same RH but higher temperature, your AH will be higher (since $$AH=RH*AH_{sat}(T)$$). In this case, the warmer air will hold more mass of water per volume. However, in a meteorolgical context, it would likely not be pertinent to point that up. The relevant quantity here is indeed RH.

To illustrate, let's say it's hot outside. You're cooling yourself off by evaporating sweat off your skin. The rate at which you can evaporate it and thus chill will be limited by the humidity already present in the air, more precisely by the relative humidity. Since RH is a measure of how close you are to saturation, it is the one that is significant in telling you how much your evaporation rate will be lowered.

In short, when it comes to measuring human comfort in a meteorolgical condition, AH would be misleading while RH is pertinent IMO. I believe people are not committing a misconception, but instead using the appropriate tool when referring to RH to compare two climatic conditions.