# At 100% relative humidity, why does the air still feels dry?

At 100% relative humidity, the dew point temperature is the same as the current temperature. One would probably conclude that water would start condensing on surfaces.

But seems that is not always the case.

This morning in Hong Kong, the RH is around 100% and temperature is around 19 degree Celsius.

But the air feels really dry, and there is no condensation on surface. In fact, we have practically a clear blue sky today. Hong Kong Observatory forecast the probability of rain less than 20%.

So, how do we understand this situation?

• Is it possible? Or is it simply faulty equipment for reading 100% RH?
• How come at dew point that water droplet is not dripping on surfaces like in spring or summer when temperature is hotter?
• Is there are measurement we can use to better tell the possibility of condensation, cloud, rain, and generally how wet the weather is than RH?
• If the air contained enough humidity to saturate it at $19 \sideset{^{\circ}}{}{\mathrm{C}} ,$ what would be the relative-humidity of that air if it were heated up closer to body-temperature?
– Nat
Commented Nov 22, 2023 at 10:56
• @Nat: The relative humidity would probably be lower right nest to a human. But that doesn't explain that there is no condensation on surfaces other than human skin. Commented Nov 22, 2023 at 11:28
• I'm not sure exactly what you mean by the second sentence there, but with respect to when there's condensation, that can happen in the opposite case: when air is hotter than the surface it contacts (rather than cold air on skin). For example, air-conditioners have cold surfaces that cool down air that flows through them. Those coils can easily collect a good bit of condensation if the air was already relatively humid. The drying effect that you're asking about is sort of the opposite, when skin's a heating surface.
– Nat
Commented Nov 22, 2023 at 13:27
• Ohh I think I get what you mean by condensation! Naw, if there were condensation, then it'd feel wet. Warming air lowers the relative-humidity, but not the absolute-humidity. This is, warming air doesn't remove water from it, but rather increases the air's capacity to hold water -- causing it to feel drier.
– Nat
Commented Nov 22, 2023 at 14:03
• Air won't generally dew at 100% relative-humidity, because it's happiest at 100% relative-humidity and has no reason to do so. Though if you walk into a sauna where the air's at 100% relative-humidity and at a higher temperature, then once that air touches your skin, your skin may slightly chill the air, lowering how much water it can hold and driving its relative-humidity above 100% -- so the air'll react by losing water, causing it to feel wet. This scenario's sort of the opposite, where your skin warms the air, so the air can hold more water and may feel drier.
– Nat
Commented Nov 23, 2023 at 5:56

Humans cannot directly sense humidity since we lack the sensory neurons for that specific purpose.

Therefore our sensation of, say, relative humidity in the air is always a combination of different sensory inputs.

These inputs consist of, for example, conductive heat from the air in contact with the skin, heat radiation from direct sunlight and the pressure of small water droplets condensing on our skin.

A prolonged contact with high humidity, be it vapor or liquid water, will also swell the skin some. On the other hand even the slightest wind will carry away moisture from the skin quite effectively. Our brains are calibrated to these different sensations of how our skin feels to the touch or how it feels to touch other object with different "skin moisture conditions" so to say, and we can interpret these sensations as different levels of humidity in the surroundings.

The human perception of air humidity is therefore not an exact sensation but rather a computation based on several factors, making it quite unreliable.

• I agree human perception can be wrong. I'm not trying to argue otherwise. But I do want a better way to correlate human perceivable humidity indicators with scientific terms (e.g. RH). For instance, condensations on table, wall is well perceivable humidity indicator. Cloud and rain fall would be another. Commented Nov 24, 2023 at 2:31
• What scientific data would correlate better to surface condensations (dew or water droplet drips) than RH? What scientific data would correlate better to cloud formation and rain? Commented Nov 24, 2023 at 2:33
• I'm not sure I understand what you mean. Condensation happens when surface is colder than dew point. Cloud formation happens when air mass cools below dew point. Commented Nov 24, 2023 at 10:44
• @KoalaYeung: Just to check, you understand that metrics like temperature and relative-humidity vary from point-to-point, right? For example, what's happening near human-skin and what's happening higher up with the clouds is significantly disconnected from what's happening in a sample of bulk-atmosphere at ground-level. Unfortunately, this does mean that the little bit of data that they tend to share on the news about the temperature/humidity outside isn't generally enough to tell someone everything about conditions.
– Nat
Commented Nov 25, 2023 at 0:13
• I understand there is a disconnection between metrics and human observable indicators. That's why I asked the question in the first place to wrap my head around the situation. There can be multiple metrics collectively indicates things that could, at least, better match observations. I asked because I want to better develop intuitions in connecting metrics to my real life experience. Commented Nov 25, 2023 at 6:30

We feel absolute humidity, i.e. how heavy is air.

----------CORRECTION NEEDED (thanks @Jeopardytempest)

Humid air is lighter than dry air. See the relevant wikipedia page. With changes in temperature temperature, the difference between densities of 100%/0% RH air increases non linearly.

At 20°C, the difference is a negligible 1%.

At 40°C, the difference is about 5%, which is something you surely notice.

When your body requires sweating, then the blockage of evo-transpiration can be noticed and it is an indirect feeling of relative humidity.

----------WRONG ANSWER LEFT HERE FOR HISTORICAL REASONS How much water can be present in air is non-linear with temperature (see here for a table to have an idea about its change).

At 0°C and 100% relative humidity, absolute humidity is 4.8 g/m3 (negligble, since air density is ~1200 g/m3).

At 20°C and 100% relative humidity, absolute humidity is 17.3 g/m3 (air density is ~1200 g/m3, it starts to be ~1% more).

At 40°C and 100% relative humidity, absolute humidity is 51.1 g/m3 (~5% of the air density, definitely noticeable, it is like the difference between swimming in sea water and in fresh water).

It is 3 times more than at 20°C, even if temperature "just" doubled (actually temperatures must be compared in K, so the temperature increase of 20°C over 293-313°K is less than 10%, and water content can increase of 300%).

When your body requires sweating, then the blockage of evo-transpiration can be noticed and it is an indirect feeling of relative humidity.

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– gerrit
Commented Dec 17, 2023 at 15:05