It is humid! What is the relative humidity? Heat Index?

Most often, the term humidity is actually relative humidity in percentage (%) rather absolute humidity. The latter refers to the vapor concentration as a ratio of mass to volume (grams/m3). The relative humidity (RH, %) is the ratio of the mass of vapor at the point of measuring to the mass of vapor at saturation of vapor temperature (100%). Another term is called dew point (unit: ℃) indicating the temperature that the vapor in that sampling air condensed. Other terms related to the humidity are here.

According to the graph below, the dew point depends on the original relative humidity (how much vapor that air contains) at which temperature. Let say, a sample air has a relative humidity of 70% at 30 ℃, using this graph, the dew point of this sample air is around 24 ℃, but the that air only have 20% RH, then the dew point is ~ 4℃.

Dewpoint RH
Wikipedia Source
Vapor holding capacity, temperature and relative humidity
Source: Wikipedia

For my case, the graph below shows the mirroring lines between the temperature and the relative humidity but this observation ignores the underlining facts that the relative humidity is calculated based on the temperature. So with a same amount of absolute vapor in a sample air, a higher temperature results in a lower relative humidity (because of that temperature, that air can hold more vapors relatively the absolute capacity of holding vapors at the latter temperature).

Relation of the temperature and relative humidity

The next question is what are the choice for a humidity sensor? There are measuring principles based on change of resistance, capacitance, and weight. Many reviews online offers better explanation, so you can browse like one in here.

As of 2018, the humidity sensor for a hobbyist are mostly resistant or capatative types. An excellent comparison posted by Robert Kandrsmith is here. The tested sensors included AM2302, SHT71, HTU21D, Si7021 (used the same driver as STH21), BME280. The accurate range is ∓3% in the range of 10-80 %RH. The weather in Hanoi could reach to a typical 80-90% and the recorded values in the range bears more uncertainty. My weather station used two version one is Si7021 but the sensor is marked at SHT21 and the other is DHT22. Adding HTU21D is not possible since the I2C address of Si7021 and DHT21D is the same (0x40), and no option to change the address is provided. (usually through copper trail and solder blob)

Many sensor

Applications of the relative humidity addressed in two main areas: laboratory and human comfort. In gravimetric analysis or analyzing of weight change especially with a sample contains water, the relative humidity has to be specified so that the weight of water vapor in the sample is taken into account.

The relative humidity can weight on human comfortability. A wetted (or high (relative) humidity) plus a high temperature with a low connection (such as no wind/or fan) reduces the evaporation of water that our body employed to cool off after exercising or have extra heat than its normal state (36.5℃). So we excrete sweat instead of vapor. So the feeling being covered by sweat and cannot get rid of it because the air around is almost saturated (I'm full, say no to more vapor, said the air).


A dried air is not helpful either. Dried air can lead to crack skin, running nose, bleeding in the respiratory track because of the imbalance of the vapor in the air. In this case, the air sucks the water out of your skin. Did you recalled running in cold, dried weather and the nose was sketchy and you smelled funny.

But not to worry about the comparison, engineers proposed a heat index as the combo term to take temperature and relative humidity into a figure number to indicate (mostly) hotness (of the air). According to Wikipedia, at 24℃ and 0%RH, the air temperature feels like 21℃, but at 24℃ and 100% RH, the air feels like 27℃.

I hope this writing could give some info to start up with relative humidity. Thanks.

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Last update: July 13, 2021