How Humidity Affects our Lives

A largely invisible yet tangible phenomenon, humidity is one of those “necessary evils” that help form our beautiful and balanced environment. In all outward appearance, humidity seems to do nothing but bad – turns everything green, attracts critters, refuses to let your sweat evaporate, rots out wood, etc. Well, let’s take a look into this sinister element of nature and see if we can’t understand it better – sweat, mildew, termites and all.

Absolute Humidity, Specific Humidity, and Relative Humidity

These terms might be familiar to you, but let’s go over them in case. Absolute humidity and specific humidity are quite similar in concept, with the former being a ratio of water weight per unit of volume – typically grams per cubic meter – and the latter being a ratio of water weight per unit of weight – typically kilogram per kilogram.

If you were to condense all the moisture in a given volume of air – say a cubic meter – the ratio of water per cubic meter is its absolute humidity. Because absolute humidity changes with temperature and pressure, it can cause confusion in more complex fields such as chemical engineering, and is therefore rarely used (in these fields). For this reason – and to distinguish it from specific humidity – it is also sometimes referred to as volumetric humidity.

Relative humidity is what most of us are more familiar with, and it is the ratio – expressed as a percentage – of existing water vapor to the maximum water vapor the air is able to hold without condensing. Percentage indicates relative humidity, whereas other ratios such as those shown above are used for other humidity units.

Dew Point Vs. Relative Humidity

This is yet another phenomenon that is related to humidity. Dew is essentially condensation, and being that condensation is the saturation of air due to temperature-change, this temperature-change is also what causes dew to form. Condensation is essentially the bringing of water vapor to dew point via the lowering of temperature. The dew point is the temperature at which water vapor will begin condensing into water.

So how is dew point and relative humidity related? Well, as the relative humidity gets higher (percentage), the closer the dew point gets to the current air temperature. In other words, maximum relative humidity – 100% – means that the current air temperature is at the dew point. The lower the relative humidity at a given temperature, the more the temperature needs to decrease in order to reach its dew point.

There is a simple equation to calculate the dew point – if you need it – as long as the relative humidity is 50% or more. This should get you pretty accurate results – within 1C. The equation is as follows: Td = T-((100-RH)/5), where Td is the dew point, T is the temperature, and RH is relative humidity. You can also calculate relative humidity with the inverse: RH = 100-5(TTd).

So now that we somewhat understand how humidity runs her business, we can take precautions to avoid suffering adversely at her hands. Why does a mattress get damp after sleeping on it directly on a cold floor? Why do closet walls (and everything between them) with no ventilation get moldy during the monsoon? Same reason dude! – Relative humidity and dew point working together to slip you a fast one.

But look on the bright side. – It makes you appreciate the good in life. Plus, not enough humidity is a problem as well. Here in Japan, people have to sometimes use dehumidifiers, but in other drier regions and countries people even have to use humidifiers in order to maintain a suitable environment for things such as musical instruments and certain precision products that would otherwise get damaged by the lack of humidity in the air. Goes both ways!