Relative Humidity

FAQs

Relative Humidity

This video is about humidity and important humidity-related factors. At the end of the video you will know the difference between absolute and relative humidity and how temperature affects humidity. Impress your friends at parties!

Relative humidity (RH) is the amount of moisture in the air relative to the maximum amount of moisture at a specified temperature. This relationship takes into account both absolute humidity and temperature. Hence when you talk about RH of 60%, you should refer to the temperature you are measuring the RH. The amount of moisture in the air at 60% RH is different at 100, 200C or 250C.

If the RH of the air is 60%, at what temperature will the air hold the most moisture? 100, 200C or 250C? If you are not sure of the answer, check out the humidity video!

RH affects our everyday lives. For example, the rate of evaporation depends on RH, not absolute humidity. Mould growth/control depends on RH also.

WITH HIGH RH:

Your laundry won't dry

You perspire more

Your hair becomes fizzy

Dust mites and mould flourish

You can detect odor better

We use hygrometers to measure RH. Hygrometers come in many forms.

In 1783, Swiss geologist and physicist Horace Benedict de Saussure constructed a mechanical hygrometer with women's hair!

Hair changes length when humidity changes. The greater the humidity, the shorter the hair becomes. The changing length of the hair will move the indicator.

As you can imagine, this is not very accurate! Luckily these days, we use digital hygrometers. Most digital hygrometers use a resistive or capacitative sensor, measuring differences in electrical currents.

One important factor when looking for hygrometers is the accuracy of the instrument. Look for a hygrometer with certified accuracy of +/-2% to +/-5%. This means the hygrometer reading is within +-2% to +/-5% of the actual RH.

Many manufacturers do not supply this information but you should always inquire.

Landed properties, especially those surrounded by beautiful mature trees in the Bukit Timah area, have higher RH for a few reasons:

  • Being close to the ground, they are affected by evaporation at the ground level. There is a little less mixing of air at 1m compared to 10m.

  • Tree covers are great for cooling air temperature, but cooler temperatures also result in higher RH.

  • Trees themselves actually humidify the air by a process called transpiration. Transpiration is the process by which the leaves lose water through the tiny pores on the surface. A tall healthy tree with thousands of leaves can breathe out 1000-2000 L of moisture in just ONE day!