Confusion over humidity

The story How UAE’s relative humidity makes it too hot to handle (April 20) left out one important factor, and that is dewpoint.

Relative humidity (RH) can be explained as the “closeness the air is to saturation”. When the RH is less than 40 per cent, it feels dry outside and when the RH is greater than 80 per cent it feels moist outside (dewpoint will determine if it is uncomfortably moist or just regularly moist). Between 40 and 80 per cent RH is comfortable if the temperature is also comfortable.

The worst combination for human comfort is a high dewpoint (18°C or above) combined with a high RH. If the dewpoint is above 18°C, it will generally always feel uncomfortably humid outside.

Obviously, the temperature could climb to over 37°C and result in a low RH, but the quantity of moisture in the air is still high and will be noticed. The optimum combination for human comfort is a dewpoint of about 15°C and a RH of between 50 per cent and 70 per cent (this would put the temperature at about 23°C). The air feels dry outside when both the dewpoint is below 15°C and the RH is less than 40 per cent. Now the dilemma, how do you differentiate the “meaning” between a high dewpoint and a high RH when they both indicate the air is humid? Dewpoint is related to the quantity of moisture in the air while relative humidity is related to how close the air is to saturation.

How you understand this difference in meaning can be a challenge. The challenge can be overcome by describing how the weather feels and relate that information to the current dewpoint and relative humidity.

Tom Plant, Dubai