Dew Point Dilemma

“Dew point” is one of those obscure meteorological stats like “barometric pressure” or “relative humidity” that interests hard-core weather watchers, pilots and scientists. It’s also a key parameter for climate control, too.

From a comfort perspective, dew point is more important than relative humidity. According to the U.S. National Oceanographic and Atmospheric Administration, another way to look at dew point is to think of it as the temperature to which the air needs to be cooled in order to achieve a relative humidity (RH) of 100 per cent. At this point, the air cannot hold any more water in the gas form. If the air were to be cooled even more, water vapor would have to come out of the atmosphere in the liquid form, usually as fog or precipitation. The higher the dew point rises, the greater the amount of moisture in the air. This directly affects how “comfortable” it will feel outside.

Often, relative humidity can be misleading. For example, a temperature of 1C and a dew point of 1C will give you a relative humidity of 100 per cent; but a temperature of 25C and a dew point of 16C will only give you a relative humidity of 50 per cent. It would feel much more “humid” on the 25C day with 50 per cent relative humidity than on the 1C day with a 100 per cent relative humidity. This is because of the higher dew point. If you want a real judge of just how “dry” or “humid” it will feel outside, look at the dew point instead of the RH. The higher the dew point, the muggier it will feel.

For automotive as well as building air conditioning, heat is removed from the passenger compartment by lowering the air temperature of the fresh or recirculated vent flow. But that’s just vent temperature as indicted by your thermometer. Humans are much more sensitive to the drop in humidity caused by dropping dew point, since skin is evaporatively cooled by perspiration. And as the dew point drops, the condensed water droplets need to be dumped overboard by the evaporator’s condensate drain.

This little tube is more important than many technicians think. If it leaks into the passenger compartment, the moisture may not only soak the sound deadener matting, but put moisture back into the passenger compartment by evaporation. Never tell a customer that damp carpets are normal on humid days. It’s also important to keep the drain clear to prevent buildup of odour and allergy-inducing mould in the evaporator case. If a bad smell is the complaint, address the condensate issue first before disinfecting the evaporator.

But why should automotive technicians care about dew point? There are two reasons: One is that automotive HVAC system performance as the consumer perceives it varies greatly with the dew point. If the system generates acceptable vent temperatures on low humidity summer days, will it perform adequately on muggy, humid days? The “it’s-not-as-cold-as-it-was-when-you-fixed-it-last-week” comeback is a possibility if you don’t address the moisture issue. In worst case scenarios it’s actually possible to blow fog through the dash vents, especially in some marginal older European systems. In A/C, moisture matters.

Want to really track A/C system performance? Find the dew point temperature for your area and use this guide instead of the less reliable “relative humidity” from the radio or TV:

General comfort levels that can be expected during the summer months:

* Less then or equal to 55: dry and comfortable

* Between 55 and 65: becoming a bit “sticky” with muggy evenings

* Greater then or equal to 65: lots of moisture in the air, becoming oppressive

Source: U.S. NOAA

Where can you find the dew point for your area? The federal government provides it at www.weatheroffice.ec.gc.ca