When talking about how muggy it is, it would seem to make sense to find out what the humidity it is.

But it’s important to remember that “humidity” is a relative term.  Hence the name “Relative Humidity”.  It is expressed as a percentage because it is the amount of water vapour in the air relative to the amount of water vapour that could be in the air at that temperature.

So, if the temperature is 4 C and the relative humidity is 90% there isn’t that much water vapour in the air because the air 4 C can’t hold that much water vapour to begin with.

However, if the temperature is 30 C and the relative humidity 90%, that is an incredible amout of water vapour since the air is capable of holding so much more at that temperature.

That means that “relative humidity” will always be dependent on the temperature.  Knowing the RH without knowing the temperature doesn’t help much when you are trying to figure out how muggy it is.

For example; a relative humidity of 70% is not very muggy if the air temp is 15 C.  However, that same relative humidity of 70% with an air temp of 25 C, is likely to feel pretty muggy to most people.

Which may leave some of you wondering if there is anything better.

The answer is; yes.

The dew point.  Or “dewpoint” as some people spell it.

The dew point is the temperature to which the air must be cooled, without removing any moisture, so that saturation will occur.  Kind of like taking a big glass and filling it halfway with water.  The glass is the air temperature and the relative humidity is half, or 50%.  If you could shrink the glass to the point where the water rose up to the rim of the glass, that would be your dew point.

For the average person all they want to know is; is it muggy outside?  Well, if you know the dew point you can tell without even knowing the air temperature.

Dew point temperature (C)	Comfort level
Below 10	Comfortable
10-15	Not bad
15-20	Muggy
20-25	Very muggy

This really only applies to summer-like air temps.  A dew point of 10 with an air temp of 10 means the air is loaded with water vapour and it’s likely to produce dew or fog.  That’s not what I would call dry air.  But not exactly “muggy” either.

Also, remember that not every perceives temperature and humidity levels the same way.  These numbers are just based on what the average person would likely perceive.  But it’s a good chart to use as a guide.

Finally, a couple of quick notes.

• Air doesn’t actually “hold” water vapour like a glass holding water.  They kind of co-exist.  However, for the sake of this discussion it’s just easier to use that old analogy.
• Besides temperature, air pressure plays a big role in determining relative humidity.  But again, to keep things simple, that’s something that’s best left for another article.

Homepage image/ Sling Psychrometer used to determine the dew point temperature.

By Howard Joseph, WeatherWatch.co.nz