WeatherWatcher: Lesson on Relative Humidity, this Summer was Muggy for us

Most of the questions I've received over the years about our weather in this area is about or revolving around the issue of relative humidity. Examples would be:

• It's 50°F outside with a relative humidity of 100%, why doesn't it feel as muggy as it does when it's 70°F and 100%?
• Why is the humidity 82% at this location but 78% at that location when they are less than 2 miles away?
• Was it more humid this summer than it was last year?

First, I'd like to explain what relative humidity means. Relative humidity is the percentage of moisture in the air before saturation at current air temperature. An example would be if the relative humidity was 100% at 50°F, it means that the air is 100% saturated, if the temperature dropped to 49.9°F some of that moisture would have to condense into fog, or water droplets. The warmer the air, the more moisture that air can hold. You might notice it's a lot more uncomfortable when it is 70°F with a relative humidity or RH for short of 100% than it is when it is 50°F at 100% RH. This is because there is much more water vapor absorbed in the air at 70°F with 100%RH, than at 50°F of the same RH value. 

The real measure of humidity really falls on whats call the dew point. The dew point is measured as a temperature in degrees.  For instance a dew point would be displayed as 52.1°F. This is the temperature at which the current moisture content in the air will condense into water droplets or ice crystals. You might notice when you take a hot shower that your bathroom mirror fogs over. That is because the mirror is colder than the dew point of the air in your bathroom.  A common household problem in the winter is high dew points inside. Your window might be 50°F, but your air temperature in the house might be 65°F. Boiling water or cooking food causes water to evaporate into your environment, increasing the dew point. In general if something is as cold or colder than the dew point, including the air, moisture will condense to it. A glass of ice water on a summer day is another good example of what happens when you have something below the dew point of the environment.

Here is a dew point calculator I like to use, it may help visualize how this works.

Now, down to the real data. Some of you have asked me about this late summer's weather, regarding muggy or humid conditions. Was it more humid this August and early September? The short answer, yes. I am measuring the muggy or humid feeling using dew point measurements, since that is a better grasp of how much moisture is in the air. 

As you can see in the graph below, I have graphed out the daily average dew point from August 1st - September 30th for the past 5 years (full station history in Shoreline) to give you a good comparison.  In general we seem to stick to dew points below 55°F, but the really uncomfortable days happen whenever the dew point exceeds 55°F or really 60°F.  August 2013 and early September 2013 had plenty of 60+°F dew points. This of course is a very good environment for thunderstorms to develop, which explains why we had many more thunderstorms than normal this summer as well.

Graphic by Carl Dinse

All this humid moisture-laden air is from a southerly to southwesterly flow that persisted late August and early September, bringing tropical air up from off the southern California coast. I also mentioned in a previous article that this is a favorable weather pattern for explosive wind storms or low pressure storm systems. The out of season wind storms we had in September were no surprise to me given how the late Summer weather pattern shaped up and is a sign of the stormy weather that may be ahead for us this fall and winter.

I will start to feature daily dew points now with my normal weekly weather reports.

With that, I will leave you with this from dictionary.com:

mug·gy [muhg-ee] 

adjective, mug·gi·er, mug·gi·est. 

(of the atmosphere, weather, etc.) oppressively humid; damp and close. 

Origin: 

1725–35; mug to drizzle (noun and v.) (< Scandinavian; compare Old Norse mugga mist, drizzle) + -y1

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