LocalSYR

Dew Point vs. Humidity...(7-22-13)

<B>( 5:00 pm Monday July 22nd)</B>It becomes humid again here in central New York early this week but should we use dew point or dew point to describe it?

The heat may have gotten the top billing last week here in central New York, and deservedly so.  We had six days in a row with 90 degree weather and that hasn’t happened since 1991.  However, it wasn’t just the heat we were dealing with but this air had a tropical feel to it making it more like Miami, New Orleans or Houston.  However, when you start talking tropical mugginess the question shows up: ‘why do you mention the dew point and not the humidity?”


How we feel because of the amount of moisture in the air is, like any other meteorological parameter, a bit subjective.  For some, the first increase in moisture in the Spring can bring an uncomfortable mugginess. For others, they relish the mid summer humidity of the past few weeks. The bottom line is this: the amount of moisture in the air affects different people in different ways.


I think a lot of people believe that using the dew point to explain how muggy it is outside is too confusing.  However, after reading this blog you will begin to think that it is humidity that is confusing.


First, dew point is an absolute measure of the amount of moisture in the end.  It is the temperature at which you have cool the air to get dew to form.  That in itself gets people all worked up; they don’t care when dew forms. That’s fine, don’t focus on the name.  Just remember these numbers:



Dew Points

Comfort Level

60 F or lower

Comfortable for most

60 F to 65 F

Slightly Humid

65 F to 70 F

Muggy

70 F or higher

Oppressive/Tropical

These are just rough guides because, as I mentioned above, everyone responds to the muggy weather differently. For some, dew points between 65 and 70 can be oppressive. From a personal standpoint, I only remember 3 numbers: below 60 for a dew point and I’m comfortable, around 65 and it is muggy and 70 or above and I’m uncomfortable.


When you see a humidity value, remember it is relative to the temperature.  If you want to be confused, just look at how the ‘relative’ humidity in Syracuse changed during one of the days during last week’s heat wave.  I picked Tuesday but I could have used just about any of the days.:



Time

Humidity

6 am

78%

9 am

67%

Noon

59%

3 pm

47%

6 pm

49%


The humidity is actually going down through the day which I think is confusing since it is during the middle of the day that it feels the most uncomfortable. That’s when you expect the humidity to be at its highest but it is just the opposite. When you ask people how it feels during the middle of the day like yesterday they might say something like this “Boy, it sure is muggy outside. The humidity must be 100%” As you can see from the numbers, however, even during the peak of one of hottest and most uncomfortable days of the year, the humidity is running only between 50-60%. Now lets show you the same chart but include the dew points:



Time

Humidity

Dew Points

6 am

78%

65 F

9 am

67%

68 F

Noon

59%

71 F

3 pm

47%

69 F

6 pm

49%

67 F


Notice our dew points hovering near 70 during the middle of the day.  The changes to dew point are tied directly to changes in the moisture in the air and won’t change with the temperature (like the humidity). With dew point separate, you get a truer measure of how it feels.


So we’ve established that a dew point of 70 F or better is oppressive for most folks in central New York.  With that as a baseline, how is this summer doing in terms of really muggy days? What I’ve done is count the number of days between June and August when the dew point cracks 70 F.  I only have data that goes back about 20 years but here is how this summer stacks up so far with some recent ‘muggy’ summers:




Summer

Days Dew Point 70+

Days Temp 90+

2013*

20

11

2010

32

15

2002

31

26

2005

29

20


*Through July 21st


It has certainly been humid so far this July but we still have a ways to go before we can crown this summer as the most humid of recent memory. Now that you have all this knowledge, embrace change and key in on the dew point, not the humidity, the rest of the summer when you want to know whether it is muggy or not. It really isn’t that confusing..
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