Fog, mild and a humidity v. Dew point article

Good morning Augusta.
Its still foggy so drive careful.
This morning it is Mostly cloudy with a chance of rain, then a chance of a
thunderstorm and a chance of rain in the afternoon. Fog early. High of 81F.
Winds less than 5 mph. Chance of rain 40%.
Tonight it will be mostly cloudy with a chance of a thunderstorm and a
chance of rain. Low of 63F. Winds less than 5 mph. Chance of rain 40% with
rainfall amounts near 0.4 in. possible.
The readings from my own instruments are:
The humidity is 87% with a Dew Point of 64.5ºF and a wind chill of 68.6ºF.
The temperature is 68.6ºF.
We have North winds between 0.0 mph and 1.1 mph.
Our Barometric pressure is 29.82 HPA 1009.8 and falling with a weather
graphic indicating rain.
The UV rating is 0 out of 16, sunset will be at 7:21 PM with Moon Rise at
11:40 p.m. and the moon phase is waning Gibbous.
For the pilots out there, Raw Metar readings are:
METAR KAUG 281053Z AUTO 00000KT 4SM BR CLR 18/17 A2991 RMK AO2 SLP127
T01780167
Visibility is 4.0 miles/6.4 kilometers with clear conditions.
We've had no rain here in the past 24 hours.
Is dew point or relative humidity a better indication of how humid the air
feels? My guess is it's relative humidity. Also is there much variation
between the dew point temperature from day to day?
A: Dew point is by far the better measurement of how humid the air feels.
This is the case because dew point is a measurement of how much humidity is
in the air, no ifs, ands or buts about it. Relative humidity tells you how
much humidity is in the air compared with how much can be in the air at the
temperature the air happens to be when you measure it.
This means that the relative humidity goes down as the temperature goes up
even though the amount of water vapor in the air (humidity) remains the
same.
To give you an idea of how this works, I used the Weather calculator on the
Web site of the National Weather Service office in El Paso, to calculate
some temperature, dew point, and relative humidity combinations.
If the temperature in the morning is 71 degrees, and the dew point is 70
degrees, the relative humidity would be 97% (rounded off). As the day warms
up the dew point would stay the same unless a new air mass arrived with
either more humid air or drier air.
Let's assume that this does not happen. If the air warmed to 95 degrees and
the dew point stayed at 71, the relative humidity would be 44% and the heat
index would be 101 degrees.
On a day like this, you'd feel sticky when you left your air-conditioned
house in the morning, and by mid afternoon, people would be walking around
saying things like, "Boy, it must be 95 degrees and 95 humidity."
No way.
The only way you'd ever see a temperature and relative humidity both of 95
would be if the dew point were 93 degrees. The only place you would find
that high a dew point outside a lab experiment would be some place like the
Persian Gulf.
A good general rule of thumb is that when the dew point is between 60 and 70
degrees F most people will think it's humid. When the dew point tops 70,
almost everyone will feel uncomfortably humid. When it tops 80, as it rarely
does anywhere in the USA, it's likely to set records.
As for day to day variation. The big variations come when a new air mass
moves in. Or, during the summer you could have the humidity (thus the dew
point) slowly increase if you have a steady wind from warm water, such as
the Gulf of Mexico, even though the weather map never shows a warm front
actually arriving.
The USATODAY.com Understanding humidity page has a lot more on this.
(Answered by Jack Williams, USATODAY.com weather editor, Nov. 30, 2002)