Warm, Humid and Humidity v. Dew Point explained
This morning it is overcast with a chance of rain, then thunderstorms and rain showers in the afternoon. High of 81F. Winds from the SSW at 5 to 10 mph. Chance of rain 70%
with rainfall amounts near 0.2 in. possible.
Tonight it will be overcast with thunderstorms and rain showers, then rain showers after midnight. Fog overnight. Low of 63F. Winds less than 5 mph. Chance of rain 90% with
rainfall amounts near 1.0 in. possible.
The readings from my own instruments are:
The humidity is 77% with a Dew Point of 58ºF and temp is 64.4ºF.
Our wind direction is South Southeast between 0.6 MPH and 2.0 MPH.
Our Barometric pressure is 30.11 HPA 1012 and falling with a weather graphic indicating rain.
The UV rating is 0 out of 16, Sunrise is 4:55 a.m. sunset is 8:27 PM and Moon Rise is at 3:48 a.m. EDT and the moon phase is Waning Crescent.
For the pilots out there, Raw Metar readings are:
METAR KAUG 250953Z AUTO 20005KT 10SM FEW060 FEW100 BKN120 17/13 A2990 RMK AO2 SLP123 T01720133
Visibility is 10.0 miles/16.1 kilometers with clouds to 6000 ft / 1828 m, and mostly cloudy to 12000 ft / 3657 m.
We received no rainfall overnight in this area.
Do you understand the subtle way that the weather reporters are trying to shift you away from "humidity" to "dew point"? Well, if you are over age 50 you probably ignore them. I grew up with air moisture being measured as humidity, and the idea that Dew Point is a much better way to explain it doesn't hold much water for me. Well, perhaps measuring that in terms of Dew Point is more accurate?
The below is lifted in whole from USA Today's web site on the topic.
Q: 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
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
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
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.
Comments
Post a Comment