Foggy, humid and dew point v. humidity
Dense Fog Advisory in effect until 9 this morning.
This morning it is partly cloudy with a chance of a thunderstorm and a chance of rain in the afternoon. Fog early. High of 82F. Winds from the SSE at 5 to 15 mph. Chance
of rain 40%.
Tonight it will be overcast with thunderstorms. Fog overnight. Low of 64F. Winds from the SSE at 5 to 10 mph shifting to the NE after midnight. Chance of rain 90% with rainfall amounts near 0.4 in. possible.
The readings from my own weather instruments are:
The humidity is 90% with a Dew Point of 62ºF and a temp of 67.5ºF.
Our wind direction is Southeast between 1.1 MPH and 2.3 MPH.
Our Barometric pressure is 29.98 HPA 1013 and falling with a weather graphic indicating rain.
The UV rating is not measurable owing to the thick fog, Sunrise is at 6:02 a.m. sunset is 7:13 PM and Moon Rise is at 2:00 p.m. EDT and the moon phase is waxing crescent.
For the pilots out there raw metar is:
METAR KAUG 021153Z AUTO 00000KT M1/4SM FG VV002 18/18 A2992 RMK AO2 SLP133 T01780178 10194 20172 53007 $
Visibility is 0.2 miles/0.4 Kilometers in thick fog with clouds up there, somewhere.
We received rain in the amount of 0.24 inches here over the past 24 hours.
In each day's post I give the humidity and the dew point. I do this because some people, like me, are die-hard humidity people and some people are of the dew point persuasion.
Those of us born in the '50's and '60's grew up in the age of humidity readings. We simply relate better to a percentage of moisture to tell us how much moisture is in the air. A temperature reading that is supposed to tell us how much moisture is in the air just doesn't give us the same information.
So, this is the explanation I found online:
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.
The USATODAY.com Understanding humidity page has a lot more on this.
(Answered by Jack Williams, USATODAY.com weather editor, Nov. 30, 2002)
Blogger's note:
Rarely have I seen an explanation that does more to confuse an issue than it does to answer it as this one does.
Dense Fog Advisory in effect until 9 this morning.
This morning it is partly cloudy with a chance of a thunderstorm and a chance of rain in the afternoon. Fog early. High of 82F. Winds from the SSE at 5 to 15 mph. Chance
of rain 40%.
Tonight it will be overcast with thunderstorms. Fog overnight. Low of 64F. Winds from the SSE at 5 to 10 mph shifting to the NE after midnight. Chance of rain 90% with rainfall amounts near 0.4 in. possible.
The readings from my own weather instruments are:
The humidity is 90% with a Dew Point of 62ºF and a temp of 67.5ºF.
Our wind direction is Southeast between 1.1 MPH and 2.3 MPH.
Our Barometric pressure is 29.98 HPA 1013 and falling with a weather graphic indicating rain.
The UV rating is not measurable owing to the thick fog, Sunrise is at 6:02 a.m. sunset is 7:13 PM and Moon Rise is at 2:00 p.m. EDT and the moon phase is waxing crescent.
For the pilots out there raw metar is:
METAR KAUG 021153Z AUTO 00000KT M1/4SM FG VV002 18/18 A2992 RMK AO2 SLP133 T01780178 10194 20172 53007 $
Visibility is 0.2 miles/0.4 Kilometers in thick fog with clouds up there, somewhere.
We received rain in the amount of 0.24 inches here over the past 24 hours.
In each day's post I give the humidity and the dew point. I do this because some people, like me, are die-hard humidity people and some people are of the dew point persuasion.
Those of us born in the '50's and '60's grew up in the age of humidity readings. We simply relate better to a percentage of moisture to tell us how much moisture is in the air. A temperature reading that is supposed to tell us how much moisture is in the air just doesn't give us the same information.
So, this is the explanation I found online:
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.
The USATODAY.com Understanding humidity page has a lot more on this.
(Answered by Jack Williams, USATODAY.com weather editor, Nov. 30, 2002)
Blogger's note:
Rarely have I seen an explanation that does more to confuse an issue than it does to answer it as this one does.
Comments
Post a Comment