What is VP2 Dewpoint based on?

[coyotefred]

Hello!

'Quick question I'm sure someone has an answer to that I couldn't find in my Davis materials or other posts.  In my Davis manual it talks about the Dewpoint number on the display as "Dewpoint (overall)."  Why the "overall"?  Obviously there's a humidity sensor in the outdoor thermo/hygro unit, but is there one in the console as well, so the "dewpoint" number on the console display is an average between the two?  Right now I'm looking at my console and I have:

Dewpoint: 61 (F)
Temp out: 74 ""
Temp in: 75 ""
Hum out: 63%
Hum in: 48%

These numbers imply two humidity sensors correct?  So the 61F dewpoint figure is what...outdoor?  indoor.... average between the two?

Thanks for any help...maybe I'm missing something obvious... 

Coyotefred

[Ugly Duckling]

Hello!

'Quick question I'm sure someone has an answer to that I couldn't find in my Davis materials or other posts.  In my Davis manual it talks about the Dewpoint number on the display as "Dewpoint (overall)."  Why the "overall"?  Obviously there's a humidity sensor in the outdoor thermo/hygro unit, but is there one in the console as well, so the "dewpoint" number on the console display is an average between the two?  Right now I'm looking at my console and I have:


These numbers imply two humidity sensors correct?  So the 61F dewpoint figure is what...outdoor?  indoor.... average between the two?

Thanks for any help...maybe I'm missing something obvious... 

Coyotefred

I don't know about on those fancy VP2 displays but on the Vue console Temps are clearly marked 'indoor' and 'outdoor'.

The VP2 temps are not marked indoor and outdoor? And dew point is just a outside number but I guess if you don't have AC and live where its hot and humid you will feel the dew point both inside or outside.

Many Happy Weather Changes,   

Marci Anna

[coyotefred]

Here is a seller's picture of the VP2 display:

http://www.scientificsales.com/v/vspfiles/assets/images/vantagepro2-console.jpg

The number I'm talking about is marked "variable display."  You can select a variety of readings here, but when it's set to "dewpoint"...that's the reading I'm talking about...   I'm just confused as to why the VP2 manual refers to this as "dewpoint (overall)" rather than something like "dewpoint (outdoor)", etc. if that's what that number indicates...

[DougW]

Don't know were the "overall" comes from but the dew point for the VP2 is calculated from the the outside temperature and humidity readings.
Currently I have:
Outside temp: 100°
Outside humidity:33%
Dew point: 66° (Console reading)

Dew Point Calculator

Hope this helps, Doug

[Ugly Duckling]

Don't know were the "overall" comes from but the dew point for the VP2 is calculated from the the outside temperature and humidity readings.
Currently I have:
Outside temp: 100°
Outside humidity:33%
Dew point: 66° (Console reading)

Dew Point Calculator

Hope this helps, Doug

When the temperature and dew point converge you can start thinking about some sort of precip, rain, snow, fog etc.

Can you imagine a temperature and dew point of 100°? That would be humid! I don't believe I have ever seen dew point above 80° and that was in the south.

Maybe Doug you living more southerly than me have seen higher dew Points?

Many Happy Weather Changes,   

Marci Anna

[DougW]

Maybe Doug you living more southerly than me have seen higher dew Points? 

 
 83.7° dew point is the highest I have recorded. (6 yrs of data) 

[dalecoy]

...  In my Davis manual it talks about the Dewpoint number on the display as "Dewpoint (overall)."  Why the "overall"? 
...

As others have pointed out, it's for the outside.

The manual talks about dew point several places.  The only place it says "(overall)" is in the specifications section.  I believe that terminology is used to indicate the specification of the maximum theoretical range of the dewpoint computation.  Which is:

-105 to +130F;
-76 to +54C

For the formulas, etc., refer to Davis Application Note 28 (Derived Variables) at http://www.davisnet.com/product_documents/weather/app_notes/AN_28-derived-weather-variables.pdf

[Ugly Duckling]

Maybe Doug you living more southerly than me have seen higher dew Points? 

 
 83.7° dew point is the highest I have recorded. (6 yrs of data) 

83.7°F Dew Point: That is HUMID!

Many Happy Weather Changes,   

Marci Anna

[dalecoy]

83.7°F Dew Point: That is HUMID!

If the outside temperature is 120 degrees F, with that dewpoint, the humidity is only 33%.  Not exactly what I would call humid.

[neondesert]

83.7°F Dew Point: That is HUMID!

If the outside temperature is 120 degrees F, with that dewpoint, the humidity is only 33%.  Not exactly what I would call humid.

Speaking from a "desert" perspective I would have to say 120 degrees and a dewpoint of 83.7 would be extremely unconformable. 
Maybe not "humid" by eastern US or tropical definitions but definitely in the dangerous category.

This combination yields a Heat Index of 154 F 
http://www.hpc.ncep.noaa.gov/html/heatindex.shtml

You'd be surprised at what feels "muggy" in this neck o' the desert!

[dalecoy]

"...but it's a dry heat....

Over the years, I've spent more than 14 years in Northern  and central New Mexico - so I understand when you say uncomfortable and dangerous.

However, "Maybe not "humid" by eastern US or tropical definitions" - OK, point taken - I should have said "33% is not high relative humidity".  The word "humid" obviously doesn't have a consistent definition.

[d_l]

I hardly use the VP2 console for any readouts other than wind and temp, but Weatherlink can display both the outdoor and indoor dew points.

One thing I've wondered about is how accurate humidity and dew point readings can be if the weather station doesn't have a FARS?

[neondesert]

"...but it's a dry heat....

For the most part...when the moisture pushes in during July, August and September, well that changes the equation somewhat. 
I for one like the increase in moisture.  Not only does it signal monsoonal rain but it also stops water from evaporating from the
foliage at an astronomical rate!
 

Over the years, I've spent more than 14 years in Northern  and central New Mexico - so I understand when you say uncomfortable and dangerous.

However, "Maybe not "humid" by eastern US or tropical definitions" - OK, point taken - I should have said "33% is not high relative humidity".  The word "humid" obviously doesn't have a consistent definition.

Agreed, it's all "relative"! 

[dalecoy]

I hardly use the VP2 console for any readouts other than wind and temp, but Weatherlink can display both the outdoor and indoor dew points.

One thing I've wondered about is how accurate humidity and dew point readings can be if the weather station doesn't have a FARS?

Well, let's see.

The humidity sensor isn't affected by temperature, so the humidity reading should be unaffected by the presence or absence of a FARS.

The purpose of the FARS is to facilitate airflow so the temperature reading will be more accurate.  But the passive shield is rather good - as I recall, the FARS just improves things in bright sun by 2 or 3 degrees.  That would have a relatively small effect on the accuracy of the computed dew point.

[d_l]

The humidity sensor isn't affected by temperature, so the humidity reading should be unaffected by the presence or absence of a FARS.

The purpose of the FARS is to facilitate airflow so the temperature reading will be more accurate.  But the passive shield is rather good - as I recall, the FARS just improves things in bright sun by 2 or 3 degrees.  That would have a relatively small effect on the accuracy of the computed dew point.

The humidity sensor is affected by stagnant air within the shield.  A 24 hour FARS is continuously sampling the air and especially the dew point changes that sometimes occur after sunset. Frankly I think the updating of the air within the sensor chamber both day and night is just as important as equilibrating the temp sensor to the outside air temp.

[dalecoy]

The humidity sensor isn't affected by temperature, so the humidity reading should be unaffected by the presence or absence of a FARS.

The purpose of the FARS is to facilitate airflow so the temperature reading will be more accurate.  But the passive shield is rather good - as I recall, the FARS just improves things in bright sun by 2 or 3 degrees.  That would have a relatively small effect on the accuracy of the computed dew point.

The humidity sensor is affected by stagnant air within the shield.  A 24 hour FARS is continuously sampling the air and especially the dew point changes that sometimes occur after sunset. Frankly I think the updating of the air within the sensor chamber both day and night is just as important as equilibrating the temp sensor to the outside air temp.

Agreed on both points (stagnant air and updating of the air).  And I do run a 24-hour FARS (actually a daytime FARS running on continuous power).

However, really stagnant air is extremely rare, and only for very short periods - the atmosphere just doesn't stay completely still.  So I don't think there's much actual effect on the humidity sensor. 

As evidence, I offer the fact that the Stevenson screen is "the standard".

[LFWX]

"...but it's a dry heat....

For the most part...when the moisture pushes in during July, August and September, well that changes the equation somewhat. 
I for one like the increase in moisture.  Not only does it signal monsoonal rain but it also stops water from evaporating from the
foliage at an astronomical rate!
 

Over the years, I've spent more than 14 years in Northern  and central New Mexico - so I understand when you say uncomfortable and dangerous.

However, "Maybe not "humid" by eastern US or tropical definitions" - OK, point taken - I should have said "33% is not high relative humidity".  The word "humid" obviously doesn't have a consistent definition.

Agreed, it's all "relative"! 

True, see "Relationship to human comfort" http://en.wikipedia.org/wiki/Dew_point

[Sigdigit]

I like the categorization Wunderground uses in their "travel" section.  They define dewpoints above 60F as "humid" and dewpoints above 70F as "sweltering".  I can't imagine dewpoints above 80 outside of a sauna.  That would be absolutely stiffling.  Maybe in a jungle somewhere.

[Skywatch]

Doesn't dew point refer to the temperature point at which air of a given saturation (relative humidity) condences into dew and/or fog?
In a hot dry desert the dew point is really low, but in the Arctic It's even lower not because it's really cold there, but the humidity is near 0. Now in a rain forest the dew point will be in the 80's and 90's because the humidity is high. This also goes with heat index because the water vapor absorbs the heat making it feal hotter. All this revolves around relative humidity and air saturation, influenced by temperature.

[neondesert]

From the source (NWS):
http://www.weather.gov/glossary/

Dew Point
(Abbrev. DWPT) - A measure of atmospheric moisture. It is the temperature to which air must be cooled in order to reach saturation (assuming air pressure and moisture content are constant). A higher dew point indicates more moisture present in the air. It is sometimes referred to as Dew Point Temperature, and sometimes written as one word (Dewpoint).

Humidity
Generally, a measure of the water vapor content of the air. Popularly, it is used synonymously with relative humidity. (emphasis added)

Relative Humidity
A dimensionless ratio, expressed in percent, of the amount of atmospheric moisture present relative to the amount that would be present if the air were saturated. Since the latter amount is dependent on temperature, relative humidity is a function of both moisture content and temperature. As such, relative humidity by itself does not directly indicate the actual amount of atmospheric moisture present. See dew point. (emphasis added)

Heat Index
The Heat Index (HI) or the "Apparent Temperature" is an accurate measure of how hot it really feels when the Relative Humidity (RH) is added to the actual air temperature.

So, regarding your question:

Doesn't dew point refer to the temperature point at which air of a given saturation (relative humidity) condences into dew and/or fog?

Pretty much, although the dew point is not dependent on relative humidity. 

In a hot dry desert the dew point is really low, but in the Arctic It's even lower not because it's really cold there, but the humidity is near 0. Now in a rain forest the dew point will be in the 80's and 90's because the humidity is high. This also goes with heat index because the water vapor absorbs the heat making it feal hotter. All this revolves around relative humidity and air saturation, influenced by temperature.

Not quite. Don't know if you saw my post last month but at the end of June the McCarran Airport ASOS recorded a dewpoint of -22°F.  The air temperature at the time was 107°F and the relative humidity was 1%.  It doesn't get much lower than that.   

The dew point in the Arctic could be considerably lower depending on the ambient temperature but you could still have a higher relative humidity.  For example, an ambient air temperature of -20°F with a dewpoint of -50°F would yield a relative humidity of about 18%   

Same for the tropics, the relative humidity is dependent on moisture content and air temperature.  So you could have an air temperature of 100°F and a dew point of 80°F
resulting in relative humidity of only 53%. 
(Relative Humidity Calculator: http://www.hpc.ncep.noaa.gov/html/dewrh.shtml)
However, this combination would be extremely dangerous as the Heat Index would be near 120°F!

The heat index is the calculated temperature the body feels when you combine the ambient temperature with either the dewpoint or relative humidity.  High moisture content retards evaporation which prevents the body from cooling efficiently. 

[Skywatch]

From the source (NWS):
http://www.weather.gov/glossary/

Dew Point
(Abbrev. DWPT) - A measure of atmospheric moisture. It is the temperature to which air must be cooled in order to reach saturation (assuming air pressure and moisture content are constant). A higher dew point indicates more moisture present in the air. It is sometimes referred to as Dew Point Temperature, and sometimes written as one word (Dewpoint).

Humidity
Generally, a measure of the water vapor content of the air. Popularly, it is used synonymously with relative humidity. (emphasis added)

Relative Humidity
A dimensionless ratio, expressed in percent, of the amount of atmospheric moisture present relative to the amount that would be present if the air were saturated. Since the latter amount is dependent on temperature, relative humidity is a function of both moisture content and temperature. As such, relative humidity by itself does not directly indicate the actual amount of atmospheric moisture present. See dew point. (emphasis added)

Heat Index
The Heat Index (HI) or the "Apparent Temperature" is an accurate measure of how hot it really feels when the Relative Humidity (RH) is added to the actual air temperature.

So, regarding your question:

Doesn't dew point refer to the temperature point at which air of a given saturation (relative humidity) condences into dew and/or fog?

Pretty much, although the dew point is not dependent on relative humidity. 

In a hot dry desert the dew point is really low, but in the Arctic It's even lower not because it's really cold there, but the humidity is near 0. Now in a rain forest the dew point will be in the 80's and 90's because the humidity is high. This also goes with heat index because the water vapor absorbs the heat making it feal hotter. All this revolves around relative humidity and air saturation, influenced by temperature.

Not quite. Don't know if you saw my post last month but at the end of June the McCarran Airport ASOS recorded a dewpoint of -22°F.  The air temperature at the time was 107°F and the relative humidity was 1%.  It doesn't get much lower than that.   

The dew point in the Arctic could be considerably lower depending on the ambient temperature but you could still have a higher relative humidity.  For example, an ambient air temperature of -20°F with a dewpoint of -50°F would yield a relative humidity of about 18%   

Same for the tropics, the relative humidity is dependent on moisture content and air temperature.  So you could have an air temperature of 100°F and a dew point of 80°F
resulting in relative humidity of only 53%. 
(Relative Humidity Calculator: http://www.hpc.ncep.noaa.gov/html/dewrh.shtml)
However, this combination would be extremely dangerous as the Heat Index would be near 120°F!

The heat index is the calculated temperature the body feels when you combine the ambient temperature with either the dewpoint or relative humidity.  High moisture content retards evaporation which prevents the body from cooling efficiently. 

How old and accurate the instruments? I've never seen the dew point at the actuall temperature with humidity lower than 95%. Don't know what the airport stations use to calculate to what a home weather station uses. And I know better than to challenge the data that of an airport station. But some of them are indeed questionable as far as accuracy. For example, why would one station report 50 degrees DWPT and another reports 28 when both have the same or close temperature, humidity, and air pressure? Doesn't make sence, but I'v seen it.

[neondesert]

How old and accurate the instruments?

According to what I could find the dewpoint temperature sensor at the McCarran ASOS was upgraded in 2005, so not that old.



As far as accuracy?  There is a quality control procedure in place. 
Here's some light reading for you if you would like to know how they are monitored.   
http://www.weather.gov/asos/pdfs/aum-toc.pdf

I've never seen the dew point at the actuall temperature with humidity lower than 95%.

I don't think you ever will.  If the dew point is the same as the ambient temperature then the humidity would be 100%

Don't know what the airport stations use to calculate to what a home weather station uses. And I know better than to challenge the data that of an airport station. But some of them are indeed questionable as far as accuracy. For example, why would one station report 50 degrees DWPT and another reports 28 when both have the same or close temperature, humidity, and air pressure? Doesn't make sence, but I'v seen it.

It depends on the airport.  For the major ones in the US it could be any one of the following:

- ASOS (Automated Surface Observing Systems) which are operated and controlled by both the FAA and the NWS.
- AWOS (Automated Weather Observing System) the oldest systems operated and controlled by the FAA.
- AWSS (Automated Weather Sensor System) similar to ASOS but operated and controlled by the FAA.

As far as your example goes, I have no doubt errors do occur.