In my ongoing "let's write yet another METAR decoder" project I've run into
a slight snag - a lot of them seem to calculate the relative humidity, but
that information isn't contained in the TAR. I assume they're calculating it
from the two temperature readings, and perhaps the pressure.

Anyone know how this is done? I've Googled around and I can't quite seem to
find the answer.

Many thanks.

Jim
--
Twitter:@GreyAreaUK
"[The MP4-12C] will be fitted with all manner of pointlessly shiny
buttons that light up and a switch that says 'sport mode' that isn't
connected to anything." The Daily Mash.

HI Jim,

You could try the website below. I think the Clapeyron Equation gives
Saturation Vapour Pressure as a function of dry bulb, and then the ratio of
the SVPs of dry bulb and dewpoint (if you know it!) gives the RH. Better to
let the computer do it!

http://www.ringbell.co.uk/info/humid.htm

Ian Bingham,
Inchmarlo, Aberdeenshire.


"Jim" wrote in message
...
In my ongoing "let's write yet another METAR decoder" project I've run
into
a slight snag - a lot of them seem to calculate the relative humidity, but
that information isn't contained in the TAR. I assume they're calculating
it
from the two temperature readings, and perhaps the pressure.

Anyone know how this is done? I've Googled around and I can't quite seem
to
find the answer.

Many thanks.

Jim
--
Twitter:@GreyAreaUK
"[The MP4-12C] will be fitted with all manner of pointlessly shiny
buttons that light up and a switch that says 'sport mode' that isn't
connected to anything." The Daily Mash.

On 2010-03-30, Ian Bingham wrote:
HI Jim,

You could try the website below. I think the Clapeyron Equation gives
Saturation Vapour Pressure as a function of dry bulb, and then the ratio of
the SVPs of dry bulb and dewpoint (if you know it!) gives the RH. Better to
let the computer do it!

http://www.ringbell.co.uk/info/humid.htm

Thanks - I think I've found a way but it's making certain assumptions about
air pressure, which your link doesn't. All I need to do now is find the
equations that page is using..!

Thanks.

Jim
--
Twitter:@GreyAreaUK
"[The MP4-12C] will be fitted with all manner of pointlessly shiny
buttons that light up and a switch that says 'sport mode' that isn't
connected to anything." The Daily Mash.

On 2010-03-30, Jim wrote:
In my ongoing "let's write yet another METAR decoder" project I've run into
a slight snag - a lot of them seem to calculate the relative humidity, but
that information isn't contained in the TAR. I assume they're calculating it
from the two temperature readings, and perhaps the pressure.

Anyone know how this is done? I've Googled around and I can't quite seem to
find the answer.

And as is usual I found it a few minutes later. Or at least I think I have.

Tc = Temperature in degrees C
Tdc is dewpoint in degrees C

First calcutate saturation vapor pressure (Es) and actual vapor pressure
(E):

Es=6.11*10.0**(7.5*Td/(237.7+Td))
E=6.11*10.0**(7.5*Tdc/(237.7+Tdc))

Relative humidity then becomes (E/Es)*100

What I _don't_ know is how these figures lose accuracy as air pressure
deviates from 'normal' since, and I quote:"The next set of formulas assumes
a standard atmospheric pressure."


http://www.wral.com/weather/blogpost/1174528/

Jim
--
Twitter:@GreyAreaUK
"[The MP4-12C] will be fitted with all manner of pointlessly shiny
buttons that light up and a switch that says 'sport mode' that isn't
connected to anything." The Daily Mash.

The website I gave does have a panel for pressure. If you put different
pressures in you can see how it affects the relative humidity. Not by a
lot, I should imagine.

Ian.

"Jim" wrote in message
...
On 2010-03-30, Jim wrote:
In my ongoing "let's write yet another METAR decoder" project I've run
into
a slight snag - a lot of them seem to calculate the relative humidity,
but
that information isn't contained in the TAR. I assume they're calculating
it
from the two temperature readings, and perhaps the pressure.

Anyone know how this is done? I've Googled around and I can't quite seem
to
find the answer.

And as is usual I found it a few minutes later. Or at least I think I
have.

Tc = Temperature in degrees C
Tdc is dewpoint in degrees C

First calcutate saturation vapor pressure (Es) and actual vapor pressure
(E):

Es=6.11*10.0**(7.5*Td/(237.7+Td))
E=6.11*10.0**(7.5*Tdc/(237.7+Tdc))

Relative humidity then becomes (E/Es)*100

What I _don't_ know is how these figures lose accuracy as air pressure
deviates from 'normal' since, and I quote:"The next set of formulas
assumes
a standard atmospheric pressure."


http://www.wral.com/weather/blogpost/1174528/

Jim
--
Twitter:@GreyAreaUK
"[The MP4-12C] will be fitted with all manner of pointlessly shiny
buttons that light up and a switch that says 'sport mode' that isn't
connected to anything." The Daily Mash.

On 2010-03-30, Ian Bingham wrote:
The website I gave does have a panel for pressure. If you put different
pressures in you can see how it affects the relative humidity. Not by a
lot, I should imagine.

Not at all, in fact. I tried everything from 1000 down to 1, didn't affect
the RH at all.

Viewing the source for the page actually did reveal the equations (which
makes a nice change) and it does _seem_ to be using the same ones I posted,
although I've not had a lot of time to doublecheck the values they're using.

Again, thanks.

Jim
--
Twitter:@GreyAreaUK
"[The MP4-12C] will be fitted with all manner of pointlessly shiny
buttons that light up and a switch that says 'sport mode' that isn't
connected to anything." The Daily Mash.

.... I realise you are doing this just as a programming exercise, but
don't get too hung up on minor differences in pressure, when your
input temperatures are given to the nearest degC.

In the METAR code, rounding occurs and will significantly affect the
computed RH .... example:-

Air temperatu 8.6
Dew point: 8.4
RH: 99%

METAR TT=09
METAR TdTd=08
RH: 93%

Air temperatu 3.4
Dew point: 2.6
RH: 94%

METAR TT=03
METAR TdTd=03
RH: 100%

If, as you wrote elsewhere in the ng, you intend releasing this to a
wider public, you'll need to heavily qualify this section as a *gross*
approximation to the RH, and IMV, a meaningless output.

Martin.


--
Martin Rowley
West Moors, East Dorset (UK): 17m (56ft) amsl
Lat: 50.82N Long: 01.88W
NGR: SU 082 023

On 2010-03-30, Martin Rowley wrote:
... I realise you are doing this just as a programming exercise, but
don't get too hung up on minor differences in pressure, when your
input temperatures are given to the nearest degC.

In the METAR code, rounding occurs and will significantly affect the
computed RH .... example:-

Air temperatu 8.6
Dew point: 8.4
RH: 99%

METAR TT=09
METAR TdTd=08
RH: 93%

Air temperatu 3.4
Dew point: 2.6
RH: 94%

METAR TT=03
METAR TdTd=03
RH: 100%

If, as you wrote elsewhere in the ng, you intend releasing this to a
wider public, you'll need to heavily qualify this section as a *gross*
approximation to the RH, and IMV, a meaningless output.

Oh aye, duly noted. I was more concerned that the pressure figure might have
made the difference between the calculated RH figure being 'broadly right'
and 'grossly wrong', but that was before I found the equation. And, as you
say, the rounding in the temps will be the major accuracy factor anyway.

Thanks for the input.

Jim
--
Twitter:@GreyAreaUK
"[The MP4-12C] will be fitted with all manner of pointlessly shiny
buttons that light up and a switch that says 'sport mode' that isn't
connected to anything." The Daily Mash.