I'm sorry, but Dr Wark is wrong if he equates dew point and wet bulb
temperature. Enc. Brit., states under the heading "psychrometer" states:
"a hygrometer composed of two similar thermometers. The bulb of one
thermometer is kept wet (by means of a thin, wet cloth wick) so that the
cooling that results from evaporation makes it register a lower
temperature than the dry-bulb thermometer. When readings are taken
simultaneously, it is possible (with the use of psychrometric tables) to
determine the relative humidity and dew-point temperature of the air. A
decrease in the humidity of the air brings an increase in the difference
between dry- and wet-bulb temperatures, called the wet-bulb
depression.". Note it states that the dew point can be determined from
the psychrometric tables.
To support this, an example is given in, for a psychrometry chart,
http://www.sp.uconn.edu/~mdarre/NE-127/NewFiles/psychrometric_inset.html :
"A sling psychrometer gives a dry-bulb temperature of 78oF and a
wet-bulb temperature of 65oF. Determine other moist air properties from
this information. Two useful air properties for environmental analysis
in agricultural buildings would be relative humidity and dewpoint
temperature. Relative humidity is an indicator of how much moisture is
in the air compared to desirable moisture conditions, and dewpoint
temperature indicates when condensation problems would occur should the
(dry-bulb) temperature drop. Find the intersection of the two known
properties, dry-bulb and wet-bulb temperatures, on the psychrometric
chart, Figure 1. The dry-bulb temperature is located along the bottom
horizontal axis. Find the line for 78oF, which runs vertically through
the chart. Wet-bulb temperature is located along diagonal dotted lines
leading to scale readings at the upper, curved boundary marked
"saturation temperature". The intersection of the vertical 78oF dry-bulb
line and the diagonal 65oF wet-bulb line has now established a "state
point" for the measured air. Now read relative humidity as 50 percent
(curving line running from left to right up through the chart) and
dewpoint temperature as 58oF (follow horizontal line, moving left,
toward the curved upper boundary of saturation temperatures)."
Note the difference between Tw wet bulb and Td dew point. This is
confirmed in another example contained in
http://info.wlu.ca/~wwwgeog/courses/gg101web/labs/gg101winter2006lab5background.pdf
and yet again at http://www.raiza.com/psycexcel1.html
Then, if you use the EZAir psychrometric software and enter some
arbitrary values, you will get answers like:
11/07/2006 Calculated Psychrometric Properties
-------------------------------------
Dry Bulb Temp = 65.0 deg F
Wet Bulb Temp = 50.0 deg F
Dew Point Temp = 34.8 deg F
Relative Humidity = 32.4%
Humidity Ratio (grains) = 29.6 grains
Humidity Ratio (W) = 0.00422 lb. water/lb. dry air
Specific Volume = 13.317 ft3 per Lb.
Enthalpy = 20.2 Btu/lb.
Even in my Physics text book (Starling and Woodall, 1950) from when I
was a student over 50 years ago, it states The temperature of the wet
bulb can in no case descend to the dew point, for, if it did,
evaporation into the surrounding layer of air (supposed also reduced to
this temperature) must cease, and heat leaking to the bulb from the stem
of the thermometer would once again cause the temperature to rise. It
follows that the wet bulb attains some temperature between the dry-bulb
reading (i.e. air temperature) and the dew point. The ratio of the
dew-point depression to the wet-bulb depression is known as Glaisher's
Factor."
I am not a physicist but I worked on hygrometric instrumentation as part
of my Insulohmeter SIR tester and I developed a modified Assmann
psychrometer to go into the climatic chamber. As none of the
psychrometric tables went as high as 95°C/95% RH, I developed
empirically the software which calculated RH from the two values (using
Pt100 sensors). For this calibration, I used dew point to determine the
RH and I know damn well that there is ALWAYS a difference between Td and
Tw (very small though it is at high temps and/or humidity.). This may
not be text book "knowledge" but real, down-to-earth, pragmatic knowledge.
Brian
Lee parker wrote:
> Brian
>
> Rather than getting into a protracted discussion with you on our opinions
> concerning this issue, I will instead simply quote from a recognized text
> "Thermodynamics" by Kenneth Wark. Dr. Wark taught Thermodynamics at Purdue
> University and his text has been used at over 20 engineering schools through
> out the country. In fact I used his text when I taught undergraduate
> thermodynamics.
>
> To quote Dr. Wark
> "It is found that, for air- water-vapor mixtures at normal temperatures and
> pressures, the wet bulb temperature, the determination of which relies on
> the heat- and mass-transfer rates, is close in value the the
> adiabatic-saturation temperature. The temperature T2 used in Eq 11-35 (the
> dew point temperature) to obtain the humidity ratio is normally the wet bulb
> temperature, and this leads to an answer of sufficiency accuracy."
>
> Your definition of the dew point and the dry bulb temperatures are
> measurement techniques not definitions. The dew point is the adiabatic
> saturation temperature. This measurement requires the adiabatic mixing of
> the principal fluid (air) with moisture which in turn requires no change in
> entropy, a violation of the Second Law. Both measurements you described
> produce a heat exchange and so both are approximations and we refer to these
> as wet bulb measurements. Probably, the most accurate technique for
> measuring dew point is to slowly move air through a channel that is
> partially filled with water. The channel is insulated and the heat transfer
> through the channel is minimized. The channel must be long enough to produce
> equilibrium. Normally this requires a very long channel, but is done when an
> extremely accurate measurement is necessary. This technique is described in
> most thermodynamic text books.
>
> I would very much like to know your bases for degrading the accuracy of the
> Psychometric Chart. These are first principal definitions. This is
> tantamount to criticizing the accuracy of Algebra, Calculus or the Periodic
> Chart. The error is not in the chart, but in the measurements used to enter
> the chart.
>
> Best regards
>
> Lee
>
> J. Lee Parker, Ph.D.
> JLP Consultants LLC
> 804 779 3389
>
>
>
>
> ----- Original Message -----
> From: "Brian Ellis" <[log in to unmask]>
> To: "TechNet E-Mail Forum" <[log in to unmask]>; "Lee parker" <[log in to unmask]>
> Sent: Monday, July 10, 2006 11:44 AM
> Subject: Re: [TN] Humidity?
>
>
>> Sorry, but dew point and wet bulb temp are not the same. Dew point is
>> the temperature at which condensation will occur on a mirror surface
>> under given static conditions. The wet bulb shows the temperature
>> depression due to the loss of heat of vapourisation of water under
>> dynamic conditions, usually by whirling a psychrometer at a given speed.
>>
>> Referring to the Smithsonian Meteorological Tables, if the measured dew
>> point is 10°C and the temp is 20°C, the RH is 53%. According to the US
>> Weather Bureau #1071, if the wet bulb temp is 10°C and the dry bulb temp
>> is 20°C, the RH is 24%. In my book 53 and 24 are not the "same
>> measurement".
>>
>> In any case, the W&D psychrometer is notoriously inaccurate (typically
>> ±15% RH errors are possible in the usual temperature range of 5°-35°C)
>> as there are many undefined variables. The dew point method is much more
>> accurate but the instruments are very costly. Absolute humidity
>> expressed as the weight of water vapour per unit volume of air (g/m3) at
>> a given temperature and barometric pressure is the only scientific
>> metric and this can be correlated to dew point at any temperature within
>> about ±1%, but not easily to relative humidity as barometric pressure
>> plays a role.
>>
>> Brian
>>
>> Lee parker wrote:
>>> Folks
>>>
>>> There has been a lot of chatter concerning dew points, relative humidity
>>> and
>>> temperature.
>>> (The dew point and wet bulb temperature are different names for the same
>>> measurement.) These three measurements are uniquely related
>>> thermodynamic
>>> properties. For instant, temperature and relative humidity uniquely
>>> define
>>> the dew point (i.e. wet bulb temperature). The relationship between the
>>> three is normally displayed in most thermodynamic text books (the kind
>>> Mechanical Engineers use) as the so called Psychometric Chart. The
>>> relationships are not linear which explainers why it is difficult to
>>> reach a
>>> relative humidity of 100%.
>>>
>>> Best regards
>>>
>>> Lee
>>>
>>> J. Lee Parker, Ph.D.
>>> JLP Consultants LLC
>>> 804 779 3389
>>>
>>>
>>> ----- Original Message -----
>>> From: "Brian Ellis" <[log in to unmask]>
>>> To: <[log in to unmask]>
>>> Sent: Friday, July 07, 2006 10:51 AM
>>> Subject: Re: [TN] Humidity?
>>>
>>>
>>> If you look at http://www.cypenv.org/weather/wx2.htm you can see today's
>>> weather measure by my weather station. I invite you to note the
>>> following:
>>> 1. temp at 1200 was ~32°C and RH was high at ~38%
>>> 2. between 1200 and 1245, the temp dropped to 28° because the sun was
>>> obscured by the formation of towering cumulus and cumulonimbus.
>>> 3. at 1245, a minor thunderstorm started
>>> 4. by 1305, 1 mm of rain had fallen and the temp had dropped to 21°C,
>>> while the RH had risen to a peak of a whopping (for here) 67% by 1325
>>> 5. Note the dewpoint was substantially constant at 14-16 °C during this
>>> episode
>>> 6. it took a couple of hours or so after the rain for "normal service to
>>> be resumed"
>>> 7. the thunderstorm was accompanied by gusty wind conditions: these
>>> winds were katabatic, coming from the direction of a nearby mountain;
>>> this is quite usual
>>> 8. as is usual during thunderstorms, the barometric pressure rose by
>>> about a hectopascal (millibar)
>>> 9. the RH has a strong inverse relationship to temperature (shown over
>>> the whole graph, whereas the dew point does not. In fact, during the
>>> storm, if there is a tendency, it goes proportionally to temperature.
>>> 10. during the period from 1330 to 1530, evaporation was high due to
>>> solar radiation and it felt very uncomfortable despite the low
>>> temperature of 24-28°C; this was because the evaporation was almost
>>> being visibly sucked to higher altitudes and forming small clouds at
>>> ~1000 to 2000 m. This is why the humidity was lower than one would
>>> instinctively think.
>>>
>>> At http://www.cypenv.org/weather/wx3.htm, you can see the same kind of
>>> thing for 6 July, but with a more violent storm, dropping 11.4 mm of
>>> rain in 25 minutes. Note the more distinct barometer blip.
>>>
>>> Note these graphs progress with time, so don't wait to look at them!
>>>
>>> I agree that 100% RH is the max: one would drown above that, except in a
>>> rare case of supersaturation, which can happen only if there are no
>>> condensation nuclei in the air, which would be almost impossible at
>>> ground level. 100% would result in very thick fog, starting at 90-95%,
>>> mist at ~85% and haze at 75% (approximations, depending on temperature,
>>> nucleation, wind, barometric pressure etc.). I believe 102% is a
>>> journalistic exaggeration (or a badly calibrated instrument!!!). I agree
>>> that a Florida summer rainstorm can look worse than a steamy tropical
>>> rain forest, but the steaminess dissipates, along with the resultant low
>>> level cloud formation almost as soon as the sun re-appears.
>>>
>>> FYI, my instruments are correct to within ±½°C, ±4% RH between 0° and
>>> 45°C, ±2 hPa, corrected to sea level, ±1 m/s after computer corrections.
>>>
>>> Anyway, the point I was trying to make but expressed badly is that when
>>> the temp and humidity are both high, the cloud formation is such that
>>> the humidity at ground level drops very rapidly if there is no further
>>> precipitation. Yes, higher humidity peaks are very possible, but they
>>> level out rapidly at high temps. I should have added "sustained"
>>> humidity! :)
>>>
>>> Brian
>>>
>>> Wrobel, Clayton wrote:
>>>> I haven't measured it myself, but the local weather often reports RH
>>>> greater than 70% here in Florida. An observation I have made: After
>>>> one
>>>> of our afternoon showers the temperature rapidly returns to 90+,
>>>> visibility is less than 1 mile doe to haze, and one can see steam
>>>> rising
>>>> from the ground. It Feels, and smells, just like a sauna.
>>>>
>>>> -----Original Message-----
>>>> From: Dehoyos, Ramon [mailto:[log in to unmask]]
>>>> Sent: Thursday, July 06, 2006 9:06 AM
>>>> Subject: Re: Humidity?
>>>>
>>>> Hi Rudy et al:
>>>> Good article about understanding humidity,
>>>> http://www.usatoday.com/weather/resources/askjack/2002-12-01-answers-dp-relhum_x.htm
>>>> ` Ramon
>>>>
>>>>
>>>>
>>>>
>>>> -----Original Message-----
>>>> From: TechNet [mailto:[log in to unmask]] On Behalf Of R Sedlak
>>>> Sent: Thursday, June 01, 2006 3:45 PM
>>>> To: [log in to unmask]
>>>> Subject: [TN] Humidity?
>>>>
>>>> Unclear how much time you have spent near the equator....
>>>>
>>>> But, I suspect not a whole lot....
>>>>
>>>> Humidity far in excess of 100% is common... or at least it feels that
>>>> way.
>>>>
>>>> I have experience periods in excess of 5 years, this in less than a
>>>> month,
>>>> where the temperature sometimes fell to 30 C, but the REPORTED humidity
>>>> was never below 90%....
>>>>
>>>> It makes you understand why suicide bombers may just be seeking some
>>>> relief.
>>>>
>>>> Rudy Sedlak
>>>>
>>>> Brian Ellis <[log in to unmask]> wrote: I have discovered that there is a
>>>> real limit to the conditions of natural temperature and humidity that
>>>> are
>>>> possible. To put it simply, if the temperature and humidity are high at
>>>> ground level, clouds will form at a given altitude, reflecting solar
>>>> radiation. This will result in a lowering of the temperature. I haven't
>>>> yet determined the exact limit conditions, but it would seem that (very
>>>> roughly), you can obtain 100% RH only if the temperature is under about
>>>> 15°C. At 25°C, the max humidity is ~85%. At 35°C, 75%. At 50°C, 55%.
>>>>
>>>> When I have more exact data, it is clear I'll be able to determine a
>>>> limit
>>>> curve, on the right side of which it will be impossible for natural
>>>> conditions to exist. However, we can create conditions in climatic
>>>> chambers which will be far to the right of the curve, and we use such
>>>> conditions in SIR/ECMR testing. Are we exaggerating when we pump up the
>>>> temperature and humidity to excessive levels, to create conditions
>>>> which
>>>> can never occur in real life? Is there even a rationale to do so?
>>>> Remember
>>>> that the inside of an enclosure with active electronics is warmer than
>>>> ambient. The real dangerous conditions are with high humidities, which
>>>> can
>>>> occur only at artificially low temperatures, such as when going into a
>>>> tropical rain forest conditions from an over-air-conditioned room
>>>> (e.g.,
>>>> 20°C to 30°C/80% RH), when condensation may occur for a short time.
>>>>
>>>> Views are welcome, please.
>>>>
>>>> Brian
>>>> --
>>>> http://www.cypenv.org Cyprus environment/energy
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