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Subject:
From:
Brian Ellis <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Brian Ellis <[log in to unmask]>
Date:
Mon, 10 Jul 2006 18:44:11 +0300
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text/plain (291 lines)
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
>> http://www.cypenv.org/worldenv World environment/energy
>> http://www.cypenv.org/weather Cyprus weather
>> http://www.cypenv.org/smf/index.php Environment/energy forums
>> http://groups.yahoo.com/group/cypnature/ Cyprus nature forum
>>
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> 
> --
> http://www.cypenv.org Cyprus environment/energy
> http://www.cypenv.org/worldenv World environment/energy
> http://www.cypenv.org/weather Cyprus weather
> http://www.cypenv.org/smf/index.php Environment/energy forums
> http://groups.yahoo.com/group/cypnature/ Cyprus nature forum
> 
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-- 
http://www.cypenv.org Cyprus environment/energy
http://www.cypenv.org/worldenv World environment/energy
http://www.cypenv.org/weather Cyprus weather
http://www.cypenv.org/smf/index.php Environment/energy forums
http://groups.yahoo.com/group/cypnature/ Cyprus nature forum

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