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
> --
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