Strangely enough, I've spent a lot of time near the equator. Let me
recount what may be considered a typical day. The sun comes up and the
temperature reaches, say 30°C-32°C by noon and the humidity will reach,
say, an uncomfortable 85%. Convection currents from the air warmed by
the hot ground (albedo ~20%) will form taken the transpired humidity
from the vegetation to cooler regions, where saturation occurs and
clouds form. These block the sun and the temp drops to ~29°C but because
the absolute humidity is constant, the relative humidity rises as the
temp drops, and the comfort level drops. Most days, this will develop
even further and you get the mother and father of a thunderstorm, taking
the temp down even more. In the intertropical convergence zone, you
never get high temperatures, like you do here or in desertic regions
(>40°C), when the RH is always low (<25%).
Also, hygrographs/meters are notoriously inaccurate at high (>80%) and
low (<25%) relative humidities, especially at temperatures outside the
range 15°C-25°C. The ONLY way of accurately measuring humidity is to
determine the absolute humidity in a laboratory and relate that to the
temperature. Wet/dry bulbs are no better, as the results of that change
with factors like airspeed.
And, of course, if the RH is higher than 100%, you will need a lifebuoy
to survive :-)
I repeat that the figures I quoted are approximate. I'm trying to
research more accurate ones, but real data is not evident. There is a
lot of contradictory data, even.
This came about as I was interested in the Heat Index and the parameters
involved, but this, too, is also full of contradictory data. The best
data I've found to date are in Steadman's three classical papers in J.
Appl. Meteorology, but these apply to what he terms "sultriness", rather
than to giving the limitative data. He admits that the limits are not
sharply defined and there are other factors involved, including
barometric pressure (which we ignore in our tests; is a conformal
coating or a solder mask polymerised at 950 hPa the same as if cured at
1050 hPa - I personally doubt it.)
I believe we still have a lot to learn.
Brian
R Sedlak wrote:
> 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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