From: Ronald Wong (ronwong@inreach.com)
Date: Wed Jan 26 2000 - 10:16:43 PST
Date: Wed, 26 Jan 2000 10:16:43 -0800 (PST) Message-Id: <l03102800b4b4785c7ed5@[209.209.17.70]> From: Ronald Wong <ronwong@inreach.com> Subject: lzielin's Two Questions
>Hi All... two questions
>
>If there a quick, in-the-head type of way for changing Celcius
>temperatures to
>Ferinheit ones FOR NEGATIVE VALUES? I have one for positive values....
>take the Celcius value multiply by 2 subtract 10% and add 32.... how about
>NEGATIVE Values?
>
>Is ther an quick,in-the-head tyupe of way to convert temperature and dewpoint
>to RELATIVE HUMIDITY?
>
>Lynne
Lynne:
A: Regarding the conversion of Celsius readings to Fahrenheit readings.
Your in-the-head way of doing it still applies. Just remember that you
are dealing with NEGATIVE numbers.
Using -40 degrees Celsius for instance:
2*C = -80
10% of -80 = -8
2*C - 10%*(2C) = -80 - (-8) = -80 + 8 = -72
thus 2*C - 10%(2C) + 32 = -40 degrees Fahrenheit.
Why this trick works is a good question to ask of your students if
they have had a course in algebra.
B: Regarding a quick way to determine the relative humidity:
I don't know of any.
Relative Humidity depends on the ratio of the partial pressure and the
saturated vapor pressure of water.
You can get the dew point temperature experimentally by cooling a polished
metal surface down until moisture starts to condense on the metal. Knowing
this temperature, you can determine the partial pressure. Knowing the
ambient temperature of the air, you can determine the saturated vapor
pressure. 100 times their ratio is the relative humidity.
How you determine these two pressures is another story.
There is no simple, linear relationship between the temperature and the two
pressures under consideration here. The relationship between temperature
and either the saturated vapor pressure or the partial pressure is usually
determined by means of a table - you might find one in your science
textbook. So I don't think there is a quick, in-the-head way to get the
relative humidity knowing just the temperature and dew point.
The quick and dirty way to get the relative humidity is to take a
thermometer and firmly tie a piece of cloth that has been soaked in water
around it's bulb. Tie a string to the loop on the other end of the
thermometer (I forgot to mention that you need a thermometer with this
loop) and twirl it around in the air. Comparing what is usually it's lower
temperature (due to evaporation) to that of a regular thermometer allows
you to determine the relative humidity.
You'll need a table to do this, but it can usually be found in a physical
science textbook. Barring that, check out the Handbook of Chemistry and
Physics.
What you have is the wet-bulb/dry-bulb method of determining the relative
humidity. The technical name for the twirling wet-bulb thermometer is
"sling psychrometer" and you can buy one for a princely sum. The official
version comes with both thermometers mounted on a sturdy frame and you
supply the water and the twirling power.
Enjoy - ron
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