WHERE TO LOCATE AN OUTSIDE THERMOMETER
Jesse Ferrell, CASI President
Many, many factors affect the accuracy of one's instrumentation. I'll
followup later with advice on the location of various weather
instruments. Today I want to concentrate on one very important one -
the thermometer.
The most likely instrument reading to become skewed is temperature
because of sunlight. What kind of shelter you have your thermometer in
depends on what type of instrument it is. Most electronic weather
stations come with either a probe (which you could house in the shelter
discussed below) or a probe encased in a sun shield (like the Davis
radiation shield, picture at
http://www.accuweather.com/web/nf/store/radiation.htm)
The radiation shield should protect the instrumentation from the sun,
and since many electronic weather stations are meant to be mounted onto
a pole other shelters don't come into play.
However, if you have a mercurial thermometer (or a thermograph) for
additional accuracy which you would like to keep outside you'll need
some protection.
Probably the ideal place to keep your instruments, in which
Universities do, and some NWS's, is in a Stevenson Screen. This is
essentially a box with slats which allow air, but not sunlight in.
They have a slatted floor to allow air in but to keep ground radiation
out.
Unfortunately Stevenson Screens are very expensive, costing hundreds of
dollars a piece. If you don't have the money you can build a similar
box for under $30 with parts from your local Hardware store. I did
this, using small wooden window shutters as the louvers for the sides
(which I think might actually be too tight). Unfortunately I never got
legs on it so the best I could do was keep it on the porch, which is
commonly unaccurate due to opening of doors and windows and heat
radiating away from the house. You can see my semi-Stevenson Screen at
the following URL: http://www.weatherwatchers.org/wxstation/setup.html
Whatever you build make sure that:
1.) It is wood -- plastic or metal boxes absorb too much heat.
2.) It is painted entirely white to refect the maximum amount of
sunlight.
3.) It never allows the sun to reach the instrument, even weak
sunlight.
4.) It allows for good air circulation.
5.) It has some type of floor to avoid ground radiation.
6.) It is approximately eye-level. This is standard for surface
temperature observations.
Below is some additional information on Stevenson Screens.
Consequently one of the URL's below from which I quote
(http://www.vision.net.au/~daly/surftemp.htm) is an excellent diatribe
on surface temperature observation and its relation to possible
worldwide temperature increase - a must-read for weather observers.
Stevenson Screen Info:
>From http://australiansevereweather.simplenet.com/jimmyd/simple/
airtemperature.htm
"Meteorologists place thermometers in Stevenson's screens that consist
of louvers allowing air but minimal sunlight to flow. In other words,
air temperature is that measured in the shade."
>From http://www.agric.gov.ab.ca/agdex/000/7100001d.html
"Thermometers to measure maximum and minimum air temperature and
sensors to determine relative humidity and dew point temperature are
placed 1.2 metres above the ground in a louvered shelter called a
Stevenson screen. The shelter protects the thermometers from direct
sunlight and precipitation."
>From http://www.vision.net.au/~daly/surftemp.htm
"MEASURING SURFACE TEMPERATURE"
"The [World Climatological] surface data comprises the combined average
of tens of thousands of thermometers world-wide in every country,
recording temperatures in standard white louvered boxes called
Stevenson Screens, usually mounted one metre above ground.
Any site needs to be properly maintained if it is to give a
continuously accurate record. This means keeping the box clean and
white, keeping the louvers clear, keeping the instruments clean, and
avoiding changes to the local micro-environment in the immediate
vicinity of the box itself.
Consider cleanliness as an example. The boxes are painted white for a
good reason, which is to reflect sunlight so that the box itself does
not heat up and give false readings. This means that the box needs an
occasional paint job and to be regularly cleaned to maintain the proper
degree of whiteness. If this is not done, the box gets progressively
dirtier over time. A dirtier box is a warmer box. Lack of simple
cleaning and painting will cause the measured temperatures to show a
warming creep over time, a creep which might get some impressionable
scientist a thousand miles away, looking only at the numbers recorded
from such a box, to get all excited about a climatic warming at the
site.
Then there’s the louvers. These allow the instruments inside the box to
be properly ventilated while being shielded from direct radiation from
outside. But such louvers are traps for blown dust and dirt, while
spiders find them ideal for cobwebs. The louvers should be cleared
regularly to maintain proper air flow to the instruments inside. If this
is not done, the box will lose ventilation efficiency and the interior
will get warmer over time. Again we have a box-induced warming creep,
giving a distant scientist the impression of climatic warming."
--
==============================================================
Jesse Ferrell - Meteorologist - j{at}weatherwatchers.org
Central Atlantic Storm Investigators - www.weatherwatchers.org
"A tornado? Why wasn't it dissipated by the weather
modification net?" --Jean-Luc Picard || ICQ No. 136862 ||