A Member asked me today what a good place to keep your weather
instruments is,
and I did a little research on the Web to confirm some ideas that I had
and
wanted to share with all the Members. This will serve as an educational
module
that I had hoped to create for sometime for the website. I welcome any
comments
to add to this document to increase its educational value.
Many, many factors affect the accuracy of one's instrumentation. 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 shelter
doesn'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 obs.
Below is some additional information on Stevenson Screens. Consequently
one of the
URL's below from which I quote (From
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 ||