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. 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 ||