Introduction
Weather data is scientific data and, if you're serious about
wanting to collect weather data of good quality, then your AWS
system and procedures need to conform to the principles of making
good scientific measurements. This isn't necessarily difficult, but
does mean that you shouldn't just accept the readings of a new weather
station at face value.
There are perhaps two guiding principles for collecting good
data: firstly, ensuring that the quality of your readings is as good
as possible, mainly by siting your weather sensors with due care; secondly, by being aware of the limitations of your data (and
everyone else's data come to that).
Sensor siting
Weather data of high precision and accuracy can only be collected
if the sensors for the various weather variables are sited and
installed with care. Mounting the sensors of a new AWS simply at the
most convenient location at your observing site will almost certainly lead to
significant errors in the data. The sensor
locations really do need to be chosen with due scientific
consideration for best results.
Siting sensors is a key topic in its own right, which is covered
in some detail at the
faq
pages for the uk.sci.weather newsgroup - see section 6C. A useful
section on the UK Met Office website also has information on sensor siting for AWS
systems in the form of downloadable PDF files.
Data limitations
All serious commercially available AWS systems have a published
specification which will show an estimated degree of accuracy for
each sensor. (If this information is difficult to find for a
particular make/model or only talks about display
resolution and not accuracy, it's often an indication that
the station is not suitable for anything other than 'gadget' use). It's worth taking a look at this specification table
for your AWS to get an idea of how reliable your data is likely to
be. It's also often possible to increase the accuracy of values of some
parameters by calibrating the sensors - a topic which is covered
elsewhere.
Remember also, that just because you can display or measure a
value of, for example, rainfall to two decimal places, does
not mean that it is accurate to that degree. It should be
possible to set up most weather stations to measure weather
parameters to 5-10% accuracy (or in the case of temperature-related
values to ±1°C
accuracy), but achieving higher accuracy, becomes increasingly difficult for nonprofessional
observers.
Comparing data - an introduction
One tempting way to assess the quality of your observations is to
compare your own AWS data with that from an established station in
your own locality. But this is a process with many pitfalls. All too
often, new AWS owners reach the conclusion that their
instrumentation is faulty because the two sets of observations are
significantly different, when in fact either the differences are
genuine (the two locations having different microclimates) or the
fault lies in the siting of the sensors, not the instrumentation
itself. Again, see the
comparing
data page for more details.
Guidelines for optimal siting of sensors
Temperature
The international standard height for measurements of air
temperature is at 1250mm or about four feet above ground level.
Temperature can vary by a surprisingly large amount with height
above the ground under certain weather conditions, and it is
essential that the 1250mm sensor height is used for comparability
with official figures. Ground level or soil temperatures should be
measured with separate sensors if required. The sensor must also be
protected from both direct sunlight and rainfall if wildly erroneous
temperature values are to be avoided, which is usually achieved by
placing the sensor inside the familiar white louvred housing of the
Stephenson Screen. The sensor
should also be positioned away from any nearby potential sources of
heat such as buildings and brick walls, in a position where free
circulation of air can occur, and over a natural surface - grass is
recommended (other surfaces such as concrete can cause significant
error). As with all sensors, the more accurate your observations are
required to be, the more care must be taken over exact siting of the
sensor.
Humidity
The humidity sensor is usually mounted alongside the temperature
sensor in most commercial AWS systems and does not therefore have
its own independent siting criteria. Where a separate sensor is
used, the same guidelines as for temperature sensor siting are
recommended. NB Humidity is not measurable to high accuracy
(typically ± 3-5%) by standard electronic sensors and may not always
quite reach 100% as a maximum reading because of sensor limitations.
Wind speed and direction
Wind speed and direction are dramatically affected near to ground
level by all physical obstructions. Even in a flat, unobstructed
location, wind speed is markedly reduced close to ground level
simply by the frictional effect of the ground surface. For
these reasons, the official sensor height for recording wind speed
is 10m above ground level in a clear unobstructed location. In
practice, it is often impossible to achieve anything like a suitable exposure of the wind
sensor, unless they live in a very rural location . The recommendation is therefore
simply to place the anemometer as high as possible given the
local circumstances and to accept that the readings will almost
certainly be significantly lower than would be measured by
'official' observations at the same location. The sensor readings
will still be valuable as part of the continuing record of weather
at that site, but will not be directly comparable with official
records of for example maximum wind speed and gusts during a gale.
Often the exposure will be worse from a certain direction and the
obvious advice is to aim for maximum exposure of the anemometer to the
commonest wind directions. Note that height above obstructions
rather than height per se is the criterion. An anemometer placed
just at
roof level on a house will often misread because of wind swirling
around the roof structure. The sensor should be placed as high above
the roof structure as it safely and economically can be (preferably
2-3m above the highest point), to avoid
potentially turbulent air below.
Rainfall
Rain rarely falls vertically, but is usually blown to a greater
or lesser degree in the wind. For consistent and accurate
measurements it's therefore important that the rain gauge is located
in an open area, for example a large lawn, where nearby objects such
as buildings, walls and trees won't deflect the entry of wind-blown
rain into the gauge. Rain shadow effects can be surprisingly large
and the standard recommendation is that the gauge be positioned at a
distance corresponding to two to four times the height of any nearby
obstruction. Where this is difficult to achieve, providing good
exposure to the most common directions of wind/rainfall should be
the priority. Gauges should be positioned in a flat area, away from
any obstructions such as fences which might cause air turbulence and
consequent non-uniform deposition of rain droplets. (NB For Davis
Vantage Pro stations the rain gauge rim will automatically be
at ~1400mm height if the temperature sensor is mounted at its
official height of 1250mm.) Most rain gauges for automatic weather stations are
of the swinging bucket type, which must be installed in an
accurately horizontal plane for correct operation.
Pressure
Since barometric pressure does not vary across a local area at
uniform altitude, pressure is generally measured by a sensor inside
the AWS console and not by an external sensor. Consequently, there
are no major concerns about siting the pressure sensor, other than
to be aware that its accuracy is only specified over a limited
temperature range. Note that pressure reduces by about 1mb for every
additional 32 feet of elevation and it is therefore essential to
know the altitude accurately of the AWS base location. Pressure is
also relatively easy to check and to calibrate. |
