Introduction
As with many things in life, they aren't always as simple
as they appear to be. I have outlined below some of the limitations of
the information displayed on this website.
Forecast
The forecast displayed at the top of the Home Page takes
into account only those conditions in the immediate vicinity of the weather
station. The situation would probably be different in another area.
Measuring
snowfall
You hear TV announcers stating things like, ‘4” of snow fell
today’, but how do they know? Measuring snowfall accurately can
be a challenging task as there are many things to take into account, e.g.
wind, slopes, warm buildings etc.
A good deal has been
written about measuring snowfall and there are a number of electronic
devices that claim to measure snowfall accurately but whether they do
I don’t know.
The surface you use
as a ‘table’ must be flat and level and 20-30 feet away from
buildings. Grass is not a good surface to use as snow lays on the top
but the ruler will slide to the ground below.
You have to consider
the frequency of the measurement especially if it is windy otherwise all
your snow could be blown off the ‘table’ before you get a
chance to measure it. If the air temperature is fluctuating the snow may
start to melt and flatten down, over the course of a day it is possible
for the snow to melt a bit or even completely and snow again several times
so if you only take one daily measurement the chances are high you will
understated the amount of snow that has fallen.
If you use a snowboard
(flat board about 16” x 16”) it should be cleaned off after
each measurement. Ideally you would use 2 snowboards, one you clean off
after each measurement to give the amount of snow fallen over a period
of time, the other you would leave to give the current snow depth. Snow
depth is not the accumulation of each new snowfall measurement as it will
take into account melting and wind etc.
It is possible to melt
the snow and then measure the amount of water produced. The rule of thumb:
10 centimetres of snow will produce one centimetre of water. You would,
however, have to be careful that your container didn’t contain any
other water, for example, it may snow for a while and then turn into sleet
or rain which would distort your measurement.
Graphs
The data for graphs are gathered at specific time intervals (sampling
time) and so may not necessarily show extremes, e.g. a windspeed graph
may indicate max. gust of 41 m.p.h. at 05:00 whereas the max. gust may
have been 46 m.p.h. but this was at 05:02 which isn’t at a graph
data gathering point. To display all the collected data would make the
graphs too large. Even the sensors themselves have a sampling time, this
varies depending upon the sensor, e.g. anemometer 2.5-3 secs., thermometer
10-12 secs.
Rainfall measurement
isn’t straight forward either
Is it raining? You look out of the window and you see that it has just
started, you then look on the weather website and it says dry. Why?
Rain will fall into a rain collector to be measured, the measuring is
often done electronically and in my automatic weather station (AWS) it
uses a tipping bucket system, see: Inside
the ISS. Until there has been sufficient rain to fill a bucket to
tipping point there would be no electronic indication of rain. It takes
2 bucket tips within 15 minutes to signal rain so it is possible for it
to rain briefly and then stop without ever tipping the buckets even though
the ground may be wet the AWS is still reporting no rain.
When there is an indication
of rain there comes a point when it stops but the weather station could
still be reporting rain. Once the rain has stopped the water is still
trickling through so ‘tipping’ events can still occur thus
registering ‘rain’.
Snow also has a side-effect on 'rainfall'. Snow will accumulate in the
rain collector. If it is a heated rain collector (which mine isn't) the
snow will begin to melt, this might be at a rate whereby it causes 2 tipping
events within 15 minutes thus indicating rain. In an unheated rain collector,
as the temperature rises and the snow inside begins to melt it could cause
an indication of rain over a period of several hours when in fact there
is none.
Windspeed - erroneous value
Very occasionally, for reasons unknown, a data spike may cause a windspeed
reading to be shown as more than 79 m.p.h. This usually shows in the records
as around 79.4 and should be ignored.
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