The weather station at Ferryhill is an automatic one (AWS). This ensures that readings can be taken every 30 minutes. The station is connected to a PC and the readings are downloaded at suitable intervals. The monthly reports are then run from this data. The actual electronic box is made by an American company called Davis Instruments. It's called The Weathermonitor II and is just about the most popular of this company's models, used all over the world, in some pretty inhospitable places, too. The company also make a more expensive model called the Vantage Pro, which comes with wireless technology, so there's no cabling to be done.
The following items are measured at Ferryhill :
Air Temperature (degC)
Barometric Pressure (mb)
Relative Humidity (%)
From Air Temperature and Relative Humidity we derive Dew
There is provision to fit a wind vane to the system, but i don't
like heights and haven't persuaded anyone to climb onto the roof
for me, so the wind readings remain blank for the moment!
The air temperature must be measured at a fixed height above the
ground and must be measured in the shade. When you hear that the
'temperature on court at Wimbledon is 40 degrees Centigrade', it is
a reading taken in the sun and as such it is meaningless. The
thermometer must be contained in a well ventilated box, known as a
screen, and situated approximately 4 feet from ground level. This
is very important as it means that different sites can then be
compared with each other.
When the air temperature falls below freezing, we are said to have
experienced an 'air frost'. When the air at ground level falls
below freezing point, we have a 'ground frost'. Because the air
closer to the ground is colder, we can experience a ground frost
without having an air frost. If the air is at 100% relative
humidity then this will show up as a white deposit on the
The highest air temperature recorded locally is 32.5 degC in August
1990. The lowest is -18.3 degC in February 1895.
Rainfall is measured using a rain gauge. There are various
different types, but I use a 'tipping bucket' variety, This counts
the number of tips and then multiplies this by the volume of the
bucket to give a total value. Rainfall amounts are these days
measured in millimeters or inches (the USA still use inches). It is
quite rare for a day to have in excess of 25.4 mm (1.00 inches) of
rain (well, in Ferryhill anyway). Rainfall for the North East of
England is on average around 26 inches per year. The driest months
are in spring and the wettest in autumn. However, a thunderstorm in
summer can totally distort the figures. For example in July 1998,
Ferryhill had over 100 mm of rain in the month, but 75% of this
fell in a 3 hour period in a torrential storm!
The Barometric Pressure is a measure of how much pressure the
atmosphere is exerting. Like rain gauges, there are also different
types of Barometer. Most homes will have one and it will be an
Aneroid variety. This consists of a corrugated box which has a
vacuum inside. As the air pressure changes, the box is slightly
deformed and this movement is transmitted via a selection of levers
to a pointer on a dial. The weathering station measures pressure
electonically. The results are essentially the same, but the
electronic way is much more sensitive. Barometric Pressure usually
varies between 960mb (in a depression) to 1040mb in an anticyclone.
Joining together points on the map with equal pressure allows us to
draw weather maps and predict the weather (in theory, it's a very
The relative humidity is the way in which the amount of moisture in
the air is represented. It is measured as a percentage and is
compared to the maximum the air can possibly hold for any given
temperature. The higher the air temperature, the more moisture can
be held, so a high temperature and a high humidity can be very
uncomfortable in summer. Rainfall in summer tends to be heavier,
because the air can hold more water. We can use changes in Relative
Humidity to detect changes of air mass and weather fronts. The
Weather station measures Relative Humidity electronically and is
very sensitive. Relative Humidity very rarely falls below 20% and
is usually up around 100% in fog.
The Dew Point is the temperature to which the air would need to be
cooled to moisture to condense out. This will show as dew on the
grass in summer, but may be 'hoar frost' in winter.