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.



Davis Weather Monitor Thermometer Screen Rain guage Hygrometer unit

The following items are measured at Ferryhill :

Air Temperature (degC)
Rainfall (mm)
Barometric Pressure (mb)
Relative Humidity (%)
From Air Temperature and Relative Humidity we derive Dew Point.

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!

Air Temperature

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 grass.

The highest air temperature recorded locally is 32.5 degC in August 1990. The lowest is -18.3 degC in February 1895.

Rainfall

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!


Barometric Pressure

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 inexact science).

Relative Humidity

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.