Valentia Observatory

Radiosonde Measurements
Measurement of upper-air parameters has been,for many years, and still remains, a very important tool for weather forecasting. Some essential uses are:

• As an input to Numerical Weather Prediction models.
• For aviation forecasting of cloud height and depth, wind shear and wind speed and direction at various heights etc.
• For general forecasting of winds, atmospheric stability, and maximum/minimum temperatures and mixing height.
• For atmospheric and ozone research and for dynamic and thermodynamic research.

Valentia Observatory has a long history of upper-air measurements. Its location on the Atlantic seaboard makes it one of the most strategically positioned stations in north-western Europe. Its radiosonde measurement records date back as far as 1943.

Radiosonde Measurements
Upper-air ascents are made from the Observatory twice daily, at 1115 and 2315 GMT. Vaisala RS80 radiosondes with LORAN C windfinding and a DigiCORA ground station are used for all ascents. The radiosonde contains four sensors, two capacitive aneroid barometers for pressure, a capacitive bead for temperature and a Humicap thin film capacitor for humidity. The outputs of these capacitors vary with the changes in the parameters being monitored and are used in conjunction with a fixed inductor. For wind finding the sonde pinpoints its own position, using the LORAN C NAVAID system of radio transmitters, and relays its location back to the ground station. From the changes in these locations the computer can determine the wind speeds and directions aloft.

A radio transmitter sends the data back to the monitoring station. The sonde transmits data for each parameter in rotation. This is achieved by means of an electrical switch, which connects each element to the transmitter in turn.
At the Observatory the data from the sonde are received by a radio receiver in the ground station and are filtered, digitised and treated by the computer software. Processing of the incoming data is performed on a personal computer using METGRAPH, a custom designed program which performs filtering and quality control checks on the data.

As the balloon ascends, the pressure decreases with height. Temperature also decreases with height, with some significant variations, e.g. there is likely to be an inversion off the ground on a clear night or at a higher level with the approach of a warm front etc. This decrease in temperature continues up to a level known as the tropopause from where it more or less levels off. The height of the tropopause changes with the prevailing conditions and is very significant in weather forecasting. Humidity varies with atmospheric conditions and is monitored only to the 200 hPa level.

After burst, the data for each ascent are quality controlled and are transmitted from the station personal computer, via a modem, to the Computer Section in Met Éireann, Headquarters, from where they are distributed nationally to the forecasting offices and internationally through the Global Telecommunications System (GTS) of the World Meteorological Organisation. The data are transmitted using a special meteorological numerical code that is universally understood.

The data file for each ascent contains the following information: height (m), pressure (hPa), Temperature (oC), Humidity (%), time since launch (s), wind speed (m/s) and wind direction (ddd). The computer software allows profiles of temperature, humidity, wind speed and direction to be generated as a function of altitude, atmospheric pressure and time.

The sondes are carried aloft by a balloon made of natural rubber filled with helium. Due to the decrease in atmospheric pressure as the balloon ascends the gas in the balloon expands. Eventually the elasticity of the rubber in the balloon is reached and it bursts. The ascents normally last for about 90 minutes and the balloons can attain heights of over 30,000 metres.

An Automatic Shipboard Aerological Programme (ASAP) type semi-automatic launcher, made by Vaisala, is used to launch radiosonde ascents. This type of launcher is very suitable for a location like that of the Observatory because it is capable of launching in wind speeds above 15 m/s. It can also be rotated so that the balloon will always be launched down wind. As the system is based on simple pneumatics it is extremely reliable and requires very little "downtime" for servicing. Its system of gas measurement, distribution and balloon filling is both simple and safe.

Ozone sonde ascents are launched manually from a custom built filling house.