Stevenson Screen Project ...

Having installed the various components of my digital weather station as per the manufacturers instructions I was informed by weather enthusiasts here in Spain that I would need to shelter the temperature & humidity sensor from direct sunshine and radiation.

“The Stevenson screen is a special shelter that prohibits direct heating by the sun's rays, while permitting air to flow through. Protecting the instruments inside, from the elements outside. It consists of a white wooden box that is fitted onto a steel frame, buried 1,2 m above ground.

· It is painted white as white is a reflector rather than an absorber and doesn't heat up as easily. This is necessary so that the heat from the sun won't affect the instruments inside.

· It has four double louvered sides, so that air can flow through in order to ensure that the air temperature can be established.

· The opening between the roof and the sides allows air to move through it freely.

· The roof is slightly slanted from front to back, allowing rain to run down.

· The screen always faces to the north in the Northern hemisphere. This is to prevent direct sunlight from falling onto the instruments inside and influencing the observations incorrectly when readings are being taken.

· It is situated 1,2 m above ground, which is the Weather Services standard level for "surface" temperature observations. This is where the relevant air temperature is measured from the heat emitted from the earth. If the box is placed on the ground it will measure the temperature of the ground and that is not what it is supposed to measure.”

DIY Solution

Preliminary research showed that purchasing a ready built Stevenson Screens was far too expensive for my pocket so I decided to go the DIY Route. Further research by searching a number of weather web forums and Internet sites provided me with a few ideas as to how to go about constructing a budget priced solution.

Fan Aspirated Radiation Screen (FARS)

A standard Stevenson Screen relies upon convection for airflow through the cabinet and across the instruments. If a fan is introduced then the airflow will be increased and the likelihood of hot spots significantly reduced.

The first step in the project was to purchase a solar powered fan that could be fitted into the FARS. Fortunately these are easily available and a UK source is CPS Solar. The model used comprises the main solar powered fan unit with a plastic filters to fit on the top and bottom of the unit.

Following the path trod by others the main components of the FARS are plastic plant pot trays that are drilled out to provide a large circular central aperture and for tie rod holes. The trays are then painted white to reflect sunlight and minimise heat absorption.

The images above show the filters and tie rods fitted to the top tray.

These images show the assembled units waiting the fan unit which just clips into the top filter panel using a bayonet connection. The arm at the top is to hold the solar panel for the temperature and humidity sensor.

A view of the top of the FARS showing solar powered fan and the temperature/humidity sensor solar panel.

Above, the completed FARS in situ.