Why Stevenson Screens are Painted White: Reasons Behind the Design
3 minute read
If you left an ordinary thermometer in a sunny spot, it would tell you the day is several degrees warmer than it really is. Weather observers spotted this problem generations ago and settled on the best solution, the Stevenson Screen.
Stevenson screens, seen on village greens, school fields, and well-kept gardens across the globe look simple but they serve a larger purpose of making sure you can count on them to give an honest reading on a hot sunny day and during the frosty winter months. If you would like the history first, our blog on the origin, purpose and usage of the Stevenson Screen is a good place to start. Metcheck has supplied meteorological instruments since 1980, so this is a subject we know very well.
The case for white paint
White reflects most of the sunlight that lands on it. During the day this keeps the instrument screen from overheating, so its temperature settles close to the air around it which is very important when you're after an accurate minimum reading overnight or true reading on a hot afternoon. If the instrument shelter were to be a dark colour, it would hold onto warmth long after the sun has gone, masking the frost you're trying to catch.
The condition of the paint matters too. A finish that has yellowed, flaked, or turned green with algae absorbs more heat than a clean white one, so a fresh coat is worth giving before the colder nights set in.
Why is a Stevenson Screen made out of wood?
Wood conducts heat slowly. A metal box would cool rapidly once the sun went down and pass the chill straight on to the instruments and it would also heat up quickly in the summer sun. Wooden walls respond more gradually in both directions, keeping the air inside closer to the true air temperature.
Louvres, roof and legs
The sides of a Stevenson Screen are made of overlapping slats at an angle, so air passes through freely while direct sun and rain stay out. The roof usually has two layers with a gap of air between them, and the floor is slatted rather than solid, which limits heat gain from above and below on a summer's day and stops the box from trapping cold air on a frosty one.
The legs lift the screen so that thermometers and hygrometers sit at a consistent height; around 1.25 metres is the most common figure. Short grass underneath is the traditional setting, since grass holds frost readily and cools quickly after dark. In the UK, the door faces north, so the sun never falls on the instrument while you take a reading.
What is kept inside a Stevenson Screen?
The two main instruments that are most often found inside are a maximum thermometer, and a minimum thermometer, which, as the name suggests, record the warmest and coolest points of the day making it useful to track the first frost of the season for example or a summer heatwave. Many weather observers like to add a wet and dry bulb hygrometer for humidity. Instrument shelter is the more widely known term and the Stevenson Screen is the best-known type.
Are Stevenson Screens still used?
Yes! Observing sites, schools, and home enthusiasts rely on them year-round, because readings taken the same way over many years can be compared fairly, in any season.
Metcheck stocks a range of instrument shelters alongside thermometers and hygrometers for professional and home weather watchers. If you'd like any help choosing which instrument is right for you, our team are happy to advise by phone, or through our "ask the expert" tab in any product listing.