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The Stevenson Screen
Thomas Stevenson — a lighthouse engineer, and Robert Louis Stevenson's father — designed this box in 1864 to solve a problem that had made a century of temperature records incomparable: every observer shielded their thermometer differently, so every station was measuring something slightly different.
The screen is a white louvred box on legs. The double louvres let air through while blocking every direct and reflected ray; the white paint reflects what lands on it; the double roof stops the sun heating the ceiling; the open slatted floor stops heat rising off the ground. Each feature removes one specific way of measuring the wrong thing.
It is the least glamorous object in this batch and the one that makes the rest of it trustworthy. A thermometer without a screen is a decoration; the same thermometer inside one produces a number another station can be compared against.
Intermediate
2 days
Instructions
1
1
Build double louvres, sloping down and out
Build double louvres, sloping down and out
Make each wall from two staggered rows of angled slats so that no straight line passes from outside to the thermometer, while air moves through freely.
Single louvres let low sun and ground-reflected light through at certain times of day, giving an error that appears and disappears with the sun and is therefore almost impossible to spot in the data. Slope them down and outward so rain and spray run off rather than in. This is the most work in the build and the reason the screen works.
Materials for this step:
Plywood Sheet2 sheets
Galvanized Nails100 pieces
Wood Glue1 pieceTools needed:
Hand Saw
Claw Hammer
Metal Ruler2
2
Paint it white, and keep repainting it
Paint it white, and keep repainting it
Paint every surface, inside and out, with white exterior paint, and repaint before it yellows or flakes.
White is not tradition, it is the measurement: a weathered or dirty screen absorbs more sunlight, warms, and reads high on sunny calm days. Published comparisons put a poorly maintained screen up to a degree above a clean one. Use ordinary white paint rather than a modern low-emissivity coating, because the screen needs to radiate its heat away at night as readily as it reflects by day.
Materials for this step:
White Exterior Paint2 litersTools needed:
Sandpaper3
3
Double roof, slatted floor, door to the pole
Double roof, slatted floor, door to the pole
Fit two roof boards with an air gap between them, leave the floor as spaced slats with a solid baffle below, and hang the door so it faces the nearer pole — north in the northern hemisphere.
The roof gap stops the sun-heated outer board radiating down onto your thermometers. The slatted floor lets cold air drain out. And the door faces away from the equator so that opening it at your reading time never lets the sun fall directly on the instruments — a real error, committed once a day, at the same time every day.
Materials for this step:
Plywood Sheet1 sheetGalvanized Nails50 piecesTools needed:
Hand Saw
Compass4
4
Set the thermometers at 1.25 metres over short grass
Set the thermometers at 1.25 metres over short grass
Mount the screen so the thermometer bulbs sit between 1.25 and 2 metres above the ground, over natural short grass, well away from buildings, paving and tarmac.
These numbers are the WMO standard and they exist so that stations can be compared. Over tarmac a screen reads up to two degrees high on a sunny afternoon; beside a wall it reads the wall's stored heat all evening. If you cannot meet the standard, record precisely what you did instead — a documented non-standard site is usable, an undocumented one is not.
Materials for this step:
Wooden Dowel4 piecesTools needed:
Measuring TapeClaw Hammer5
5
Prove the screen is doing something
Prove the screen is doing something
For a week, run a second thermometer outside the screen in the open and record both at the same moments.
On a calm sunny afternoon the unscreened one will read several degrees higher; on a clear night it will read lower, because it radiates to the sky. Two rows of numbers you collected yourself are worth more than any assurance in this blueprint, and they tell you exactly how wrong your records were before you built the box.
Materials for this step:
Thermometer1 piece6
6
The errors the screen removes, in numbers
The errors the screen removes, in numbers
Loading Jupyter Notebook...
7
7
Write the site description down once, properly
Write the site description down once, properly
Record the latitude, longitude, altitude, screen height, ground surface, and the distance and height of everything within thirty metres. Photograph it in four directions.
This is called metadata and it is what lets someone interpret your record in twenty years, including you. It also lets you explain a step in your own data: the year the hedge grew, or the neighbour laid a patio. Station history explains more apparent climate signals than climate does.
Tools needed:
Measuring TapeMaterials
6- 2 sheetsPlaceholder
- 150 piecesPlaceholder
- 1 piecePlaceholder
- 2 litersPlaceholder
- 4 piecesPlaceholder
- 1 piecePlaceholder
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