Storm Station 247

The Field Book/Vol. VII, The Instruments/FB-INS-009

Stevenson screen and siting

A thermometer in the open measures the sun and the ground as much as the air. A screen shades it, lets the air through, and holds it at a fixed height over grass, and the ground around it is classed by how far it stands from anything that makes or reflects heat. That is what makes a temperature from one station comparable with another.

Plate FB-INS-009The stationRevision 1, 2026-09-25Status draftAlso called thermometer screen, instrument shelter, cotton region shelter, radiation shield, MMTS, beehive shield, aspirated shield, siting class, screen height

A painted elevation of three white temperature shields standing on short grass under a blue sky, heights marked from the ground to 8 feet: a louvred wooden screen on four legs with a double roof and double slatted walls, its door facing north, and inside it a maximum and a minimum thermometer at 5 feet; a round stack of white plates on a post, the plate shield the cooperative stations use, its sensor at 5 feet (1.5 metres); and a white tube with a fan on top drawing air up past its sensor, the fan aspirated shield of the airport stations; with dashed lines at 4.1 and 6.6 feet (1.25 and 2 metres), the band every one of them holds its sensor in.
Fig. A Three ways to shield a thermometer. Shade it, let the air through, and hold it at a fixed height over grass.MaximizeThe drawing, to scale

What it is

A thermometer left in the open does not measure the air. In sunshine it absorbs light and reads warm; at night it gives off heat to the clear sky and reads cold; near a wall or a road it reads the wall and the road. The screen is the answer: a white, louvred box that shades the thermometer, lets the air flow through it, and holds it at a fixed height over grass. Thomas Stevenson designed the double louvred version in the 1860s, and it still carries his name.

The United States has used three kinds. The louvred wooden screen, known in the United States as the cotton region shelter, held the cooperative observers' thermometers for a century. Since the 1980s most cooperative stations use the MMTS, a round stack of white plates around an electronic sensor. The airport stations use a fan aspirated shield, a tube that draws air steadily past the sensor. Fig. A draws all three.

What it is not

It is not the whole of a good temperature. A perfect screen in a parking lot still reads the parking lot. Where the screen stands matters as much as the screen, and the siting classes below are how that is judged.

It is not a thermometer on a porch or a car. Those read their own surroundings, and they are not comparable with a screened reading or with each other.

Lookalikes

A thermometer on a porch or a car
It measures its own surroundings: the wall, the sun on it, the engine. It is not a screened temperature, and it is not comparable with one.
The grass minimum
A thermometer laid just above the grass on a clear night, read for frost. It reads colder than the screen on purpose.
A road surface temperature
Road weather stations measure the pavement itself for ice. It is a different quantity from the air above it, What an instrument is FB-INS-001.

The machine

Shade and air

A screen does two jobs that pull against each other. It must block sunlight and the ground's heat, which argues for solid walls, and let the outside air flow freely over the sensor, which argues for no walls at all. The louvred screen does both with slatted walls set at an angle, doubled so that no straight line of light reaches inside, and a double roof with an air gap under the top. It is painted white and the door faces north, so that opening it in the Northern Hemisphere lets no sunlight in.

The weakness of a screen that relies on the wind is a still, sunny day. With little wind, the air inside warms more than the air outside, and the reading runs high. A fan fixes that: the aspirated shield keeps the same flow over the sensor in any wind, which is why the airport stations use one.

The height

Air changes quickly with height close to the ground, and most quickly in the first few feet. On a clear, calm night the grass cools by giving off heat to the sky, and the air at the grass can be 4 to 9 °F (2 to 5 °C) colder than the air at head height. On a sunny afternoon the order reverses, and the air just above the grass is the hottest. Fig. C draws both. Between 4.1 and 6.6 ft (1.25 and 2 m), where the World Meteorological Organization places the sensor, the two curves are steadiest, so a small difference in the height of two stations makes little difference to what they read. The cooperative program fixes it at 5 ft (1.5 m), give or take a foot.

The siting classes

The ground around the screen is judged by distance. For temperature, a class 1 site has no pavement, building, water or other heat source within 330 ft (100 m). Class 2 allows them beyond 100 ft (30 m), class 3 beyond 33 ft (10 m). Class 4 has them closer, and class 5 is worse still. The World Meteorological Organization gives the uncertainty the siting may add: up to 1.8 °F (1 °C) at class 3, 3.6 °F (2 °C) at class 4, and 9 °F (5 °C) at class 5. Fig. B draws the rings to scale. The class describes the surroundings, not the instrument, and a good sensor in a class 5 site is still a class 5 reading.

A painted plan, drawn to scale, of a temperature screen standing in grass with rings around it at 33, 100 and 330 feet (10, 30 and 100 metres): a paved parking lot whose nearest corner is 66 feet (20 metres) away, a building 200 feet (61 metres) to the north, and a pond 400 feet (122 metres) to the south east, outside every ring; beside it the rule that the nearest heat source sets the class, class 1 beyond 330 feet, class 2 beyond 100 feet, class 3 beyond 33 feet, class 4 or 5 closer; the finding that this site is class 3 because of the lot; and the uncertainty siting may add, up to 1.8, 3.6 and 9 degrees Fahrenheit (1, 2 and 5 degrees Celsius) at classes 3, 4 and 5.
Fig. B The siting classes. The nearest heat source to the screen sets the class of its temperature.MaximizeThe drawing, to scale
A painted chart of temperature against height in the lowest 10 feet (3 metres) over grass, split into a dark night half and a warm day half: a pale curve for a typical clear, calm night, coldest at the grass at 34 degrees Fahrenheit and 41 degrees Fahrenheit (5 degrees Celsius) at 5 feet; a red curve for a typical sunny afternoon, hottest at the grass at 95 degrees Fahrenheit and 86 degrees Fahrenheit (30 degrees Celsius) at 5 feet; frost on the grass under the night half; and dashed lines at 4.1 and 6.6 feet (1.25 and 2 metres), the band where both curves have steadied.
Fig. C Why the height is fixed. The lowest feet of air on a clear, calm night and a sunny afternoon.MaximizeThe drawing, to scale

The drawings

Each figure drawn as an engineering sheet, its parts numbered, to print at 11 by 17.

Elevation of three temperature shields over short grass, heights to scale: a louvred wooden screen on its stand, cut away to show the thermometers inside, with double louvred walls, a double roof and its door facing north; a round plate shield of stacked white plates on a post, the cooperative observer's MMTS, its sensor at 5 feet (1.5 metres); and a fan aspirated shield, a white tube drawing air past the sensor, as the airport stations use; with the band from 4.1 to 6.6 feet (1.25 to 2 metres) in which every one of them holds its sensor.1122334455667788AABBCCDDEESTORM STATION 247THE FIELD BOOKPLATE FB-INS-009-AELEVATION OF A LOUVRED SCREEN, A PLATE SHIELD AND A FAN ASPIRATED SHIELD, EACH AT THE HEIGHT ITS NETWORK USES, OVER SHORT GRASSFIG. A THREE WAYS TO SHIELD A THERMOMETER0 ft1 ft2 ft(0.6 m)3 ft4 ft(1.2 m)5 ft6 ft(1.8 m)7 ft8 ft(2.4 m)6.6 ft (2 m)4.1 ft (1.25 m)MAXMINLOUVRED SCREENcotton region shelterDOOR FACES NORTH5 ft (1.5 m)PLATE SHIELDthe MMTS, cooperative stationsFANFAN ASPIRATED SHIELDairport stationsKEYThe band, 4.1 to 6.6 ft (1.25 to 2 m)1234567TITLEStevenson screen and siting, three ways to shield a thermometerVOL. VII THE INSTRUMENTS · THE STATIONTYPEELEVATIONSCALEHEIGHTS TO SCALE, INSTRUMENTS ENLARGEDREVREV 1 DRAFT SHEET 1 of 3DATE2026-09-25IDFB-INS-009-ADRAWN AS linework on paperSOURCES WMO, NWS
Fig. A, the drawing Three ways to shield a thermometer. Elevation of a louvred screen, a plate shield and a fan aspirated shield, each at the height its network uses, over short grass HEIGHTS TO SCALE, INSTRUMENTS ENLARGEDMaximizeThe sheet, SVG, 11 by 17The painting
  1. The louvred screen, cut away
  2. Double louvred walls
  3. The thermometers inside
  4. The door, facing north
  5. The plate shield (MMTS)
  6. The fan aspirated shield
  7. The band from 4.1 to 6.6 ft (1.25 to 2 m)
Plan of the ground around a temperature screen, drawn to scale: rings at 33 feet (10 metres), 100 feet (30 metres) and 330 feet (100 metres) from the screen; a parking lot whose nearest edge is 66 feet (20 metres) away, which makes the site class 3; a building 200 feet (61 metres) away, which alone would allow class 2; and a pond 400 feet (122 metres) away, outside every ring; with the class each ring sets and the uncertainty siting may add at classes 3, 4 and 5.1122334455667788AABBCCDDEESTORM STATION 247THE FIELD BOOKPLATE FB-INS-009-BPLAN OF THE GROUND AROUND A SCREEN, WITH THE DISTANCES THAT SET ITS SITING CLASS FOR TEMPERATUREFIG. B THE SITING CLASSES, IN PLAN33 ft (10 m)100 ft (30 m)330 ft (100 m)PARKING LOT66 ft (20 m)BUILDING200 ft (61 m)POND400 ft (122 m)THE NEAREST HEAT SOURCE SETS THE CLASSbeyond 330 ft (100 m): class 1beyond 100 ft (30 m): class 2beyond 33 ft (10 m): class 3closer: class 4 or 5HERE: THE LOT, 66 ft (20 m)THIS SITE IS CLASS 3Siting may add up to1.8 °F (1 °C) at class 33.6 °F (2 °C) at class 49 °F (5 °C) at class 50200 ft (61 m)N1234567TITLEStevenson screen and siting, the siting classesVOL. VII THE INSTRUMENTS · THE STATIONTYPEPLANSCALETO SCALE, AS THE BARREVREV 1 DRAFT SHEET 2 of 3DATE2026-09-25IDFB-INS-009-BDRAWN AS linework on paperSOURCES WMO, NWS
Fig. B, the drawing The siting classes. Plan of the ground around a screen, with the distances that set its siting class for temperature TO SCALE, AS THE BARMaximizeThe sheet, SVG, 11 by 17The painting
  1. The screen
  2. 33 ft (10 m), the class 3 ring
  3. 100 ft (30 m), the class 2 ring
  4. 330 ft (100 m), the class 1 ring
  5. A parking lot 66 ft (20 m) away
  6. A building 200 ft (61 m) away
  7. A pond 400 ft (122 m) away
Sounding of the lowest 10 feet (3 metres) of air over grass on a typical clear, calm night and a typical sunny afternoon, height and temperature to scale: at night the air is coldest at the grass, about 34 degrees Fahrenheit (1 degree Celsius), and warmer upward, 41 degrees (5 degrees) at 5 feet (1.5 metres); in the afternoon it is hottest at the grass, about 95 degrees (35 degrees), and cooler upward, 86 degrees (30 degrees) at 5 feet; the band from 4.1 to 6.6 feet (1.25 to 2 metres) where both curves are steadiest.1122334455667788AABBCCDDEESTORM STATION 247THE FIELD BOOKPLATE FB-INS-009-CTEMPERATURE AGAINST HEIGHT IN THE LOWEST 10 FT (3 M), ON A CLEAR CALM NIGHT AND A SUNNY AFTERNOON, OVER GRASSFIG. C WHY THE HEIGHT IS FIXED0 ft2 ft(0.6 m)4 ft(1.2 m)6 ft(1.8 m)8 ft(2.4 m)10 ft(3 m)30 °F(-1 °C)40 °F(4 °C)50 °F(10 °C)60 °F(16 °C)70 °F(21 °C)80 °F(27 °C)90 °F(32 °C)100 °F(38 °C)CLEAR, CALM NIGHTSUNNY AFTERNOON41 °F (5 °C) AT 5 ft86 °F (30 °C) AT 5 ft34 °F95 °F6.6 ft (2 m)4.1 ft (1.25 m)123456TITLEStevenson screen and siting, why the height is fixedVOL. VII THE INSTRUMENTS · THE STATIONTYPESOUNDINGSCALEHEIGHT TO SCALE, TEMPERATURE TO SCALEREVREV 1 DRAFT SHEET 3 of 3DATE2026-09-25IDFB-INS-009-CDRAWN AS linework on paperSOURCES WMO, NWS
Fig. C, the drawing Why the height is fixed. Temperature against height in the lowest 10 ft (3 m), on a clear calm night and a sunny afternoon, over grass HEIGHT TO SCALE, TEMPERATURE TO SCALEMaximizeThe sheet, SVG, 11 by 17The painting
  1. The night curve
  2. The afternoon curve
  3. The band from 4.1 to 6.6 ft (1.25 to 2 m)
  4. The screen's reading at night, 41 °F (5 °C)
  5. The screen's reading in the afternoon, 86 °F (30 °C)
  6. The grass

Ingredients

  • Shade: a white housing that keeps sunlight and the ground's heat off the sensor
  • Ventilation: louvres, or a fan, that keep the air inside the same as the air outside
  • A fixed height over a natural surface, so every station samples the same layer of air
  • Distance from anything that makes or holds heat: pavement, buildings, water and machinery

Scales

time
a minute for an electronic sensor to follow the air; the day's highest and lowest, for a cooperative observer's thermometers
horizontal
the ground within 330 ft (100 m) of the screen, which its siting class describes
vertical
4.1 to 6.6 ft (1.25 to 2 m), the band a screen holds the sensor in
orlanski
micro

Signatures

surface
a daily high that rises with the paving around a station; not with the weather; a night low that is warmer than the stations around it; a jump in the record when a shield was changed or a station moved

The numbers

QuantityValue, and the kind of number it is
Sensor height4.1 to 6.6 ft (1.25 to 2 m) above the ground, over short grass or the natural surfaceStandard, Guide to Instruments and Methods of Observation (WMO-No. 8)
Cooperative station height5 ft (1.5 m) above the ground, give or take 1 ft (0.3 m)Standard, Directive 10-1315
Airport station heightAbout 5 ft (1.5 m), in a fan aspirated shieldTypical, Automated Surface Observing System (ASOS) User's Guide
Screen doorFacing away from the sun at the time of reading: north in the Northern HemisphereStandard, Guide to Instruments and Methods of Observation (WMO-No. 8)
Class 1 sitingNo artificial heat source, reflecting surface or water within 330 ft (100 m); flat, open ground under low natural vegetationStandard, Guide to Instruments and Methods of Observation (WMO-No. 8)
Class 2 sitingNone within 100 ft (30 m)Standard, Guide to Instruments and Methods of Observation (WMO-No. 8)
Class 3 sitingNone within 33 ft (10 m); siting may add up to 1.8 °F (1 °C) to the reading's uncertaintyStandard, Guide to Instruments and Methods of Observation (WMO-No. 8)
Class 4 sitingHeat sources within 33 ft (10 m), covering less than half the ground there; up to 3.6 °F (2 °C) addedStandard, Guide to Instruments and Methods of Observation (WMO-No. 8)
Class 5 sitingWorse than class 4; up to 9 °F (5 °C) addedStandard, Guide to Instruments and Methods of Observation (WMO-No. 8)
Grass and screen at nightOn a clear, calm night the grass can be 4 to 9 °F (2 to 5 °C) colder than the air at screen heightTypical, Wallace 2006

How the station sees it

Every temperature on this site came from a sensor in some kind of shield, and the record shows when the shield or the surroundings changed. A station whose daily highs creep up while its neighbours' do not has often gained a parking lot, not a warmer climate. A jump on the day an MMTS replaced a louvred screen is the change of shield, not the weather. The instrument pages keep each station's history so that such a step can be seen for what it is.

How it is warned

The screen is not warned; its readings are what heat and cold warnings are judged against. A heat advisory or a freeze warning names temperatures measured the way this plate draws: in a shield, at screen height, over open ground.

See also

Sources

  1. World Meteorological Organization. Guide to Instruments and Methods of Observation (WMO-No. 8).
  2. National Weather Service. Directive 10-1315, Cooperative Station Observations.
  3. National Weather Service. Automated Surface Observing System (ASOS) User's Guide.
  4. Office of the Federal Coordinator for Meteorology. Federal Meteorological Handbook No. 1, Surface Weather Observations and Reports.
  5. Wallace, J. M. and P. V. Hobbs. Atmospheric Science, An Introductory Survey, 2nd ed. (2006).

Definition after the Guide to Instruments and Methods of Observation (WMO-No. 8). Plate FB-INS-009, revision 1, 2026-09-25. The number is permanent; cite it.