Storm Station 247

The Field Book/Vol. VII, The Instruments/FB-INS-001

What an instrument is

An instrument is a sensor, set in a place, read on a clock and kept in a record. Where it stands decides most of its error: wind at 33 ft (10 m) over open ground, temperature between 4.1 and 6.6 ft (1.25 and 2 m) over grass in a ventilated shield, a gauge clear of anything that shelters it. Those rules are what make one station comparable with another.

Plate FB-INS-001The stationRevision 1, 2026-09-24Status draftAlso called weather station, surface observation, exposure, siting, siting class, calibration, representativeness, anemometer, radiation shield, rain gauge

A painted surface weather station over short grass in open country: a mast 33 feet (10 metres) tall carrying a cup anemometer and a wind vane on a crossarm at its top; a white louvred radiation shield holding the temperature and humidity sensor at 5 feet (1.5 metres) on the mast; a precipitation gauge with a level opening on its own stand; and a grey equipment enclosure at the ground holding the logger, the barometer and the transmitter.
Fig. A A surface station taken apart. The mast and its sensors over short grass: wind at 33 ft (10 m), temperature and humidity at 5 ft (1.5 m).MaximizeThe drawing, to scale

What it is

An instrument is a sensor, set in a place, read on a clock and kept in a record. Remove any one of the four and its number cannot be trusted. A thermometer in the sun reads the sun. A gauge whose clock has drifted puts the rain in the wrong hour. A reading that was never kept cannot be checked.

Fig. A draws a surface weather station taken apart. Each sensor stands where the WMO Guide to Instruments and Methods of Observation puts it, so that one station can be compared with another:

  1. Wind, at 33 ft (10 m) above open ground, high enough that the ground's roughness has less hold on it.
  2. Temperature and humidity, between 4.1 and 6.6 ft (1.25 and 2 m) above short grass, inside a shield that keeps the sun and the rain off the sensor and lets the air through.
  3. Precipitation, in a gauge with a level opening, clear of anything that could shelter it.
  4. Pressure, read inside the enclosure, where the pressure is the same as outside, then reduced to sea level so stations at different heights can be mapped together.
  5. The logger, which stamps each reading with the time in UTC, and the transmitter, which sends it on.

What it is not

It is not the weather everywhere near it. A reading describes the air at the sensor, and a station stands for the ground around it only as far as that ground is like its own. A station on a sunny airport apron and one in a shaded valley a mile away can both be right and differ by several degrees.

It is not exact. Accuracy, how close a reading is to the truth, is different from resolution, the smallest step it reports, and from drift, how its error grows with time. A thermometer that reports tenths of a degree may be accurate only to half a degree. Calibration against a reference, traceable to a national standard, is what ties a sensor to the truth.

Lookalikes

A forecast grid point
A number for a box of model atmosphere, not a measurement at a place. It can be close to a station's reading and still not be one.
A radar or satellite estimate
A remote measurement: an instrument, but one that infers the quantity at a distance, WSR-88D radar FB-INS-005.
The analysed temperature on the live map
An analysis: station readings joined into a surface. It is made from instruments and is not one.

The machine

Exposure. Most of a station's error comes from where it stands, not from the sensor. A temperature sensor has to see the air and not the sun, so it sits in a louvred or aspirated shield, Stevenson screen and siting FB-INS-009. It stands over grass, because pavement heats more under the sun and gives the heat back all night. Obstacles shelter the wind for a long way downwind of them, so the Guide asks that nothing stand closer to a wind sensor than ten times its own height, Fig. B. Wind also carries drops and flakes over a gauge's opening, so every gauge catches too little in wind, and snow most of all.

Siting class. The Guide grades a site for each variable from class 1, the best, to class 5, where the surroundings can add large errors. A class describes the place, not the sensor: a perfect thermometer beside a car park is still a class 5 temperature.

Time. Every reading carries its time and its averaging. A reported wind is an average over a stated interval, two minutes in United States surface reports and ten minutes in most other countries, and a gust is the highest short burst within it. Rain is an amount over an interval. Two instruments that average differently report different numbers for the same weather, and both are right.

A painted plan of a weather station in open grass: the station in the middle; a building 20 feet (6 metres) high to one side, which must stand at least 200 feet (60 metres) away, ten times its height; a tree 39 feet (12 metres) high to the other side, which must stand at least 390 feet (120 metres) away; and a paved road along one edge, a source of heat kept away from the temperature sensor. Not to scale.
Fig. B Where it stands. A station in open grass, each obstacle at least ten times its own height away, and a road kept at a distance.MaximizeThe drawing, to scale

The drawings

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

Elevation of a surface weather station over short grass, heights to scale and sensors enlarged: a mast to 33 feet (10 metres) carrying a cup anemometer and a wind vane on a crossarm; a louvred radiation shield on its own post holding the temperature and humidity sensor at 4.9 feet (1.5 metres), inside the band from 4.1 to 6.6 feet (1.25 to 2 metres); a precipitation gauge with its opening level; and an enclosure holding the logger, the barometer and the transmitter, with a solar panel and an antenna.1122334455667788AABBCCDDEESTORM STATION 247THE FIELD BOOKPLATE FB-INS-001-AELEVATION OF A SURFACE WEATHER STATION, EACH SENSOR AT THE HEIGHT THE WMO GUIDE GIVES ITFIG. A A SURFACE STATION TAKEN APART051015202530350246810ft(m)33 ft (10 m)6.6 ft (2 m)4.1 ft (1.25 m)5 ft (1.5 m)OPENING LEVELLOGGERBAROMETERRADIOSOLAR PANELEACH HEIGHT FROM THE WMO GUIDE. THE SENSORS ARE DRAWN LARGER THAN THEIR SCALE.KEYTemperature, 4.1 to 6.6 ft (1.25 to 2 m)1234567TITLEWhat an instrument is, a surface station taken apartVOL. VII THE INSTRUMENTS · THE STATIONTYPEELEVATIONSCALEHEIGHTS TO SCALE, SENSORS ENLARGEDREVREV 1 DRAFT SHEET 1 of 2DATE2026-09-24IDFB-INS-001-ADRAWN AS linework on paperSOURCES WMO, Office of the Federal Coordinator for Meteorology, NWS
Fig. A, the drawing A surface station taken apart. Elevation of a surface weather station, each sensor at the height the WMO Guide gives it HEIGHTS TO SCALE, SENSORS ENLARGEDMaximizeThe sheet, SVG, 11 by 17The painting
  1. Cup anemometer and wind vane, at 33 ft (10 m)
  2. The mast
  3. Temperature and humidity sensor
  4. Radiation shield, louvred and ventilated
  5. Precipitation gauge
  6. Logger, barometer and transmitter
  7. Short grass, the natural surface
Plan of a weather station and its surroundings, to scale: the mast at the centre of a small fenced plot; a building 20 feet (6 metres) high standing 262 feet (80 metres) away, outside the circle of 197 feet (60 metres), ten times its height, that the wind sensor needs clear; a tree 39 feet (12 metres) high standing 490 feet (150 metres) away, outside its own circle of 390 feet (120 metres); and a paved road 330 feet (100 metres) to the south, a source of heat kept away from the temperature sensor.1122334455667788AABBCCDDEESTORM STATION 247THE FIELD BOOKPLATE FB-INS-001-BPLAN OF A STATION'S SURROUNDINGS, DRAWN TO THE WMO GUIDE'S DISTANCESFIG. B WHERE IT STANDSBUILDING, 20 ft (6 m) HIGHTREE, 39 ft (12 m) HIGH260 ft (80 m)490 ft (150 m)200 ft (60 m), 10 × 20 ft390 ft (120 m), 10 × 39 ftPAVED ROAD, 26 ft (8 m) WIDETHE RULE FOR WINDNothing closer to the mast than tentimes its own height.THE RULE FOR TEMPERATUREOver grass, in a shield, clear of paving,walls and anything that heats or shades it.THE RULE FOR RAINNothing closer than twice, and better fourtimes, its height above the gauge.N0330 ft(100 m)123456TITLEWhat an instrument is, where it standsVOL. VII THE INSTRUMENTS · THE STATIONTYPEPLANSCALETO SCALE, AS THE BARREVREV 1 DRAFT SHEET 2 of 2DATE2026-09-24IDFB-INS-001-BDRAWN AS linework on paperSOURCES WMO, Office of the Federal Coordinator for Meteorology, NWS
Fig. B, the drawing Where it stands. Plan of a station's surroundings, drawn to the WMO Guide's distances TO SCALE, AS THE BARMaximizeThe sheet, SVG, 11 by 17The painting
  1. The station plot
  2. A building 20 ft (6 m) high
  3. Ten times the building's height, 197 ft (60 m)
  4. A tree 39 ft (12 m) high
  5. Ten times the tree's height, 390 ft (120 m)
  6. A paved road, a source of heat

Ingredients

  • A sensor, which turns a property of the air into a signal
  • An exposure, the place and the housing that let the sensor read the air and nothing else
  • A clock and an averaging rule, so each number means a stated interval at a stated time
  • A record, kept and checked, so that the number can be traced and compared

Scales

time
seconds for a single sample; a two minute average for a reported wind; an hour or a day for rain
horizontal
the ground around the sensor, as far as that ground is like its own; a few hundred metres to a few kilometres
vertical
0 to 33 ft (0 to 10 m) for a surface station
orlanski
micro

Signatures

surface
each reading with its time; its averaging interval and the station's height; a missing reading shown as missing; never filled

The numbers

QuantityValue, and the kind of number it is
Wind sensor height33 ft (10 m) above open groundStandard, Guide to Instruments and Methods of Observation (WMO-No. 8)
Open ground for windNo obstacle closer than 10 times its own heightStandard, Guide to Instruments and Methods of Observation (WMO-No. 8)
Temperature and humidity height4.1 to 6.6 ft (1.25 to 2 m) above the ground, in a ventilated radiation shield, over short grass or the natural surfaceStandard, Guide to Instruments and Methods of Observation (WMO-No. 8)
Precipitation gaugeNo obstacle closer than twice, and preferably four times, its height above the gauge's openingStandard, Guide to Instruments and Methods of Observation (WMO-No. 8)
Siting classesClass 1, the best, to class 5, for each variable; the class describes the surroundings, not the sensorStandard, Guide to Instruments and Methods of Observation (WMO-No. 8)
Reported windA two minute average in United States surface reports; a ten minute average in most other countriesStandard, Federal Meteorological Handbook No. 1
TimeCoordinated Universal Time (UTC)Standard, Federal Meteorological Handbook No. 1
Severe windA measured gust of 58 mph (50 knots, 26 m/s) or moreStandard, Directive 10-511

How the station sees it

This is the plate behind every instrument in the atlas. Each station's page names its network, where it stands, when it last reported and its rank, which carries a date and falls when the station stops reporting. The networks are built for different jobs: airport stations to a federal standard, reporting every hour; cooperative observers reading temperature and rain once a day, some for more than a century; volunteers with rain and snow gauges; road stations sited beside the pavement, for the road. The atlas keeps them apart so each reading can be weighed for what it is.

How it is warned

Instruments are not warned. They confirm a warning and score it afterwards: a measured gust of 58 mph (50 knots, 26 m/s), a report of hail an inch across, an hour of rain in a gauge. Warnings say where their evidence came from, radar indicated or observed, and a station's reading is observed.

See also

Sources

  1. World Meteorological Organization. Guide to Instruments and Methods of Observation (WMO-No. 8).
  2. Office of the Federal Coordinator for Meteorology. Federal Meteorological Handbook No. 1, Surface Weather Observations and Reports.
  3. National Weather Service. Automated Surface Observing System (ASOS) User's Guide.
  4. National Weather Service. Directive 10-511, WFO Severe Weather Products Specification.

Definition after the Guide to Instruments and Methods of Observation (WMO-No. 8). Plate FB-INS-001, revision 1, 2026-09-24. The number is permanent; cite it.