Storm Station

The Field Book/Vol. VII, The Instruments/FB-INS-003

Personal weather station

A personal weather station is a set of consumer sensors that a person mounts at home: a thermometer and hygrometer, a wind cup or sonic sensor, a rain gauge and a barometer, reporting to the internet every few minutes. Tens of thousands of them fill the gaps between official stations, and how well each one reads depends mostly on where it stands.

Plate FB-INS-003The stationRevision 1, 2026-09-25Status draftAlso called PWS, home weather station, backyard weather station, hobby station, Citizen Weather Observer Program, CWOP, amateur weather station

Elevation of a backyard drawn to scale, height and distance alike: a house 25 feet tall with paving beside it and a tree 20 feet tall; the rain gauge 40 feet beyond the tree, twice its height; the thermometer in its shield at 5 feet, 100 feet (30 metres) from the paving; and the wind sensor on a 33 foot (10 metre) mast 250 feet from the house, ten times its height.1122334455667788AABBCCDDEE© 2026 STORM STATION 247THE FIELD BOOKPLATE FB-INS-003-AELEVATION OF A BACKYARD WITH A 25 FT HOUSE AND A 20 FT TREE, AND WHERE EACH SENSOR MUST STAND TO MEET THE WMO'S EXPOSURE RULESFIG. A WHERE EACH SENSOR STANDSHOUSE, 25 FTPAVINGTREE, 20 FT40 FT: 2 × THE TREE100 FT (30 M) FROM PAVINGWIND, 33 FT (10 M)250 FT: 10 × THE HOUSEDRAWN TO SCALE, HEIGHT AND DISTANCE ALIKE1234TITLEPersonal weather station, where each sensor standsVOL. VII THE INSTRUMENTS · THE STATIONTYPEELEVATIONSCALETO SCALEREVREV 1 DRAFT SHEET 1 of 2DATE2026-09-25IDFB-INS-003-ADRAWN AS linework on paperSOURCES WMO, Gunn
Fig. A Where each sensor stands. Elevation of a backyard with a 25 ft house and a 20 ft tree, and where each sensor must stand to meet the WMO's exposure rules TO SCALEMaximizeThe sheet, SVG, 11 by 17
  1. The rain gauge, twice the tree's height away
  2. The thermometer, 100 ft from the paving
  3. The wind mast, ten times the house's height away
  4. The paving

What it is

A personal weather station is a set of consumer sensors mounted at a home, a school or a farm. It usually has a thermometer and hygrometer in a small shield, a wind cup and vane or a sonic sensor, a tipping rain gauge and a barometer indoors. A console or a small computer sends a reading to the internet every few minutes. Tens of thousands of them stand across the country, far more than the official stations, and a network run with the Weather Service, the Citizen Weather Observer Program, passes many of their reports on to forecasters.

What it is not

It is not held to a standard. An airport station is sited, maintained and checked to a federal handbook, ASOS FB-INS-002; a home station stands wherever its owner can put it. Its sensors can be very good, and its readings can still be far off, because the site decides most of what it measures.

Lookalikes

An airport station
Sited, maintained and checked to a federal standard, ASOS FB-INS-002.
A cooperative observer
A volunteer with standard instruments supplied and inspected by the Weather Service, reading once a day, the long record behind the climate.

The machine

Siting

Fig. A draws a backyard to scale and the rules for each sensor, from the WMO's guide to instruments. The wind sensor belongs at 33 ft (10 m), at least ten times an obstacle's height from it. Behind a house 25 ft tall, that is 250 ft away. The rain gauge needs every object at least twice its height away: 40 ft from a tree 20 ft tall. The thermometer belongs at about 5 ft over grass, in a shield, at least 100 ft (30 m) from paving, walls and water for a good siting class, Stevenson screen and siting FB-INS-009. Few yards allow all three, and a roof, where many stations go, suits none of them.

The rain shadow

Rain does not fall straight down in wind. It falls at an angle set by the wind and the drop's own fall speed, and anything tall throws a rain shadow downwind. Fig. B draws a tree 20 ft tall. Drops 0.04 in (1 mm) across fall 13.2 ft/s (4.03 m/s) in still air, and their shadow reaches 11 ft beyond the tree in a 5 mph wind and 44 ft in a 20 mph wind. A gauge at twice the tree's height stays out of the shadow of those drops in winds up to 18 mph, and of larger 0.08 in (2 mm) drops up to 29 mph. Closer in, it catches less rain than fell.

The sun

A small shield in sunlight, with little wind to cool it, warms its thermometer above the air, and paving and walls add their own heat. That is why the home station that reads hottest in a town on a sunny afternoon is often the one worst sited, not the one in the hottest air.

Section through a tree 20 feet tall with the wind from the left: for drops 0.04 inches (1 millimetre) across, falling 13.2 feet a second, the edge of the tree's rain shadow reaches 11 feet beyond it at 5 miles an hour, 22 feet at 10, 33 feet at 15 and 44 feet at 20; a rain gauge 40 feet away, twice the tree's height, stays out of the shadow of those drops in winds up to 18 miles an hour, and of 0.08 inch (2 millimetre) drops up to 29.1122334455667788AABBCCDDEE© 2026 STORM STATION 247THE FIELD BOOKPLATE FB-INS-003-BSECTION THROUGH A TREE IN WIND, THE EDGE OF ITS RAIN SHADOW DRAWN FOR WINDS OF 5 TO 20 MPH, 0.04 IN (1 MM) DROPS FALLING AT THEIR MEASURED STILL AIR SPEEDFIG. B THE RAIN SHADOW OF A TREETREE, 20 FTWIND5 MPH: 11 FT10 MPH: 22 FT15 MPH: 33 FT20 MPH: 44 FT40 FT: THE GAUGE AT 2 × THE TREE0.04 IN (1 MM) DROPS FALL 13.2 FT/S (4.03 M/S); 0.08 IN (2 MM) DROPS 21.3 FT/S (6.49 M/S)THE GAUGE STAYS OUT OF THE SHADOW OF 0.04 IN (1 MM) DROPS IN WIND UP TO 18 MPH, 0.08 IN (2 MM) DROPS UP TO 29 MPHDRAWN TO SCALE; STILL AIR FALL SPEEDS123TITLEPersonal weather station, the rain shadow of a treeVOL. VII THE INSTRUMENTS · THE STATIONTYPESECTIONSCALETO SCALEREVREV 1 DRAFT SHEET 2 of 2DATE2026-09-25IDFB-INS-003-BDRAWN AS linework on paperSOURCES WMO, Gunn
Fig. B The rain shadow of a tree. Section through a tree in wind, the edge of its rain shadow drawn for winds of 5 to 20 mph, 0.04 in (1 mm) drops falling at their measured still air speed TO SCALEMaximizeThe sheet, SVG, 11 by 17
  1. The treetop
  2. The shadow's edge at 20 mph
  3. The gauge, twice the tree's height away

Ingredients

  • Sensors: temperature and humidity in a small shield, wind, rain and pressure
  • A site: the yard, the roof and what stands around them
  • A connection that sends each reading to the internet, and to networks that collect them

Scales

time
a reading every few seconds to every few minutes
horizontal
a point in a yard; its value depends on the ground within a few hundred feet (100 m)
vertical
from the ground to the top of its mast, often a roof
orlanski
micro

Signatures

surface
afternoon highs above every neighbour on sunny days; from a shield in the sun or near paving; wind that never gusts; from a mast in the lee of a roof; rain totals below a gauge in the open; from a gauge under a tree

The numbers

QuantityValue, and the kind of number it is
Wind exposureThe wind sensor at 33 ft (10 m), at a distance of at least ten times the height of any obstacleStandard, Guide to Instruments and Methods of Observation (WMO-No. 8)
Rain gauge exposureAny object at least twice its height above the gauge's rim away from the gaugeStandard, Guide to Instruments and Methods of Observation (WMO-No. 8)
Thermometer sitingAt 4.1 to 6.6 ft (1.25 to 2 m) over grass; class 2 siting needs no paving, building or water within 100 ft (30 m)Standard, Guide to Instruments and Methods of Observation (WMO-No. 8)
Raindrop fall speed0.04 in (1 mm) drops fall 13.2 ft/s (4.03 m/s) and 0.08 in (2 mm) drops 21.3 ft/s (6.49 m/s) in still airTextbook, Gunn 1949

How the station sees it

This site does not carry personal stations. Their readings can be compared with the standard stations it does carry: the nearest airport station for the temperature and wind, the cooperative observers for the day's high, low and rain, and the community rain gauges read each morning. A home station that stays within a degree or two of those, and whose rain totals track theirs over weeks, is well sited.

How it is warned

Personal stations are not used to issue warnings. Forecasters watching a storm may still look at them, because a dense network of home stations can show a gust front or a flooding downpour between the official stations.

See also

Sources

  1. World Meteorological Organization. Guide to Instruments and Methods of Observation (WMO-No. 8).
  2. Gunn, R. and G. D. Kinzer. The Terminal Velocity of Fall for Water Droplets in Stagnant Air, Journal of Meteorology 6 (1949).

Definition after the Guide to Instruments and Methods of Observation (WMO-No. 8). Plate FB-INS-003, revision 1, 2026-09-25. The number is permanent; cite it.