Observing station and source scope
This transparent Joshi-only chapter explains the instruments and station model. Oxford has no matching chapter. A station observation is valuable only when the element is measured with the correct instrument, unit and exposure.
Element, instrument and unit
Pressure, temperature, dew point, humidity, precipitation, wind, cloud base, visibility or RVR and upper-air variables each have a standard observing method. The instrument name alone is not enough: units and representative exposure matter.
Quality first
A well-calibrated instrument can still give a poor observation if it is shielded, too close to an obstacle or read at the wrong time. The model compresses observations, so quality errors become code errors.
Pressure, temperature, humidity and rain
A barometer measures pressure. Thermometers measure air temperature and dew point systems support moisture assessment. Hygrometers or related methods describe relative humidity or mixing ratio. Rain gauges measure precipitation.
Exposure and interpretation
Temperature instruments need representative exposure away from artificial heating. Pressure must be reduced or coded according to the observing procedure. Dew point and humidity should be read with temperature because their difference affects condensation risk.
Aviation consequence
Pressure affects altimeter setting. Temperature affects performance. Dew-point spread supports fog and cloud assessment. Rainfall supports the present and recent weather picture but does not by itself describe runway condition.
Wind, visibility, cloud and upper air
Anemometers measure wind speed and wind vanes support direction. Visibility or RVR systems observe the visual environment. Ceilometers or ceilographs help determine cloud base. Upper-air systems measure wind, pressure, temperature and humidity with height.
Representative observations
Wind exposure must avoid local shelter and obstruction. A visibility sensor gives a measurement at its site, while fog and haze may vary across an aerodrome. Cloud base requires vertical context, particularly near terrain.
Upper-air relevance
Surface weather does not give the complete flight profile. Upper-air wind and temperature support route level, icing, turbulence and performance planning.
Station-model layout
A station model is a compact plotted summary. Its standard positions hold wind, cloud amount, temperature, dew point, pressure, pressure change, weather, cloud type, cloud base, rainfall and visibility information.
Read positions in a fixed order
Start with wind staff and centre circle, then temperature, dew point and pressure. Next decode current and past weather, cloud types, cloud base, rainfall and visibility. This prevents a symbol from being read in the wrong field.
Wind and pressure
Wind direction and speed use the staff and feathers. Pressure is coded in abbreviated form and needs the correct leading digits restored from the synoptic context.
Symbols and code tables
Joshi provides current and past weather symbols, cloud symbols and code tables for cloud base h, rainfall RRR and visibility VV. The tables must be used rather than guessed from a partial pattern.
Cloud and weather
Cloud family, amount and present weather are separate elements. Past weather gives history rather than the current condition. Read both when judging a trend.
h RRR and VV
The h, RRR and VV codes convert compact numbers into cloud-base, rainfall and visibility values. Their ranges and exceptions are table-based, so consult the relevant table for each code.
Worked station-model decisions
A complete station-model decode begins with layout, then converts every coded element using the correct table. Two models can show similar weather symbols but imply different aviation decisions because wind, visibility, cloud base and pressure differ.
Worked method
First decode wind and cloud amount. Then temperature and dew point. Restore pressure from the context. Decode present and past weather, cloud type, h, RRR and VV. Finally relate the result to runway, approach and trend risk.
Decision use
A low visibility code with a low cloud-base code can be approach-limiting. A strong wind staff with precipitation and pressure change can signal a rapidly changing terminal environment.
Code tables and two worked models
| Code | Use | Reading rule |
|---|---|---|
| N | Total cloud | 0 to 8 oktas; circle fill shows amount. |
| h | Cloud base | Use Joshi's h table; code gives the prescribed height band. |
| RRR | Rainfall | Use the rainfall table and period shown on the model. |
| VV | Visibility | Use the VV code table, not the cloud-base table. |
| ww/W | Present/past weather | ww is current weather; W is the previous weather period. |
Model A: wind shaft from 330 with one full and one half barb is 330 degrees at 15 kt. A nearly filled circle means 7 to 8 oktas. Temperature 24 C and dew point 22 C indicate a small spread. Pressure code 012 is restored as 1012.0 hPa when the synoptic context supports that value. If VV gives 2000 m and h gives a low base, this is an approach and alternate concern.
Model B: a shaft from 090 with two full barbs is 090 degrees at 20 kt. A half-filled circle means 4 oktas. A present-weather rain symbol is current rain; a past-weather symbol records what occurred earlier. Decode RRR and h from their separate tables, then combine wind, cloud base and visibility before deciding the runway and approach effect. Wind barbs are half barb 5 kt, full barb 10 kt and pennant 50 kt.
Instrument and symbol matrix
| Element | Instrument | Usual unit |
|---|---|---|
| Pressure | Barometer | hPa |
| Air temperature and dew point | Thermometer and dew-point system | C |
| Humidity | Hygrometer | RH or mixing ratio |
| Precipitation | Rain gauge | mm |
| Wind | Vane and anemometer | degrees and kt |
| Cloud base | Ceilometer | ft or m |
| Visibility/RVR | Visibility system | m |
Cloud symbols identify genus and amount; present-weather symbols describe the observation now, while past-weather symbols describe the preceding period. The station-model temperature is plotted top left, dew point bottom left, and sea-level pressure top right. The three-hour pressure tendency is plotted below the pressure, so a number and its tendency must not be interchanged.
Station-model plotting positions
The wind staff leaves the station circle towards the direction from which wind blows. Total cloud covers the centre circle. Temperature is top left, dew point bottom left, mean sea-level pressure top right and pressure tendency below it. Present weather, past weather, cloud types, cloud base h, rainfall RRR and visibility VV occupy their standard surrounding fields. This fixed layout is the key to decoding twelve separate observations without mixing them.
Cloud and weather reading
Cloud amount on the station circle uses oktas. Cloud genus symbols, present-weather symbols and past-weather symbols are read from separate Joshi tables. A rainfall code does not identify the weather type, and a visibility code does not identify the cloud base. The examiner can deliberately place similar-looking values in different model fields, so layout must be recognised before any table is used.
When restoring pressure from the final three digits, choose the value near 1000 hPa that fits the synoptic situation. For example, 012 normally becomes 1012.0 hPa rather than 901.2 hPa. The pressure tendency symbol and value show the three-hour change; use both with wind and weather to judge whether the station is improving or deteriorating.
Two-table rule
Use the station-model layout table first to identify the field, then the appropriate code table to convert it. This prevents the common error of reading h as VV, or reading present weather as past weather. The final aviation statement must include wind, visibility, cloud base, weather and pressure trend because those elements drive runway, approach and alternate judgement.
Observation quality: screen temperature, wind exposure, rain-gauge siting and visibility-sensor location determine whether the model represents the aerodrome. A coded value cannot correct a badly exposed observation, so station practice is part of the meaning of the decoded model.
The model is a compact observing record, not a forecast. Its value for a pilot comes from combining the coded surface facts with the current forecast, not from extrapolating a single plot beyond its observation time.