Automated airport weather stations use a variety of sophisticated equipment to observe the weather.

Wind speed and direction

 
ASOS Ice Free Wind Sensor

A majority of older automated airport weather stations are equipped with a mechanical wind vane and cup system to measure wind speed and direction. This system is simple in design: the wind spins three horizontally turned cups around the base of the wind vane, providing an estimation of the wind's speed, while the vane on top turns so that the face of the vane offers the least resistance to the wind, causing it to point in the direction the wind is coming from and thus providing the wind direction.

The new generation of sensors use sound waves to measure wind speed and direction. The measurement is based on the time it takes for an ultrasonic pulse to travel from one transducer to another, which varies depending on - among other factors - the wind speed. The transit time is measured in both directions for several (usually two or three) pairs of the transducer heads. Based on those results, the sensor computes wind speed and direction. Compared to mechanical sensors, the ultrasonic sensors offer several advantages such as no moving parts, advanced self-diagnostic capabilities and reduced maintenance requirements.

NWS and FAA ASOS stations and most of new AWOS installations are currently equipped with ultrasonic wind sensors.

Unlike all other measurements, which are made near between 3 and 9 feet (1 and 3 meters) above the ground, wind speed and direction are measured at 30 feet (10 meters).

Visibility

 
ASOS Visibility Sensor

To determine visibility, automated airport weather stations use one of two sensor types:

The forward scatter sensor uses a beam of infrared light which is sent from one end of the sensor toward the receiver, but offset from a direct line to the receiver by a certain angle. The amount of light scattered by particles in the air and received by the receiver determines the extinction coefficient. This is then converted to visibility using either Allen's or Koschmieder's law.

In a transmissometer, a beam of visible light is transmitted from its transmitter to receiver head. The extinction coefficient is derived from the amount of light lost in the air.

There also are sensors that, to a certain degree combine a transmissometer with a forward scatter sensor.

Forward scatter sensors are more popular due to their lower price, smaller size and lower maintenance requirements. However, transmissometers are still used at some airports as they are more accurate at low visibilities and are fail-safe, i.e. in case of failure report visibility lower than actual.

Current sensors are capable of reporting visibility in a wide range. For aviation purposes, the reported values are rounded down to the nearest step in one of the following scales:

  • M1/4 (less than 1/4 mile), 1/4, 1/2, 3/4, 1, 1-1/4, 1-1/2, 2, 2-1/2, 3, 4, 5, 7, 10 and 10+ (greater than 10 miles)
  • In steps of 50 m when the visibility is less than 800 m; in steps of 100 m when it is 800 m or more, but less than 5 km; in kilometer steps when the visibility is 5 km or more but less than 10 km; and 10 km when visibility is 10 km or more.

Present weather (falling precipitation)

 
ASOS Present Weather Sensor

Automated airport weather stations use a Light Emitting Diode Weather Identifier (LEDWI) to determine if and what type of precipitation is falling. The LEDWI sensor measures the scintillation pattern of the precipitation falling through the sensor's infrared beam (approximately 50 millimeters in diameter) and determines from a pattern analysis of the particle size and fall velocity whether the precipitation is rain or snow. If precipitation is determined to be falling, but the pattern is not conclusively identified as either rain or snow, unknown precipitation is reported. Automated airport weather stations are not yet able to report hailice pellets, and various other intermediate forms of precipitation.

Obscurations to vision

Automated airport weather stations do not have a separate sensor for detecting specific obscurations to vision. Instead, when visibility is reduced below 7 statute miles, the system uses the reported temperature and dew point to determine an obscuration to vision. If relative humidity is low (i.e., there is a large difference between the temperature and dew point), haze is reported. If relative humidity is high (i.e., there is a small difference between the temperature and the dew point),mist or fog is reported, depending on the exact visibility. Fog is reported when visibility is 1/2 mile or less; mist is reported for visibilities greater than 1/2 mile but less than 7 miles (11 km). If the temperature is below freezing, humidity is high and visibility is 1/2 mile or less, freezing fog is reported.

Cloud coverage and ceiling

 
ASOS CT12K ceilometer

Automated airport weather stations use an upward-pointing laser beam ceilometer to detect the amount and height of clouds. The laser is pointed upward, and the time required for reflected light to return to the station allows for the calculation of the height of the cloud base. Because of the limited coverage area (the laser can only detect clouds directly overhead), the system computer calculates a time-averaged cloud cover and ceiling, which is reported to external users. To compensate for the danger of rapidly changing sky cover, the averaging is weighted toward the first 10 minutes of the 30-minute averaging period. The range of the ceilometer is 12,000 feet (3,700 m); clouds above that height are not detectable by automated stations at present.

Temperature and dew point

 
ASOS HO-1088 thermometer

Automated airport weather stations use a temperature/dew point sensor (hygrothermometer) designed for continuous operation which normally remains on at all times, except during maintenance.

The measurement of temperature is simple compared to the dew point. Operating under the principle that electrical resistance varies with temperature, a platinum wire resistive temperature device measures the ambient air temperature. The current ASOS thermometer is designated the HO-1088, though some older systems still utilize the HO-83.

 
ASOS DTS-1 dew point sensor

In contrast, the dew point measurement is considerably more complex. The original dew point sensor deployed on ASOS systems utilized a chilled mirror that is cooled to the point where a fine film of condensation forms on the mirror's surface. The temperature of the mirror at this condition is equal to the dew point temperature. The hygrometer measures the dew point by directing a light beam from a small infrared diode to the surface of the mirror at an angle of 45 degrees. Two photo transistors are mounted so they measure a high degree of reflected light when the mirror is clear (direct) and scattered light when the mirror is clouded with visible condensation (indirect). With the formation of condensation on the mirror, the degree of cloudiness of the mirror surface increases with the direct transistor receiving less light and the indirect transistor more light. The output from these photo transistors controls the mirror cooling module which is an electronic heat pump that operates much like a thermocouple in reverse, producing a heating or cooling effect. When the sensor is first activated, the mirror is clear. As the mirror surface temperature is cooled to the dew point temperature, condensations forms on the mirror. The electronics continuously tries to stabilize the signal levels to the power amplifier to maintain the mirror temperature at the dew point. If the dew point of the air changes or if the circuit is disturbed by noise, the loop makes the necessary corrections to restabilize at the dew point and maintaining continuous operation.

The NWS has replaced most if not all chilled mirror sensors because of problems. NWS ASOS now use Vaisala's DTS1 sensor, which measures humidity only. ASOS then uses the ambient air temperature from the thermometer and the humidity obtained from the DTS1 to determine dew point.

Older AWOS systems used a Lithium Chloride Dew Point sensor. Current AWOS systems use capacitive relative humidity sensors, from which Dew Point is calculated. [5]

Barometric pressure and altimeter setting

Data from a barometric pressure sensor are used to calculate altimeter setting. Pilots rely on this value to determine their altitude. To ensure safe separation from terrain and other obstructions, high degree of accuracy and reliability is required from a pressure sensor.

Most aviation weather stations use two (required for an AWOS) or three independent pressure transducers. The transducers may or may not share their associated tubing and external ports (designed to minimize effect of wind/wind gusts). Should the reported pressures differ by more than a preset maximum, the pressure values are discarded and altimeter setting is not reported or is reported as "missing."

Altimeter setting is calculated based on barometric pressure, site elevation, sensor elevation and - optionally - air temperature.

Altimeter setting is reported in inches of mercury (in steps of 0.01 inHg) or whole hectopascals, rounded down.

Precipitation accumulation

 
ASOS heated tipping bucket precipitation gauge

The original precipitation accumulation measuring device used for automated airport weather stations was the heated tipping bucket. The upper portion of this device consists of a 1-foot (0.30 m) diameter collector with an open top. The collector, which is heated to melt any frozen precipitation such as snow or hail, funnelswater into a two-chamber, pivoting container called a bucket. Precipitation flows through the funnel into one compartment of the bucket until 0.01-inch (0.25 mm) of water (18.5 grams) is accumulated. That amount of weight causes the bucket to tip on its pivots, dumping the collected water and moving the other chamber under the funnel. The tipping motion activates a switch (either a reed switch or a mercury switch), which sends one electrical pulse for each 0.01-inch (0.25 mm) of precipitation collected.

Because of problems the heated tipping bucket has with properly measuring frozen precipitation (particularly snow), the All Weather Precipitation Accumulation Gauge (AWPAG) was developed. This sensor is essentially a weighing gauge where precipitation continuously accumulates within the collector, and as the weight increases, precipitation is recorded. Only select NWS ASOS units have been equipped with the AWPAG. [6]

Lightning (thunderstorms)

 
ASOS Thunderstorm Sensor

Many automated airport weather stations within the United States use the National Lightning Detection Network (NLDN) to detect lightning via the Automatic Lightning Detection and Reporting System (ALDARS). The NLDN uses 106 sensors nationwide to triangulate lightning strikes. Data from the detection grid is fed into ALDARS, which in turn sends messages to each automated airport station informing it of the proximity of any lightning strikes. Lightning strikes within 5 miles (8.0 km) of the station result in a report of a thunderstorm at the station (TS). Lightning strikes more than 5 miles (8.0 km) but less than 10 miles (16 km) from the station result in a report of a thunderstorm in the vicinity of the station (VCTS). Lightning more than 10 miles (16 km) but less than 30 miles (48 km) from the station results only in a remark of distant lightning (LTG DSNT).[7]

However, some stations now have their own lightning sensor to actually measure lightning strikes at the site rather than requiring an external service. This thunderstorm sensor works by detecting both the flash of light and momentary change in the electric field produced by lightning. When both of these are detected within a few milliseconds of each other, the station registers a possible lightning strike. When a second possible lightning strike is detected within 15 minutes of the first, the station records a thunderstorm.[8]