Automated airport weather stations are automated sensor suites which are designed to serve aviation

Aviation

Aviation is the design, development, production, operation, and use of aircraft, especially heavier-than-air aircraft. Aviation is derived from avis, the Latin word for bird.-History:...

 and meteorological

Meteorology

Meteorology is the interdisciplinary scientific study of the atmosphere. Studies in the field stretch back millennia, though significant progress in meteorology did not occur until the 18th century. The 19th century saw breakthroughs occur after observing networks developed across several countries...

 observing needs for safe and efficient aviation operations and weather forecasting

Weather forecasting

Weather forecasting is the application of science and technology to predict the state of the atmosphere for a given location. Human beings have attempted to predict the weather informally for millennia, and formally since the nineteenth century...

. Automated airport

Airport

An airport is a location where aircraft such as fixed-wing aircraft, helicopters, and blimps take off and land. Aircraft may be stored or maintained at an airport...

 weather station

Weather station

A weather station is a facility, either on land or sea, with instruments and equipment for observing atmospheric conditions to provide information for weather forecasts and to study the weather and climate. The measurements taken include temperature, barometric pressure, humidity, wind speed, wind...

s have become the backbone of weather observing in the United States

United States

The United States of America is a federal constitutional republic comprising fifty states and a federal district...

 and Canada

Canada

Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...

, and are becoming increasingly prevalent worldwide due to their efficiency and cost-savings.

System types within the United States

In the United States, there are several varieties with somewhat subtle but important differences. These include the Automated Weather Observing System (AWOS), Automated Surface Observing System (ASOS), and Automated Weather Sensor System (AWSS).

Automated Weather Observing System (AWOS)

The Automated Weather Observing System (AWOS) units are operated and controlled by the Federal Aviation Administration

Federal Aviation Administration

The Federal Aviation Administration is the national aviation authority of the United States. An agency of the United States Department of Transportation, it has authority to regulate and oversee all aspects of civil aviation in the U.S...

 (FAA) in the United States, as well as by state and local governments and some private agencies; the American National Weather Service

National Weather Service

The National Weather Service , once known as the Weather Bureau, is one of the six scientific agencies that make up the National Oceanic and Atmospheric Administration of the United States government...

 (NWS) and Department of Defense

United States Department of Defense

The United States Department of Defense is the U.S...

 (DOD) play no role in their operation or deployment.

These systems are among the oldest automated weather stations and predate ASOS. They generally report at 20-minute intervals and do not report special observations for rapidly changing weather conditions. There are several varieties of AWOS depending upon the sensor systems which are installed; the most common type is the AWOS-III, which observes temperature

Temperature

Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...

 and dewpoint in degrees Celsius, wind

Wind

Wind is the flow of gases on a large scale. On Earth, wind consists of the bulk movement of air. In outer space, solar wind is the movement of gases or charged particles from the sun through space, while planetary wind is the outgassing of light chemical elements from a planet's atmosphere into space...

 speed

Speed

In kinematics, the speed of an object is the magnitude of its velocity ; it is thus a scalar quantity. The average speed of an object in an interval of time is the distance traveled by the object divided by the duration of the interval; the instantaneous speed is the limit of the average speed as...

 and direction

Direction (geometry, geography)

Direction is the information contained in the relative position of one point with respect to another point without the distance information. Directions may be either relative to some indicated reference , or absolute according to some previously agreed upon frame of reference Direction is the...

 in knots, visibility

Visibility

In meteorology, visibility is a measure of the distance at which an object or light can be clearly discerned. It is reported within surface weather observations and METAR code either in meters or statute miles, depending upon the country. Visibility affects all forms of traffic: roads, sailing...

, cloud

Cloud

A cloud is a visible mass of liquid droplets or frozen crystals made of water and/or various chemicals suspended in the atmosphere above the surface of a planetary body. They are also known as aerosols. Clouds in Earth's atmosphere are studied in the cloud physics branch of meteorology...

 coverage and ceiling up to twelve thousand feet, and altimeter setting. Recently, additional sensors which have become available for AWOS systems include present weather, freezing rain and thunderstorm (lightning).

Automated Surface Observing System (ASOS)

The Automated Surface Observing System (ASOS) units are operated and controlled cooperatively in the United States by the NWS, FAA and DOD. After many years of research and development, the deployment of ASOS units began in 1991 and was completed in 2004.

These systems generally report at hourly intervals, but also report special observations if weather conditions change rapidly and cross aviation operation thresholds. They generally report all the parameters of the AWOS-III, while also having the additional capabilities of reporting temperature and dewpoint in degrees Fahrenheit, present weather

Weather

Weather is the state of the atmosphere, to the degree that it is hot or cold, wet or dry, calm or stormy, clear or cloudy. Most weather phenomena occur in the troposphere, just below the stratosphere. Weather refers, generally, to day-to-day temperature and precipitation activity, whereas climate...

, icing

Atmospheric icing

Atmospheric icing occurs when water droplets in the atmosphere freeze on objects they contact. This can be extremely dangerous to aircraft, as the built-up ice changes the aerodynamics of the flight surfaces, which can increase the risk of a subsequent stalling of the airfoil...

, lightning

Lightning

Lightning is an atmospheric electrostatic discharge accompanied by thunder, which typically occurs during thunderstorms, and sometimes during volcanic eruptions or dust storms...

, sea level pressure and precipitation

Precipitation (meteorology)

In meteorology, precipitation In meteorology, precipitation In meteorology, precipitation (also known as one of the classes of hydrometeors, which are atmospheric water phenomena is any product of the condensation of atmospheric water vapor that falls under gravity. The main forms of precipitation...

 accumulation.

Besides serving aviation needs, ASOS serves as a primary climatological

Climate

Climate encompasses the statistics of temperature, humidity, atmospheric pressure, wind, rainfall, atmospheric particle count and other meteorological elemental measurements in a given region over long periods...

 observing network in the United States, making up the first-order network of climate stations. Because of this, not every ASOS is located at an airport; for example, one of these units is located at Central Park

Central Park

Central Park is a public park in the center of Manhattan in New York City, United States. The park initially opened in 1857, on of city-owned land. In 1858, Frederick Law Olmsted and Calvert Vaux won a design competition to improve and expand the park with a plan they entitled the Greensward Plan...

 in New York City

New York City

New York is the most populous city in the United States and the center of the New York Metropolitan Area, one of the most populous metropolitan areas in the world. New York exerts a significant impact upon global commerce, finance, media, art, fashion, research, technology, education, and...

 and another is located on Cabbage Hill near Pendleton, Oregon

Pendleton, Oregon

Pendleton is a city in Umatilla County, Oregon, United States. Pendleton was named in 1868 by the county commissioners for George H. Pendleton, Democratic candidate for Vice-President in the 1864 presidential campaign. The population was 16,612 at the 2010 census...

, for the sole purpose of providing climatological observations.

Automated Weather Sensor System (AWSS)

As with the AWOS, the Automated Weather Sensor System (AWSS) units are operated and controlled by the FAA in the United States; the NWS and DOD play no role in their operation or deployment.

The reporting characteristics of the AWSS are very similar to those of ASOS.

Observing equipment

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

Wind speed and direction

A majority of automated airport weather stations are equipped with a standard wind vane

Weather vane

A weather vane is an instrument for showing the direction of the wind. They are typically used as an architectural ornament to the highest point of a building....

 and cup system to measure wind speed and direction. This system works similarly to many older wind measurement systems; 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 tendency for these systems to freeze up during winter weather has resulted in the development of a new ultrasonic wind sensor. The new ultrasonic sensors use ultrasound

Ultrasound

Ultrasound is cyclic sound pressure with a frequency greater than the upper limit of human hearing. Ultrasound is thus not separated from "normal" sound based on differences in physical properties, only the fact that humans cannot hear it. Although this limit varies from person to person, it is...

 to determine horizontal wind speed and direction. The measurement is based on the time it takes for an ultrasonic pulse to travel from one transducer

Transducer

A transducer is a device that converts one type of energy to another. Energy types include electrical, mechanical, electromagnetic , chemical, acoustic or thermal energy. While the term transducer commonly implies the use of a sensor/detector, any device which converts energy can be considered a...

 to another, which varies depending on the wind speed. The transit time is measured in both directions for three pairs of the transducer heads. Using the two measurements for each of the three ultrasonic paths at 60° angles to each other, the sensor is able to compute the wind speed and direction. Only NWS and FAA ASOS stations are currently equipped with the ultrasonic wind sensors.

Visibility

Automated airport weather stations use a forward scatter sensor

Sensor

A sensor is a device that measures a physical quantity and converts it into a signal which can be read by an observer or by an instrument. For example, a mercury-in-glass thermometer converts the measured temperature into expansion and contraction of a liquid which can be read on a calibrated...

 which determines the local air clarity and translates it into prevailing visibility. The sensor uses a beam of xenon light which is sent from one end of the sensor toward the receiver, but offset from a direct line to the receiver by 45 degrees

Degree (angle)

A degree , usually denoted by ° , is a measurement of plane angle, representing 1⁄360 of a full rotation; one degree is equivalent to π/180 radians...

. The amount of light deflected by particles in the air into the receiver determines the clarity of the air. Computer

Computer

A computer is a programmable machine designed to sequentially and automatically carry out a sequence of arithmetic or logical operations. The particular sequence of operations can be changed readily, allowing the computer to solve more than one kind of problem...

 algorithm

Algorithm

In mathematics and computer science, an algorithm is an effective method expressed as a finite list of well-defined instructions for calculating a function. Algorithms are used for calculation, data processing, and automated reasoning...

s then translate the amount of scattered light into a prevailing visibility. The current sensors available for deployment cannot detect differences in visibility that are less than 1/4 mile or greater than 10 miles (16.1 km). Thus, visibilities are only reported at a maximum of 1/4 mile increments, with visibility significantly below 1/4 mile being reported as "M1/4" (less than 1/4 mile); visibilities above 10 miles (16.1 km) are reported as equal to 10 miles (16.1 km).

Present weather (falling precipitation)

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

Infrared

Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...

 beam (approximately 50 millimeters in diameter) and determines from a pattern analysis of the particle size and fall velocity whether the precipitation is rain

Rain

Rain is liquid precipitation, as opposed to non-liquid kinds of precipitation such as snow, hail and sleet. Rain requires the presence of a thick layer of the atmosphere to have temperatures above the melting point of water near and above the Earth's surface...

 or snow

Snow

Snow is a form of precipitation within the Earth's atmosphere in the form of crystalline water ice, consisting of a multitude of snowflakes that fall from clouds. Since snow is composed of small ice particles, it is a granular material. It has an open and therefore soft structure, unless packed by...

. 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 hail

Hail

Hail is a form of solid precipitation. It consists of balls or irregular lumps of ice, each of which is referred to as a hail stone. Hail stones on Earth consist mostly of water ice and measure between and in diameter, with the larger stones coming from severe thunderstorms...

, ice pellets

Ice pellets

Ice pellets are a form of precipitation consisting of small, translucent balls of ice. Ice pellets usually are smaller than hailstones. They often bounce when they hit the ground, and generally do not freeze into a solid mass unless mixed with freezing rain...

, 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 dewpoint to determine an obscuration to vision. If relative humidity is low (i.e, there is a large difference between the temperature and dewpoint), haze

Haze

Haze is traditionally an atmospheric phenomenon where dust, smoke and other dry particles obscure the clarity of the sky. The World Meteorological Organization manual of codes includes a classification of horizontal obscuration into categories of fog, ice fog, steam fog, mist, haze, smoke, volcanic...

 is reported. If relative humidity

Relative humidity

Relative humidity is a term used to describe the amount of water vapor in a mixture of air and water vapor. It is defined as the partial pressure of water vapor in the air-water mixture, given as a percentage of the saturated vapor pressure under those conditions...

 is high (i.e, there is a small difference between the temperature and the dewpoint), mist

Mist

Mist is a phenomenon of small droplets suspended in air. It can occur as part of natural weather or volcanic activity, and is common in cold air above warmer water, in exhaled air in the cold, and in a steam room of a sauna. It can also be created artificially with aerosol canisters if the...

 or fog

Fog

Fog is a collection of water droplets or ice crystals suspended in the air at or near the Earth's surface. While fog is a type of stratus cloud, the term "fog" is typically distinguished from the more generic term "cloud" in that fog is low-lying, and the moisture in the fog is often generated...

 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 (11.3 km). If the temperature is below freezing

Freezing

Freezing or solidification is a phase change in which a liquid turns into a solid when its temperature is lowered below its freezing point. The reverse process is melting....

, humidity is high and visibility is 1/2 mile or less, freezing fog is reported.

Cloud coverage and ceiling

Automated airport weather stations use an upward-pointing laser

Laser

A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...

 beam ceilometer

Ceilometer

A ceilometer is a device that uses a laser or other light source to determine the height of a cloud base. Ceilometers can also be used to measure the aerosol concentration within the atmosphere.-Optical Drum 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

Ceiling (cloud)

- ICAO :The height above the ground or water of the base of the lowest layer of cloud below 6000 meters covering more than halfthe sky.- United Kingdom :...

, 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 12000 feet (3,657.6 m); clouds above that height are not detectable by automated stations at present.

Temperature and dew point

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

Resistance thermometer

Resistance thermometers, also called resistance temperature detectors or resistive thermal devices , are sensors used to measure temperature by correlating the resistance of the RTD element with temperature. Most RTD elements consist of a length of fine coiled wire wrapped around a ceramic or glass...

 measures the ambient air temperature.

In contrast, the dew point measurement is considerably more complex. The dew point sensor contains 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 HO-83 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 HO-83 and the humidity obtained from the DTS1 to determine dewpoint.

Older AWOS systems used a Lithium Chloride Dew Point sensor. Current AWOS systems use capacitive relative humidity sensors, from which Dew Point is calculated.
http://www.nws.noaa.gov/ops2/Surface/documents/DewPoint0816.pdf

Altimeter and sea level pressure

The automated airport weather station pressure

Pressure

Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...

 sensor is the most reliable and accurate of all the automated sensors. The pressure sensor configuration consists of two separate pressure transducers. Each pressure transducer is a highly accurate pressure measurement instrument that uses advanced microcomputer

Microcomputer

A microcomputer is a computer with a microprocessor as its central processing unit. They are physically small compared to mainframe and minicomputers...

-based electronics

Electronics

Electronics is the branch of science, engineering and technology that deals with electrical circuits involving active electrical components such as vacuum tubes, transistors, diodes and integrated circuits, and associated passive interconnection technologies...

 and firmware, resulting in 99.98% accuracy. The capacitive sensors are located on a tray at the bottom of the computer cabinet and are permanently evacuated to a vacuum

Vacuum

In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...

 on one side to make each an absolute or barometric pressure sensor. The pressure sensors share a common 3/8-inch Tygon

Tygon Tubing

Tygon is a brand name for a family of flexible tubing consisting of a variety of base materials. Tygon is a registered trademark of Saint-Gobain Corporation. It is an invented word, owned and used by Saint-Gobain and originated in the late 1930s. Tygon products are produced in three countries, but...

 sensor tube for sensing barometric pressure. Since the ambient barometric pressure is input to each pressure transducer through a shared Tygon sensor tube, each pressure transducer receives the same barometric pressure input level. This configuration ensures reliable reporting of barometric pressure information. A transducer assembly internal to each unit converts the pressure level into an electrical signal level. This level is then monitored and translated by a microprocessor

Microprocessor

A microprocessor incorporates the functions of a computer's central processing unit on a single integrated circuit, or at most a few integrated circuits. It is a multipurpose, programmable device that accepts digital data as input, processes it according to instructions stored in its memory, and...

-based circuit

Electronic circuit

An electronic circuit is composed of individual electronic components, such as resistors, transistors, capacitors, inductors and diodes, connected by conductive wires or traces through which electric current can flow...

 within the pressure sensor to produce a barometric pressure value. During normal operation, the automated station reads the pressure value from each pressure transducer, compares the values to verify the accuracy of the measured data and calculates the barometric pressure.

Precipitation accumulation

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 (0.3048 m) diameter collector with an open top. The collector, which is heated to melt any frozen precipitation such as snow or hail

Hail

Hail is a form of solid precipitation. It consists of balls or irregular lumps of ice, each of which is referred to as a hail stone. Hail stones on Earth consist mostly of water ice and measure between and in diameter, with the larger stones coming from severe thunderstorms...

, funnels water

Water

Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

 into a two-chamber, pivoting container called a bucket

Bucket

A bucket, also called a pail, is typically a watertight, vertical cylinder or truncated cone, with an open top and a flat bottom, usually attached to a semicircular carrying handle called the bail. A pail can have an open top or can have a lid....

. Precipitation flows through the funnel

Funnel

A funnel is a pipe with a wide, often conical mouth and a narrow stem. It is used to channel liquid or fine-grained substances into containers with a small opening. Without a funnel, spillage would occur....

 into one compartment of the bucket until 0.01 inch (0.254 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

Switch

In electronics, a switch is an electrical component that can break an electrical circuit, interrupting the current or diverting it from one conductor to another....

 (either a reed switch

Reed switch

The reed switch is an electrical switch operated by an applied magnetic field. It was invented at Bell Telephone Laboratories in 1936 by W. B. Ellwood. It consists of a pair of contacts on ferrous metal reeds in a hermetically sealed glass envelope...

 or a mercury switch

Mercury switch

A mercury switch is a switch whose purpose is to allow or interrupt the flow of electric current in an electrical circuit in a manner that is dependent on the switch's physical position or alignment relative to the direction of the "pull" of earth's gravity, or other inertia.Mercury switches...

), which sends one electrical

Electricity

Electricity is a general term encompassing a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognizable phenomena, such as lightning, static electricity, and the flow of electrical current in an electrical wire...

 pulse for each 0.01 inch (0.254 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.

Icing (freezing rain)

Automated airport weather stations report freezing rain

Freezing rain

Freezing rain is the name given to rain that falls when surface temperatures are below freezing. The raindrops become supercooled while passing through a sub-freezing layer of air, many hundred feet , just above the surface, and then freeze upon impact with any object they encounter. The resulting...

 via the resonant frequency

Resonance

In physics, resonance is the tendency of a system to oscillate at a greater amplitude at some frequencies than at others. These are known as the system's resonant frequencies...

 of a vibrating rod. The resonant frequency decreases with increasing accretion (additional mass) of ice

Ice

Ice is water frozen into the solid state. Usually ice is the phase known as ice Ih, which is the most abundant of the varying solid phases on the Earth's surface. It can appear transparent or opaque bluish-white color, depending on the presence of impurities or air inclusions...

, hoarfrost

Frost

Frost is the solid deposition of water vapor from saturated air. It is formed when solid surfaces are cooled to below the dew point of the adjacent air as well as below the freezing point of water. Frost crystals' size differ depending on time and water vapour available. Frost is also usually...

, freezing fog, freezing drizzle

Drizzle

Drizzle is a light rain precipitation consisting of liquid water drops smaller than those of rain, and generally smaller than 0.5 mm in diameter. Drizzle is normally produced by low stratiform clouds and stratocumulus clouds. Precipitation rates due to drizzle are on the order of a millimetre...

, rime, or wet snow.

To report freezing rain, the system combines the sensor output from the freezing rain sensor with data from the LEDWI. The LEDWI must provide a positive indication of unknown precipitation or rain before the system can transmit a report of freezing rain. If the LEDWI reports either no precipitation or snow, the system will ignore the input from the freezing rain sensor. The sensor is designed to detect and report icing from all weather conditions.

Lightning (thunderstorms)

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

Lightning detector

A lightning detector is a device that detects lightning produced by thunderstorms. There are three primary types of detectors: ground-based systems using multiple antennas, mobile systems using a direction and a sense antenna in the same location , and space-based systems.The device was invented in...

 nationwide to triangulate

Triangulation

In trigonometry and geometry, triangulation is the process of determining the location of a point by measuring angles to it from known points at either end of a fixed baseline, rather than measuring distances to the point directly...

 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 km) of the station result in a report of a thunderstorm

Thunderstorm

A thunderstorm, also known as an electrical storm, a lightning storm, thundershower or simply a storm is a form of weather characterized by the presence of lightning and its acoustic effect on the Earth's atmosphere known as thunder. The meteorologically assigned cloud type associated with the...

 at the station (TS). Lightning strikes more than 5 miles (8 km) but less than 10 miles (16.1 km) from the station result in a report of a thunderstorm in the vicinity of the station (VCTS). Lightning more than 10 miles (16.1 km) but less than 30 miles (48.3 km) from the station results only in a remark of distant lightning (LTG DSNT).

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.

Data dissemination

Data dissemination is usually via an automated VHF airband

Airband

Airband or Aircraft band is the name for a group of frequencies in the VHF radio spectrum allocated to radio communication in civil aviation, sometimes also referred to as VHF, or phonetically as "Victor"...

 radio frequency

Radio frequency

Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...

 (108-137MHz) at each airport

Airport

An airport is a location where aircraft such as fixed-wing aircraft, helicopters, and blimps take off and land. Aircraft may be stored or maintained at an airport...

, broadcasting

Broadcasting

Broadcasting is the distribution of audio and video content to a dispersed audience via any audio visual medium. Receiving parties may include the general public or a relatively large subset of thereof...

 the automated weather observation. This is often via the Automatic Terminal Information Service

Automatic Terminal Information Service

Automatic Terminal Information Service, or ATIS, is a continuous broadcast of recorded noncontrol information in busier terminal areas. ATIS broadcasts contain essential information, such as weather information, which runways are active, available approaches, and any other information required by...

 (ATIS). Most automated weather stations also have discrete phone numbers to retrieve real-time observations over the phone or through a modem.

In the United States, the AWOS/ASOS Data Acquisition System (ADAS), a computer system run by the FAA, polls the systems remotely, accessing the observations and disseminating them worldwide electronically in METAR

METAR

METAR is a format for reporting weather information. A METAR weather report is predominantly used by pilots in fulfillment of a part of a pre-flight weather briefing, and by meteorologists, who use aggregated METAR information to assist in weather forecasting....

 format.

Limitations requiring human augmentation

At present, automated airport weather stations are unable to report a variety of meteorological conditions. These include:

• shallow or patchy fog
• blowing dust
  Dust
  Dust consists of particles in the atmosphere that arise from various sources such as soil dust lifted up by wind , volcanic eruptions, and pollution...
  
  
• smoke
  Smoke
  Smoke is a collection of airborne solid and liquid particulates and gases emitted when a material undergoes combustion or pyrolysis, together with the quantity of air that is entrained or otherwise mixed into the mass. It is commonly an unwanted by-product of fires , but may also be used for pest...
  
  
• falling ash
• volcanic eruptions
  Volcano
  2. Bedrock3. Conduit 4. Base5. Sill6. Dike7. Layers of ash emitted by the volcano8. Flank| 9. Layers of lava emitted by the volcano10. Throat11. Parasitic cone12. Lava flow13. Vent14. Crater15...
  
  
• tornado
  Tornado
  A tornado is a violent, dangerous, rotating column of air that is in contact with both the surface of the earth and a cumulonimbus cloud or, in rare cases, the base of a cumulus cloud. They are often referred to as a twister or a cyclone, although the word cyclone is used in meteorology in a wider...
  
  es
• precipitation that is not in the form of rain or snow, such as hail, ice pellets and snow grains
• multiple forms of precipitation falling at the same time
• depth of new snowfall
• total snow depth
• in-cloud and cloud-to-cloud lightning
• clouds that are not directly above the station
• clouds that are more than twelve thousand feet above ground level
• cloud type

Because many of these can pose dangers to aircraft and all of these are of interest to the meteorological community, most of the busier airports also have part-time or full-time human observers who augment, or provide additional information to, the automated airport weather station's observations. Research is on-going to allow the automated stations to detect many of these phenomena.

Automated stations can also suffer from mechanical breakdown, requiring repair or replacement. This can be either due to physical damage (either natural or human caused), mechanical wear, or severe icing during winter weather. During system outages, human observers are often required to supplement missing or non-representative observations from the automated station. Research is also ongoing to produce more robust systems which are less vulnerable to natural damage, mechanical wear and icing.

External links

• Automated Surface Observing System - NWS
• Canada AWOS frequencies
• Social & Economic Benefits of ASOS from "NOAA Socioeconomics" website initiative
• Example Commercial AWOS Supplier
• ASOS on NOAA's Observing System Architecture (NOSA)