Automated Surface Observing System ASOS

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Automated Surface Observing System (ASOS)
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Links to ASOS Information

• http//www.nws.noaa.gov/ost/asostech.html
• http//www.nws.noaa.gov/asos/index.html
• http//www.nws.noaa.gov/asos/pdfs/aum-toc.pdf

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ASOS Instruments
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ASOS Instruments

• Tipping Bucket (Rain Gauge)
• Hygrothermometer (Temperature/Dew Point Sensor)
• Present Weather Identifier (LEDWI)
• Wind Speed and Direction Sensors
• Data Collection Package
• Ceilometer (Cloud Height Indicator)

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ASOS Instruments (cont.)

• Freezing Rain Sensor
• Thunderstorm Sensor
• Visibility Sensor
• Acquisition Control Unit (ACU) includes Pressure
  Sensor

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ASOS Tipping Bucket Rain Gauge
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Liquid Precipitation Accumulation Sensor (Heated
Tipping Bucket Rain Gauge)

• Wind shield to prevent blowing snow from falling
  into the collection funnel.
• 12 inch diameter collector funnel.
• Pivoting dual chamber tipping bucket.
• The bucket tips when a chamber holds the
  equivalent of 0.01 inches of rain.
• Electronic switch that counts the number of tips
  per minute.

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Rain Gauge (cont.)

• Drain pan and tube.
• Heating elements to prevent freezing during cold
  weather and melt frozen precipitation.
• The heating elements are around the underside of
  the collection funnel and the drain tube.

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ASOS Hygrothermometer
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Hygrothermometer

• Dew Point Temperature is the temperature at
  which air becomes saturated when it is cooled at
  a constant pressure.

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Hygrothermometer (cont.)

• Temperature is measured by a platinum wire
  Resistive Temperature Device (RTD).
• The RTD is based on the principle that electrical
  resistance is a function of temperature.

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Hygrothermometer (cont.)

• Dew point temperature is measured with a chilled
  mirror device. When a beam of light is
  transmitted onto the mirror and a sensor measures
  the reflected light. The mirror is cooled and
  droplets or frost forms at the dew point
  temperature. The sensor measures a change in the
  reflected light and the dew point temperature is
  recorded.

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Chilled Mirror Device
Light source (LED)
Photo Resistors
Chilled Mirror
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Hygrothermometer (cont.)

• Temperature and dew point temperature are
  continually measured and sample values are
  provided approximately six times per minute
  (every 10 seconds).
• The ACU uses these samples to compute a
  one-minute average of the temperature and dew
  point temperature each minute.

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Hygrothermometer (cont.)

• Once each minute the ACU calculates a five-minute
  average of the temperature and dew point
  temperature based on the last five one-minute
  averages.
• At the appropriate time the current temperature
  and dew point temperature are reported to the
  nearest tenth of a degree Celsius.

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Present Weather (Precipitation) Identifier (LEDWI)
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Present Weather Identifier (cont.)

• The Present Weather (Precipitation)
  Identification Sensor is also known as the Light
  Emitting Diode Weather Identifier (LEDWI).
• The LEDWI contains a coherent light transmitter
  and a photo diode receiver.

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Present Weather Identifier (cont.)

• The transmitter emits a coherent infrared beam
  directed at the receiver.
• When the receiver records a change in the
  infrared radiation reaching it, then ASOS uses
  the characteristics of the received radiation to
  determine precipitation type and intensity.

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Wind Speed and Direction Sensors
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Wind Sensors (cont.)

• Wind Speed is measured by a rotating three-cup
  anemometer that drives a photointerrupter device.
  The starting threshold for the device is at 2
  knots and wind speeds of 2 knots or less are
  reported as calm.

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Wind Speed Conversions

• 1 knot 1 nautical mile per hour
• 1 knot 1.15 statute miles per hour
• 1 knot 0.51 meters per second
• 1 knot 1.85 kilometers per hour

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Wind Sensors (cont.)

• Wind direction is measured with a directional
  vane assembly coupled to a precision
  potentiometer.
• Federal standards for wind sensors specify a
  height of 10 meters.

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Wind Sensors (cont.)

• Wind speed and wind directions are recorded every
  five seconds.
• ASOS reports a two-minute average of the
  five-second average of wind speed and wind
  direction once a minute.
• Peak Wind is the greatest five-second average
  wind speed exceeding 25 knots in the past hour.

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Cloud Height Indicator
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Cloud Height Indicator (cont.)

• A Laser Ceilometer is used to measure the height
  of the bottom of the clouds.
• A gallium arsenide laser projects a beam of
  infrared radiation upwards.
• The instrument measures the elapsed time until
  the beam is reflected back to the sensor and
  since the radiation travels at the speed of light
  it converts time to the height of the clouds.

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Cloud Height Indicator (cont.)

• Because of scattering by the atmosphere, this
  instrument has an effective vertical range to
  12,600 feet and it only reports ceilings to a
  height of 12,000 feet.
• Measured heights of clouds may be augmented by
  human observers at some locations.

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Freezing Rain Sensor
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Freezing Rain Sensor (cont.)

• The Freezing Rain Sensor consists of a small
  cylindrical ultrasonically vibrating probe.
• Accumulation of ice on the probe decreases the
  frequency at which it vibrates.

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Freezing Rain Sensor (cont.)

• If the Freezing Rain Sensor indicates that ice is
  accumulating and the Precipitation Identification
  Sensor indicates that rain is falling, then ASOS
  reports the present weather as Freezing Rain.
• The Freezing Rain Sensor can detect freezing rain
  when the rate is as low as 0.01 inch per hour.

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Thunderstorm (Lightning Detection) Sensor
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Thunderstorm Sensor (cont.)

• The Lightning Detection Sensor reports a
  thunderstorm when and optical flash and an
  electrical field change (i.e. a radio signal)
  occur within milliseconds of each other.

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Thunderstorm Sensor (cont.)

• The sensor can detect cloud-to-ground and
  cloud-to-cloud lightning strikes.
• All strikes are counted, but only cloud-to-ground
  strikes are used to generate an estimate of the
  range.
• Strikes are grouped in three range bins 0 to 5
  miles, 5 to 10 miles, and 10 to 30 miles.

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Thunderstorm Sensor (cont.)

• The ASOS algorithm reports a thunderstorm when
  two lightning reports occur within 15 minutes of
  each other in an attempt to reduce false alarms.
• ASOS generates a TS report if the lightning is
  within 5 miles of the sensor.
• ASOS generates a VCTS report if the lightning is
  between 5 and 10 miles of the sensor.

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Other Sources of Lightning Reports

• Some airports have the FAA Automated Lightning
  Detection and Reporting System (ALDARS).
  Lightning information from ALDARS is provided
  through the National Lightning Detection Network.

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Forward Scatter Visibility Sensor
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Visibility

• Visibility is expressed in terms of the
  horizontal distance at which a person can see and
  identify specified objects.
• Obstructions to visibility include mist (BR),
  haze (HZ) and fog (FG).

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Visibility Sensor (cont.)

• The visibility sensor uses a pulsed xenon flash
  lamp that transmits light in the blue portion of
  the visible spectrum twice a second.
• A detector looks north to detect the amount of
  the blue light that is forward scattered into it
  and an algorithm converts the result to a
  visibility report.

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ACU and Pressure Sensor
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Pressure Sensor (cont.)

• The pressure sensor is located in a tray near the
  bottom of the ACU.
• The pressure sensor uses digital pressure
  transducers, that contain capacitive sensors.
• A vacuum on one side of the sensor allows it to
  measure the pressure on the other side of the
  sensor.

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Pressure Sensor (cont.)

• The pressure sensors are the most reliable and
  accurate sensor in ASOS.
• Note ASOS measures the actual air pressure. An
  algorithm converts the measured pressure to the
  sea level pressure and altimeter settings that
  are reported in the METAR observations.