On the surface, you may wonder why an airport needs an AWOS in an era when weather data is available to everyone on the screen as soon as you turn on your phone and aircraft are being built, which are more technologically advanced than ever. But, when it comes to flying, some things will never change, and one of those things is that knowing the weather conditions is vital to airport procedures. Whether performing land or take-off operations or monitoring air traffic overhead, a slight change in weather conditions can change the whole advance. Those weather conditions must be accurate to the airport, not just the general area.

Conditions such as heavy fog, fast-changing crosswinds, or icy rain on the runway can dramatically differ over a relatively short distance. Still, an AWOS system situated by the runway reports the current conditions the pilots will be navigating. AWOS systems also support air traffic control when managing heavy air traffic during inclement, severe, or fast-changing weather. Knowing how much traffic they can safely handle and whether they should divert to a nearby airport where weather conditions are more favorable saves them time and money. It protects the safety of everyone in the sky.

An AWOS is made up of a collection of weather sensors and equipment. These sensors measure weather conditions such as wind speed and direction, temperature and humidity, precipitation type and accumulation, visibility and ceiling, ice accretion, and thunderstorm conditions with lightning strikes. Collected weather data is analyzed and reported in automated reports. These reports are broadcast to airline and airport personnel to keep them informed of current weather conditions at the airport. Anyone can listen to an AWOS report by calling the posted telephone number at an airport for their AWOS system.

What configuration of weather sensors an AWOS sensor supports will vary by the level of AWOS an airport has. AWOS systems are customizable and configurable to an airport’s specific weather demands and budgetary concerns, with the most basic systems monitoring barometric pressure and altimeter settings and the most complex measuring everything up through freezing rain and runway surface conditions.

Some standard weather sensors that make up an AWOS system include a mechanical or ultrasonic wind sensor to measure wind speed and direction. Ultrasonic wind sensors compute measurements based on the time it takes for an ultrasonic pulse to travel from one transducer to another, which varies depending on many factors, including wind speed. This transit time is then measured in both directions for several of the transducer heads. Based on these results, the sensor computes wind speed and direction.

Ceilometers measure cloud height and coverage in the skies immediately overhead at the airport. Ceilometers work through an upward-pointing laser beam that detects the amount and height of clouds overhead. The laser is pointed directly upward, and the time required for the reflected light to return to the station allows for calculating the height of the cloud base.

A tipping bucket rain gauge is commonly used in AWOS systems to measure precipitation accumulation, whether it is from rain or snowfall. The upper portion of a tipping bucket rain gauge consists of a collector with an open top. The top is grated to keep debris from accumulating in the sensor and blocking water collection. The collector, which is usually heated to melt any frozen precipitation such as snow or hail so accurate readings can be collected, funnels water into a two-chamber, pivoting container called a bucket. As water accumulates in the bucket, the 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 that sends an electrical pulse for each 0.01-inch (0.25 mm) of precipitation collected.

Visibility sensors use forward scatter technology with a beam of infrared light sent from one end of the sensor toward the receiver. Still, they offset slightly from a direct line to the receiver by an angle. The amount of light scattered by particles in the air and received by the receiver determines the visibility.

AWOS systems also include lightning sensors, ice accretion sensors, present weather sensors, temperature and humidity gauges, and barometric pressure readings. All these different sensors report to a central computer program that produces the weather reports with up-to-the-minute readings.

While both collect and report weather data, there are some key differences between these two systems.

➤ AWOS, or Automated Weather Observing System, generate and transmit minute-by-minute updates of the conditions at the airport and report on a variety of meteorological factors. AWOS has nine levels of reporting starting at AWOS A, to AWOS IV-Z. However, most airports will have some sort of an AWOS III. While AWOS reports are automated, they may include human observations issued as SPECI reports. If the AWOS is unmonitored, an “AUTO” tag is included in the textual output.
The US has federally owned and operated AWOS and non-Federal AWOS that are commercially manufactured.

➤ ASOS, or Automated Surface Observing Systems, are primarily operated and controlled by the NWS, DOD, and, occasionally, the FAA. They have a level of reporting comparable to an AWOS-III and AWOS IV Z. They offer hourly reports on barometric pressure, wind speed and direction, DA, visibility, sky condition, ceiling height, precipitation, and in some cases thunderstorm and freezing rain detection. ASOS systems are the National Weather Service’s primary climatology network for observing and reporting the weather in the United States and may be found in locations other than airports if they serve to improve weather awareness in areas pertinent to the public.

Mesotech's FAA Certified Airport Weather Advisor™ systems are available in the AWOS capability levels defined by the FAA.

Each Airport Weather Advisor™ system has a set of common components for data collection and processing and the AWOS level defines what sensing and reporting capabilities it has. Most airports opt for one of the AWOS III (AWOS 3) levels because the addition of the Ceilometer.

AWOS LEVEL CAPABILITIES INCLUDE:

➤ AWOS A: Altimeter settings
➤ AWOS A/V: Altimeter setting and visibility
➤ AWOS I: Wind speed and direction, temperature, dew point, altimeter setting, and density altitude
➤ AWOS II: Wind speed and direction, temperature, dew point, altimeter setting, density altitude, and visibility
➤ AWOS III / AWOS-3: Wind speed and direction, temperature, dew point, altimeter setting, density altitude, visibility, precipitation accumulation, cloud height, and sky condition
➤ AWOS IIIP / AWOS-3P: Wind speed and direction, temperature, dew point, altimeter setting, density altitude, visibility, precipitation accumulation, cloud height, sky condition, and present weather
➤ AWOS IIIPT / AWOS-3PT: Wind speed and direction, temperature, dew point, altimeter setting, density altitude, visibility, precipitation accumulation, cloud height, sky condition, present weather, and thunderstorm/lightning
➤ AWOS IV Z / AWOS-4Z: Wind speed and direction, temperature, dew point, altimeter setting, density altitude, visibility, precipitation accumulation, cloud height, sky condition, present weather, thunderstorm/lightning, and freezing rain

AWOS systems are operated, maintained, and controlled by certified maintainers and technicians working for aviation service providers. The Federal Aviation Administration, or FAA, regulates and monitors most AWOS systems through the Non-Federal program. This system oversees their technical specifications, commissioning, operation, and maintenance. The Non-Federal program also oversees the non-federally employed technicians who maintain the AWOS systems in the field and the manufacturers who engineer and build them. FAA-certified AWOS systems are required by law to be operated and maintained at the same standard level as Federally owned and operated systems.