By Mark Dorsten —
Automated weather observation products are typically sited in reports prepared by Professional Engineers.
To gain a better understanding of how the underlying data is collected and analyzed, I will refer to Aviation Weather Services publication AC 00-45H section 3.1.3 General Types of Observations
3.1.3 General Types of Observations.
There are three general types of surface observations:
- Manual Observation. Weather observations done by a human weather observer who is certified by the NWS or the FAA.
- Automated Observation. Automated observations are derived from instruments and algorithms without human input or oversight. In the United States, there are two main kinds of automated observing systems: the automated surface observing system (ASOS) and the Automated Weather Observing System (AWOS). Detailed information on ASOS and AWOS can be found in the Aeronautical Information Manual (AIM), Chapter 7, Safety of Flight, Section 1, Meteorology. Automated METARs and SPECIs contain AUTO in the report (see paragraph 3.1.5.4).
- Augmented Observation. At select airports in the United States, the automated observing system will have input and oversight by human weather observers or tower controllers certified in weather observing. These are referred to as augmented stations. Human observers report weather elements that are beyond the capabilities of the automated system and/or are deemed operationally significant. The weather elements observed and reported by the human observer vary, depending on the selected airport. AUTO is not used in augmented reports.
The data sampling is described in 3.1.5.5 Wind Group and 3.1.5.5.1 Wind Gusts:
3.1.5.5 Wind Group.
METAR KOKC 011955Z AUTO 22015G25KT 180V250 3/4SM R17L/2600FT +TSRA BR OVC010CB 18/16 A2992 RMK AO2 TSB25 TS OHD MOV E SLP132
- Wind is the horizontal motion of air past a given point. It is measured in terms of velocity, which is a vector that includes direction and speed. It indicates the direction the wind is coming from.
- In the wind group, the wind direction is coded as the first three digits (220) and is determined by averaging the recorded wind direction over a 2-minute period. It is coded in tens of degrees relative to true north using three figures. Directions less than 100 degrees are preceded with a 0. For example, a wind direction of 90 degrees is coded as 090. A wind from the north is coded as 360.
- Immediately following the wind direction is the wind speed coded in two or three digits (15). Wind speed is determined by averaging the speed over a 2-minute period and is coded in whole knots (kts) using the units, tens digits, and, when required, the hundreds digit. When wind speeds are less than 10 kts, a leading 0 is used to maintain at least a two-digit wind code. For example, a wind speed of 8 kts will be coded 08KT. The wind group is always coded with a KT to indicate wind speeds are reported in knots. Other countries may use kilometers per hour (KPH) or meters per second (MPS) instead of knots.
3.1.5.5.1 Wind Gust.
- Wind speed data for the most recent 10 minutes is examined to evaluate the occurrence of gusts. Gusts are defined as rapid fluctuations in wind speed with a variation of 10 kts or more between peaks and lulls. The coded speed of the gust is the maximum instantaneous wind speed.
- Wind gusts are coded in two or three digits immediately following the wind speed. Wind gusts are coded in whole knots using the units, tens, and, if required, the hundreds digit. For example, a wind out of the west at 20 kts with gusts to 35 kts would be coded 27020G35KT.
Based on the NOAA methodology provided above, when an Engineering Report quotes AWOS or ASOS data, it must be remembered the wind speed is a 2-minute sampling taken close to the top of the hour. Wind gusts suffer a similar admission of data since they use a 10-minute sampling period.
With a 2-minute period, an AWOS or ASOS omits 96.7% of the wind speed data. We can infer eyewitness accounts of high wind periods including wind shifts are credible and may not be reflected in Engineering Reports if the changes occurred within the 58-minutes of data omitted from the wind speed calculations.
A similar situation exists with wind gust reporting. With a sampling period of 10-minutes, 83.3% of the data is omitted from the report.
If the PE preparing the report does not understand the limitations of the automated systems, erroneous conclusions are quite possible.
All of the published information included in this memorandum is included in the FAA Private Pilot training program. It is covered in the FAR/AIM, written tests, Ground School and CFI