ASPM Weather Processing
Weather data are retrieved from three sources:
Meteorological Aviation Routine Weather Report (METAR): Published hourly without quality control. A typical METAR contains data for the temperature, dew point, wind speed and direction, precipitation, cloud cover and heights, visibility, and barometric pressure. A METAR may also contain information on precipitation amounts, lightning, and other information that would be of interest to pilots.
Automated Surface Observing System (ASOS): Hourly data with some quality controls. Available next day but not current through the end of the prior day. ASOS is a joint effort of the National Weather Service (NWS), the Federal Aviation Administration (FAA), and the Department of Defense (DOD). ASOS is designed to support weather forecast activities and aviation operations
Quality Controlled Local Climatological Data (QCLCD): Quality controlled month-to-date file also not current through the end of the prior day. The month is finalized on the 6th of the following month regardless of it being complete. Therefore, occasionally ASPM does not receive QCLCD data for the last day or two of the month.
Data Processing Logic
ASPM uses QCLCD when available due to its top-level of quality control, followed by ASOS, and then METAR to fill in next-day data when the other two sources are not current.
Each data source is loaded using the best available data. QCLCD is read first, then missing QCLCD data are filled in by ASOS, and missing ASOS is filled in with METAR data.
Weather records are typically published hourly at a minimum, with special interim reports when weather changes during the hour. Each record is assigned to a quarter hour and is carried over a maximum of three quarters, or until the next reported record. If more than one record is present in a quarter hour, the record with the worst weather is selected, as evaluated by the Ceiling and Visibility values.
From the quarter hour records, the record with the worst weather is selected (also based on Ceiling and Visibility) and will become the hourly record used by the ASPM morning process for the purpose of determining IMC/VMC conditions and evaluating weather impact in the Weather Factors module.
Hours for which no weather data are available from any of the three data sources are identified by a value of 999 for Sky Conditions and 30nm for Visibility. The highest legitimate value for Visibility is 10nm, although values of 15 and 20 do occur.