About the MRCC Hours At Or Below Freezing GIS Product
Greatest Number of Consecutive Hours at or Below 32°F:
Purpose: Determine the longest period of freezing temperatures over the (1) last 24 hours; (2) last 7 days; and (3) last 30 days.
Data: Hourly surface observations (top-of-the-hour) from ASOS/AWOS network available via METAR reports from the MRCC cli-MATE data access system (Station > Hourly > METAR).
Spatial Domain: Continental United States
MAPS: The calculation is run once a day around 10 a.m. CT; Maps are created around 11:30 a.m. CT and often posted by 11:40 a.m. CT.
TIME SERIES: Data are pulled and processed real-time when the user selects this option/link.
"Bull's Eye" Anomaly Discussion
The freeze maps available through VIP are developed from Global Historical Climatology Network (GHCN) surface observation data. This data set is based on a select series of stations selected on length of record, homogeneity of record, number of site location moves, and spatial homogeneity across the globe. Occasionally temperature “anomalies” or “bull’s eyes” can appear on our maps. These can occur for several reasons. Some of the GHCN locations are Automated Weather Observing System (AWOS) or Automated Surface Observing System (ASOS). These sites are most commonly located at airports around the country and provide hourly data, in addition to daily observation data we use in our maps.
Because AWOS and ASOS systems are automated, immediate quality control beyond basic systematic removal of measurements does not occur, and abnormally high or low temperature reports that are still within reason to the computer system can slip through and are sent out for data collection. Because we collect the data before it goes through the normal GHCN quality control procedures are conducted, some invalid data may come through. When this occurs, it can result in some of the “bull’s eyes” on our maps. It is known by National Weather Service forecasters that regularly monitor AWOS and ASOS data that some locations always report several degrees warmer or colder than a majority of other locations. This occurs because of instrument calibration windows being several degrees (meaning a temperature is considered accurate ± 1.8°F) (Nadolski 1998). So if several instruments measure 1.6°F below actual temperature while another measures 1.6°F above actual temperature, that is a difference of 3.2°F! Forecasters can adjust for knowledge of temperature discrepancies in their products, however the raw data is still sent directly to our maps.
Applicable to all observing systems is process of station siting. Station siting is the process of locating a weather observation station based on the criteria such as 1) placing it in a location with natural ground cover (not concrete or asphalt, top of a root, etc.); 2) placing it in a location with no or minimal slope (no hills) because this can result in pooling of colder air, shadowed areas, or changes to wind; and 3) placing the rain gauge so that the top of the rain gauge is no less than twice the height away from a building (~10-12 feet); and placing the site away from a water body. If not all criteria are met for a location, the best possible placement is found with notes usually made about placement choice (National Weather Service Directives). However, all locations are still subjected to larger-scale microclimate influences. A microclimate is a climate that may be several acres in size, such as a farm at the bottom of a broad valley. Because the farm is in a broad valley, it meets all siting requirements, but it is likely more susceptible to cooler temperatures, fog, or higher winds.
If you are using our Freeze Maps products and notice a bull’s eye on the map, we ask that you please take it into consideration in your analysis and do not consider it an accurate measurement, rather consider it with caution. If you notice a prolonged recurrence of a temperature anomaly and you’d like to report it, the following website is available for you to do so: https://datzilla.srcc.lsu.edu/datzilla/. This website will report the error directly to individuals who will assess the problem with more detail.
— — —
Nadolski, V.L., 1998: Automated Surface Observing System (ASOS) User’s Guide. National Oceanic and Atmospheric Administration, Department of Defense, Federal Aviation Administration, and the United States Navy. 74 pp. http://www.nws.noaa.gov/asos/pdfs/aum-toc.pdf (Accessed 12 March 2015).