Greatest Number of Consecutive Hours at or Below 32°F

Purpose: This product identifies the longest continuous period of freezing temperatures over the (1) last 24 hours, (2) last 7 days, and (3) last 30 days.

Data Source: Non-quality-controlled, top-of-the-hour surface observations from first-order U.S. Automated Surface Observing System (ASOS) stations via the Applied Climate Information System (ACIS) Hourly data feed.

Missing Data: This tool applies a threshold of 10% for missing data in each time period (24 hours, 7 days, and 30 days). If more than 10% of the available hours in a period are missing, the tool will display a pop-up stating "missing data."

Spatial Domain: Continental United States

Timescale: Calculations are performed daily around 11 a.m. ET (10 am CT), with maps generated and posted around 11:30 a.m. ET.

• Last 24 hours - Maps for the last 24 hours include data from 11 a.m. ET the previous day until 11:00 AM ET today.
• Last 7 days - Maps from the last seven days include data from 11 a.m. ET, seven days before 11:00 AM on the current day.
• Last 30 days - Maps from the last 30 days include data from 11 a.m. ET, thirty days before 11:00 AM on the current day.

"Bull's Eye" Anomaly Discussion

Occasionally, "bull’s eye" temperature anomalies appear on the maps due to the following reasons:

• Non-quality-controlled Data - Hourly data are based on raw decoded METAR observations, which have not been fully quality-controlled.
• Calibration Inconsistencies - ASOS instruments have calibration windows with slight variances in accuracy (±1.8°F). This means two instruments could measure temperature slightly off, with a potential difference of up to 3.2°F across sites. While forecasters account for these variances, the raw data is directly fed into the freeze maps without adjustment.
• Station Siting - The placement of weather stations plays a crucial role in data accuracy. Ideally, stations are placed on natural ground (not near asphalt or concrete), on flat terrain, away from buildings and bodies of water, and positioned to minimize microclimate influences such as pooling cold air or wind barriers. However, real-world conditions may limit optimal placement, leading to site-specific microclimate effects.
• Microclimate Considerations - Even when stations meet siting requirements, larger-scale microclimate effects may still influence temperature readings. For instance, a weather station at the bottom of a broad valley may record cooler temperatures or higher winds, even if it is sited correctly.

If a "bull’s eye" anomaly is spotted on a freeze map, it should be regarded cautiously and not taken as an entirely accurate measurement. Users are encouraged to report recurring anomalies to the MRCC for further investigation.

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Resource: 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 03 October 2024).