69% of US surface weather stations now near heat sources:
61% are class four (below), 8% are class five:
Station quality ratings obtained from NOAA/NCDC via this source:
Climate Reference Network Rating Guide - adopted from NCDC Climate Reference Network Handbook, 2002, specifications for siting (section 2.2.1) of NOAA's new Climate Reference Network:
Class 1 (CRN1)- Flat and horizontal ground surrounded by a clear surface with a slope below 1/3 (<19deg). Grass/low vegetation ground cover <10 centimeters high. Sensors located at least 100 meters from artificial heating or reflecting surfaces, such as buildings, concrete surfaces, and parking lots. Far from large bodies of water, except if it is representative of the area, and then located at least 100 meters away. No shading when the sun elevation >3 degrees.
Class 2 (CRN2) - Same as Class 1 with the following differences. Surrounding Vegetation <25 centimeters. No artificial heating sources within 30m. No shading for a sun elevation >5deg.
Class 3 (CRN3) (error >=1C) - Same as Class 2, except no artificial heating sources within 10 meters.
Class 4 (CRN4) (error >= 2C) - Artificial heating sources <10 meters.
Class 5 (CRN5) (error >= 5C) - Temperature sensor located next to/above an artificial heating source, such a building, roof top, parking lot, or concrete surface."
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