2-meter Relative Humidity (%) - Relative Humidity (RH) values are displayed in increments of 5 percent (%) beginning at 5% through 45%. Burnt orange contours are displayed for RH values of 30% or less (with hatched areas indicative of 20% or less) while thin pink contours show RH values from 30% to 45%. The maximum threshold of 45% for display purposes is deemed appropriate because low RH is an indicator of high fire danger, and most areas of the United States do not have major wildfire difficulties with RH levels higher than 45%. The SPC fire weather verification scheme uses a range of minimum RH values that varies from 20% in the desert Southwest and interior sections of the western United States to 35% in parts of the southeastern United States.

Brotak and Reifsynder (1997) found that 93% of fires examined coincided with a low relative humidity in the lower atmosphere (dew-point depression > 10C at 850mb). Moisture in the atmosphere, whether in the form of water vapor, cloud droplets, or precipitation, is the primary weather parameter that affects fuel moisture content and thus the flammability of wildland fuels. The amount of moisture that fuels can absorb from or release to the air depends largely on relative humidity (Whiteman, 2000).

For in-depth information on the generation of Nam 2-meter Relative Humidity fields see the following website:

http://www.emc.ncep.noaa.gov/mmb/research/FAQ-eta.html#ETA9