Several fires were burning in northern Louisiana and the surrounding region on 6 August 1997. Such fires exhibit a distinct "hot spot" signature (black pixels indicating hot tempertures) on the GOES-8 and GOES-9 shortwave infrared (3.9 micron) imagery shown above.
Maximum Fire Pixel Temperature GOES-9 / GOES-8
Mean Background Pixel Temperature GOES-9 / GOES-8
304.1 K / 324.5 K
300.5 K / 303.5 K
310.7 K / 317.0 K
300.9 K / 303.9 K
309.2 K / 320.0 K
300.8 K / 303.6 K
Differences between the mean background brightness temperatures detected by GOES-8 and GOES-9 are on the order of 3 degrees K, yet the maximum brightness temperatures for each of the 3 fires are much warmer as observed from GOES-8. This is due in part to the viewing geometry for GOES-8 (located at 75 W longitude) versus GOES-9 (located at 135 W longitude) and the associated atmospheric attenuation differences. The primary reason for the large fire pixel brightness temperature disparity can be attributed to the fact that the GOES-8 field-of-view is smaller than that of GOES-9 in this region. Any given fire in this part of the United States occupies a larger fraction of the GOES-8 field-of-view, resulting in a higher overall fire pixel brightness temperature.
The GOES-8 and GOES-9 shortwave IR images below show a large (10,000 acre) wildfire which was spreading rapidly in the Los Padres National Forest area of southern California on the same day. In this case, the viewing geometry is such that a given fire would occupy a larger fraction of the GOES-9 field-of-view, yeilding a warmer fire pixel brightness temperature than GOES-8. However, the maximum temperature measured for this particular fire was 325.0 K from GOES-9 and 328.8 K from GOES-8. The shortwave IR detector on the GOES-9 satellite becomes "saturated" at a brightness temperature of around 325 K -- even a pixel that should appear warmer would still have a value of 325 K. The GOES-8 shortwave IR detector saturates around 335 K, so for this fire it was able to detect a warmer pixel brightness temperature than GOES-9 (in spite of the less favorable viewing geometry).