Blog from August, 2024

A recent wildfire in Jasper, Alberta, Canada, which began on 07/24/24, highlighted the impact of cloud on active fire detections. As of 08/06/24, the wildfire had consumed 34,000 hectares of Jasper, including much of the downtown area. While some limited detected fire activity was captured by MODIS and VIIRS to the south/southeast of Jasper on the daytime observations between 12:30PM and 3:00PM PDT on 07/024/24, no additional detections were captured for several consecutive days following these initial detections. FIRMS received multiple questions from users about the wildfire in Jasper and, therefore, this blog explains the reasons for the limited active fire detections during this period.  

FIRMS provides active fire detections from both polar orbiting satellites and geostationary satellites. The polar orbiting satellites each typically collect two observations per day over Jasper, Alberta, based on the latitude of Jasper – once during the day and again about 12 hours later in the night.  As mentioned above, on 07/24/24 there was some limited fire activity detected by MODIS and VIIRS further south/southeast of Jasper captured on the daytime observations between 1:30PM and 4:00PM MDT.  However, if users view the MODIS and VIIRS imagery that were acquired on 07/24/24 (the imagery can be loaded by selecting the VIIRS and MODIS Corrected Reflectance imagery within the Dynamic Imagery dropdown), they will notice that the Jasper area was under heavy cloud cover.  Unfortunately, the satellite sensors cannot discern fire activity under heavy clouds or smoke cover and, therefore, under these conditions there will be very limited active fire detection. Thick cloud continued to persist for several days and continued to impact the active fire detections. The images below, acquired on 07/25/24, 07/26/24, and 07/28/24 show thick cloud over the area: 

07/25/24

07/26/24

07/28/24

FIRMS also provides active fire detections from geostationary satellites. The GOES geostationary satellites (GOES-16 and GOES-18) that provide coverage over this area, conduct observations at a higher temporal frequency (multiple times per hour), but at a coarser spatial resolution (2km). However, if there is heavy cloud cover or thick smoke, these satellites also cannot detect fire activity. Users can explore the GOES West Geocolor imagery over the area of Jasper to assess the cloud cover (the FIRMS interface needs to be toggled to ADVANCED mode to access these imagery). The following FIRMS FAQs also provide additional information on situations when active fires may not be detected - https://www.earthdata.nasa.gov/faq/firms-faq#ed-not-detected and https://www.earthdata.nasa.gov/faq/firms-faq#ed-modis-fire-size.