The relationship between forest fire occurrence and 500 mb longwave ridging

Abstract

Abstract In general terms the probability of forest fires is increased by weather conditions that: a. maximize the drying of forest fuels,b. give a minimum of rain and yet still allow enough convective activity to produce lightning and spread going fires. The hot sunshine and dry weather of summer high pressure systems fulfill these conditions. At the earth's surface however high pressure is transitory while at the 500 millibar level, ridging is much more persistent. Such longwave ridges aloft may persist for as long as 80 days or even more but show oscillations in their strength. For the severe forest fire season of 1974 in northwestern Ontario, the changes in amplitude of the persistent longwave ridge over the area were examined. Using a diagnostic Hovmoller diagram, periodic oscillations of eleven days were discovered in its intensity which agreed closely with the periodicity in the Fire Weather Index, a measure of the forest's potential for fires. If such longwave behaviour is predictable then long term general statements about forest fire occurrence can be made.