General Weather Processes That Effect Fires

Objectives: Discuss general circulation patterns that influence fire behavior and fire seasons. Understand gradient or general winds. Understand how cold fronts affect fire behavior.

Circulation Around the earth...

but generally influenced General or Gradient Winds Large scale winds caused by high and low pressure systems, but generally influenced and modified in the lower atmosphere by terrain.

Weather fronts designate the boundry between two Pressure cells usually move across the United States from west to east. Weather fronts designate the boundry between two air masses of different temperature and characteristics.

L = air moves counter- clockwise and toward the center of the cell. H = air moves clockwise and outward from the center of the cell.

COLD FRONT A cold front is the boundary line between a cooler air mass which is replacing a warmer air mass. COLD FRONT

COLD FRONT WINDS W-NW airflow associated SE-SW with a cold front airflow ahead of a cold front

INDICATORS OF A COLD FRONT PASSAGE 1. Line of cumulus clouds approaching from the west or northwest. 2. Large clouds of dust can precede the arrival of a cold front. 3. Shift in winds from the southeast, south to the southwest & increase in velocity. 4. Winds will be strongest, erratic and gusty as the front reaches you. 5. Winds will continue to shift as the front passes, generally resulting in strong, gusty, cool winds from the west or northwest.

COLD FRONTS ARE POTENTIALLY DANGEROUS ON FIRES 1. Wind direction will abruptly change. 2. Strong southerly winds ahead of the front will drive the fire head to the north or northeast. 3. Winds shifting to the west or northwest after the front passes will drive the fire to the east or southeast 4. Rapid drop in relative humidity within 24 hours of the frontal passage

FIRE BEFORE COLD FRONT PASSAGE SE-SW airflow ahead of a cold front

FIRE AFTER COLD FRONT PASSAGE W-NW airflow associated with a cold front

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