WILDFIRE: LATEST LEARNINGS- v01.6

WILDFIRE: LATEST LEARNINGS

[Home] [Index] [News]

[Previous] [Next] Click to view: [General content] ...then... [Operational content] ...then... [Expert content]

[Science]

.
General learning outcome:

GENERAL FIRE CONCEPTS:

Specific learning outcome:

The interaction of wind & terrain is a fundamental concept for understanding fire behaviour.

Topic:

The various forms of interactions of wind and terrain: lee-slope eddies, wind channelling, fire channelling, thermal winds, low-level jets and general wind-terrain interactions.

Operational Awareness for Advanced Firefighters & Fire Behaviour Analysts


Lee-slope eddies occur at the top of a ridge, when the upslope wind separates from the ground when the lee slope begins. After descending and re-attaching the air flow back upslope, in the opposite direct to the prevailing wind. These eddies are dominant when wind speeds exceed 20 km/hr. Below that speed they may or may not be present. Above 30 km/hr they are almost certain.
Wind channelling occurs when the prevailing winds blow across a steep valley. Some air flow is deflected down into the valley, and may then flow sideways, up or down the valley. Thus fires influenced by these winds may move in unexpected directions.
Thermal winds occur in rugged terrain when solar heating occurs on some slopes and not on others. Heated air rising over the former may be replaced by cooler air from the latter. This can produce circulations within valleys. These localised winds are dominant when prevailing winds are light, especially at night. Through the day, heating of the various aspects in a gully will change, as will the heating of a mountain range change between morning and evening.
Low-level jets occur when night-time inversions separate prevailing winds from the friction caused by rugged terrain. These stronger winds may be experienced by high ground that sticks up through the inversion.
General wind-terrain interaction involves wind accelerating up windward slopes, deflecting around side slopes and forming eddies on lee slopes. Subtle changes in prevailing wind direction may shift specific sites into quite different local wind fields, causing major changes in local fire behaviour.
.
This website is dedicated to bringing forward the latest research findings. They are provided to allow firefighters to be made aware of critical safety issues ahead of the material being included in the national training curriculum.
Material prepared by Rick McRae for AFAC Research Dissemination Pilot Study, March 2012.