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Operational Awareness for Advanced Firefighters & Fire Behaviour Analysts | ||||||
The thermal expansion of middle layers of the air can push warm, dry air downwards towards the surface. The highest ground, above say 1500m ASL, will be impacted by this first. Upper air frequently has very low dew point temperatures, unaffected by the evaporation of water from the surface. Thus even though the temperature at these heights may be much lower than in the lowlands, the relative humidity can be very low – say 10%.This can reduce fuel moisture content to dangerous conditions. If, as is common on high ground, the wind speeds are elevated, then the FDI can be elevated as well. Most importantly this often happens between midnight and sunrise, inverting the typical daily FDI cycle. Records show that this can happen on between one day in six and one day in nine. Thus over the course of a campiagn fire it is very likely to happen. . |