Relative Humidity, (RH)
The amount of moisture in the air expressed as a percentage of how much it can hold at given temperature.
The higher the air temperature the greater the capacity to hold moisture. NORMALLY;
RH higher in am. RH drops through day.
RH rises towards evening.
The amount of ambient heat energy in the air.
Warmer air can hold more moisture therefore RH is usually lower.
Affects fuel drying rates.
Affects fuel ignition.
Affects firefighter performance.
Most influential weather factor. Wind Speed determines magnitude of effect.
Wind Direction determines WHERE the effect is greatest on the fire.
Change of wind direction = change of fire head.
Transfers heat, fire brands, rate of spread.
A wind-driven wildfire shown in image above.
Any form of moisture that comes from the atmosphere.
QUANTITY and DURATION are important.
Light rain will not materially affect moisture content of large logs but will affect fine fuels, such as lichen.
Precipitation sometimes does not moisten the underside of fuels.