Basic Forest Fire Suppression Course - Online Lessons


3.1 Diurnal Effects on Weather

The normal diurnal effect (daytime period) on relative humidity, temperature and wind will influence fire behavior during the day.

The Relative Humidity / Air Temperature relationship is a constantly changing process. At night when the RH values are usually higher and the air temperature is lower, fire activity is usually at it`s lowest level (between midnight and sunrise). As the sun heats the face of the earth, temperatures begin to rise and the RH values begin to drop, fire risk will be most likely between 10 am and 6 pm.

Although fire risk will be greatest between 10am and 6 pm, fire behaviour will be most aggressive by mid to later afternoon

Relative humidity is lowest Temperature is highest

There is a very distinct relationship between Temperature and Relative Humidity. This is noticeable in the graph indicating the diurnal effects, however there is another situation that should be closely monitored. As air temperature readings increase and relative humidity readings decrease during the day, burning conditions will become more intense. Certain atmospheric conditions can produce severe fire behavior reaching a point when the air temperature reading and the relative humidity reading will read the same (ie. Temp. 30° C , RH 30%), at this point a condition has been achieved called “CROSS OVER”. A fire burning under this condition will exhibit extreme behaviour and should be treated with extreme caution. This condition should not be treated lightly and may continue for some time. It can become even worse if the relative humidity reading continues to drop (monitor the situation closely).

Precipitation does not follow a normal daily cycle. Dew may form in the early morning hours, normally between 4 am. and 6 am.


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