WeatherWeather Wise


Weather Wise: Burning Conditions

Burning Conditions
Posted at 6:48 PM, Apr 12, 2021

HELENA — A rash of early season wildland fires have hit Montana, many of which were debris burns and prescribed burns that got out of control in hazardous weather conditions.

However, if people are looking to do some burning, this week is the week to do it.

Generally cloudy, cool weather, with higher humidity and light precipitation for days are ideal conditions for keeping burns under control. Preferably, burning with snow on the ground really limits potential fire growth and spread.

Temperature has an overriding effect on a fire's behavior. Below 32 degrees and even grass has a difficult time burning. Above 80 degrees and burning is hazardous. The sweet spot is between 40 and 60 degrees with light wind.

A general guideline is the 60:40 rule: burn with a temperature of less than 60, humidity greater than 40%, and a wind speed of 5 to 15 mph.

Another important thing to remember is the rule of halves related to changes in humidity and fire behavior. When air temperature increases by 20 degrees, the humidity typically decreases by 50%. So if temperature changes from 60 degrees with 40% humidity to 80 degrees, the humidity changes to 20% which would pose a safety risk.

Do not burn if a frontal passage or wind shift is likely within 12 hours.

If conditions are not right, wait until everything is right. If you started the fire and it is not going well, put it out.

Finally, never leave the fire until it is completely out, and you feel safe putting your hands in the ground where the burning embers were.