Learning to Burn
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Learning to Burn
Learning to Burn
Module Objective: Knowing the weather conditions needed for the burn
Weather is an important factor to consider when planning your burn. It can change very quickly with the potential to catch you out. The choice of weather conditions is key to controlling the fire and meeting the objectives of the burn.
For a given site, the topography and fuel type/loading are generally fixed constraints, while the weather parameters are the variables that a manager can choose for a desired fire. The main weather factors to monitor are temperature, relative humidity, wind, rainfall and atmospheric stability. These will have an influence on how a fire behaves (how much fuel is available to burn, how fast the fire will travel, and how hot the burn will).
As a farmer, being familiar with local weather and how the weather changes throughout the is a critical factor in successful prescribed burning.
It is extremely important to have the best, most up-to- date weather information available when planning a prescribed burn.
Weather factors can make a fire ineffective, difficult to control, or behave in unpredictable ways.
Understanding how weather factors influence fire behaviour can increase your ability to predict fire direction, behaviour, and rate of travel, which will reduce the risk of wildfire and increase burn crew safety.
Abrupt changes in weather predicted for the days following the burn (abrupt changes in wind direction, rising wind speeds, very low humidity, fronts, and thunderstorms) will require extra vigilance to ensure that smouldering fuels do not reignite and start wildfires.
ALWAYS CHECK WEATHER BEFORE BURNING, IF IN DOUBT, DO NOT BURN
A general understanding of the separate and combined effects of several weather, vegetation and terrain factors on fire behaviour and fuel condition is essential to plan and execute a good prescribed burn.
Only burn when weather and fuel conditions permit safe containment of fire and maintenance of cool, controllable burning conditions.
Carefully assess weather and vegetation (fuel) conditions that influence behaviour, predictability and above all, safety and containment of the fire. At all times during the burn be aware of changes in weather and fuel conditions.
Based on these constraints, aim to conduct burning in a safe and controlled manner likely to minimise the risk and nuisance o operatives and other people, and in a manner that does not harm the productivity of the land being burnt
| Element | Description |
| Air Temperature | Air temperature will affect the dryness of the fuels through its influence on relative humidity. The dryness of fuels will be reflected in the intensity of the fire (how hot it will burn). Fuels in a warm environment are pre-heated by the air temperature and the sunlight, drying it out and making ignition easier.Ambient air temperature is measured in °C. |
| Relative humidity | Humidity affects the moisture content of the vegetation you are trying to burn. This will influence whether the vegetation will ignite, how fast and hot it will burn. This is the amount of moisture in the air. When relative humidity is low, fuels dry and ignite easier. Embers remain hot longer and travel further, increasing the risk of spot fires. Wet fuels lose moisture to dry air (i.e. on sunny days). Relative humidity is measured as a percentage. The flammability of fine vegetations (grass) will be more sensitive to changes in RH compared to thicker vegetation. Changes in moisture content can lag two hours behind in changes with RH in a forest, compared to 30 minutes in open grassland. Example is dew in the morning, and how long it can take to dry out. |
| Dew Point | Is the temperature at which dew forms. It is the point to which temperature must drop before RH reaches 100% and water starts to condense out of the air. Relative humidity and dew point are the most important factors, since they will have a controlling influence on fuel moisture. Even if the fuel was dry yesterday, a strong dewfall on the morning of the burn may delay things. Low dew points on the kestrel, is an indications that the RH will not rise significantly overnight and the fuel moisture contents will remain low. Under these conditions , a prescribed burn will not self-extinguish overnight, especially subject to strong winds. Example: kestrel dew point says 0ºC (so you need 0 degrees for the dew to form), BUT the current and forecast air temperature overnight will remain at 20ºC, so therefore no dew will form overnight. |
| Wind Speed | Wind helps dry out fuels, it directly affects the speed at which a fire spreads and direction the fire will travel. Wind is the element that catches people out the most when they are using fire. Wind speed is recorded in km/hr. |
| Rainfall | Fuels absorb moisture in continuous rain or high humidity. In short periods of heavy rain, water tends to run off and not be absorbed by fuels. Precipitation is measured in mm. |
Technical weather information can be obtained from the (fireweather.niwa.co.nz) website
An example is below but will likely be different to that outlined in your permit conditions, follow those rules and do not burn outside of the prescriptions.
Check through module 9&10 for consistency + any other modules (checklists etc)
| Weather type | Ideal conditions |
| Wind speed | 8 – 20 km/h |
| Wind direction | Steady and away from sensitive areas |
| Temperature | 13 – 27 degrees C |
| Humidity | 40% – 70% (no less than 30%) 30 – 55% for optimal smoke management |
| Cloud cover | Clear sky to 70% cover; 30 – 70% cloud cover is optimal for smoke management |
| Mixing height | 1800 ft or higher |
| Transport wind speed | 13 – 30 km/h throughout the mixing height |
Know what weather conditions are needed for your burn