Module 11: Right days for burning

Putting your plan into action

Module Objective: Knowing the best times to burn to meet your burn objectives safely and effectively.

Seasons

To conduct a prescribed burn safely and effectively it’s important to closely look at your local weather patterns. The best time of the year to burn is generally autumn and spring, and potential late summer. This is when the temperatures start to cool and before the autumnal winds start to strengthen and make burning potentially dangerous.

The time of the year to burn depends on the state of the fuel (vegetation) to be burnt and appropriate weather conditions. In damper areas, burning may be confined to the driest months (January and February). Where the fuel has been treated to increase its flammability and where ample good burning weather is available, burning ma be done in March or April, when less hazardous weather conditions prevail.

Generally, it is necessary to take advantage of suitable weather as it occurs, at the same time striking a balance between obtaining a clean burn and keeping the hazard of burning within manageable bounds.

Time of day

Timing of the burn is the most critical element for obtaining the desire results from the burn. This depends on your burn objectives, if you want a low intensity fire or a fierce quick burning fire. If possible, plan prescribed burn operation¬¬ so that the entire job can be completed within a standard workday.

Have a good understanding of what the weather patterns are going to be on the day, and these are well established before you light the match. Even so, you need to be prepared and have a plan in place should the weather conditions change. Consider not burning if the weather is too variable, or a change in the weather occurs.

You must only light up during daylight hours. Some permit requirements may request the fire is extinguished by nightfall.

Weather patterns are important when deciding what time of day to start a burn.

New Zealand wind tends to change direction before 1pm with the majority increasing in force during the morning, reaching a maximum sometime after midday. The change in force is at times quite sudden.

You need to be familiar with your local and regional weather patterns. Fire generally burns quickly during mid-afternoon when relative humidity is low, and the temperature is higher. In some environments, winds shift or increase during the day.

Examples of this are:

• coastal areas – the onshore coastal breezes arrive most afternoons
• Hilly or mountainous areas – cool katabatic winds push down valley systems in the evening.
• An approaching front can change the wind direction, wind speed, rainfall, temperature and humidity levels.

Although these are the general conditions, weather can change at any time of the day, and you must monitor the weather forecast regularly

There are situations where an early burn is acceptable, where:

• Fire and Emergency approves and notes it in the fire conditions on the fire permit
• the area to be burned is surrounded by non-combustible materials (what do you mean? Like a fire break? Green paddocks?)
• a safety burn is required to widen a firebreak, but only after a risk assessment has been completed. Do you mean a burn out, to blacken/remove vegetation? And if so, when do they do this? On the day before they do the burn? Week before etc?

(FENZ – what do you mean by early burn? Like when there is dew on the ground, i.e. 5am? Some farmers like to burn then, and would say this is far more safer to do so, than wait till the afternoon. So lets make confirmation on this please.

Depending on your burn objectives, the fuel type, terrain and current season and weather, the burn could start between:

For low intensity burns:

• morning when dew is still present - check
• Prescribed fires are typically ignited between 10 a.m. and noon, after any morning dew has had a chance to evaporate and any surface inversion from the previous night has lifted
• when there is a gentle breeze

For hot burns:

• between 1pm and 6pm.
• Under normal circumstances, between 1pm and 3pm,
• but under hazardous conditions, between 4pm and 6pm.check

There are several reasons for advocating 1pm to 3pm as a window for lighting under normal conditions. They are:

1. Poor burning is usually encountered when an earlier start is made.
2. Usually the bulk of burning is done during the best burning period of the day from 1pm to 4pm, irrespective of the starting time.
3. It is always considered safer to be burning away rather than towards the most hazardous time of the day. A breakaway at mid-day with the driest part of the day still to come, is a far less attractive proposition than one at 3pm when the cooler, damper conditions, are not too far away.
4. The wind (direction and strength) is seldom reliable earlier in the day, while very often it becomes steady after noon-day.
5. Men fresh on the job with the hottest part of the day passing behind them, are better able to deal with emergencies than are those who have been on the job two, three or four hours, and are still faced with the highest period of hazard.
6. A speedy burn under good burning conditions is generally safer and certainly less costly than a prolonged burn due to a too early start.
7. Burning conditions are usually better during the day than at night because wind speeds are higher, wind directions are steadier, and an unstable atmosphere favours the vertical dispersion of smoke. These conditions make smoke management much easier during the day.

The later start times – 4pm to 6pm, apply to difficult areas that may be burnt only during periods of high fire hazard. The late start after the heat of the day towards evening when temperatures lower and the humidity increases will often result in a satisfactory burn under reasonably safe conditions.

It is not unusual in some localities to find there are perhaps only 2-3 days during the year when conditions are ideal for good burning.

Notes:

• If the forecast is for poor night-time dispersion, halt ground ignition before 3 p.m. Halt ignition before 4 p.m. to allow adequate time for the fire to burn out before atmospheric dispersion conditions deteriorate. At night smoke tends to stay near the ground and collects in depressions.
• Also, relative humidity is higher at night, resulting in spotty burning and an increased likelihood of fog formation. However, on some winter nights when a strong cold front is moving through the area, winds remain strong and persistent and relative humidity does not rise greatly. These conditions can provide good prescribed burning conditions especially when cooler temperatures are needed. Whenever night burning is done, keep a close watch on the weather and smoke drift.

Burning is not recommended when:

• air temperature exceeds 27C
• wind speeds exceed 32 km/h
• relative humidity is less than 20 percent,
• variable or no-wind conditions.
• burning is not recommended within 12 hours of a predicted wind shift.

VC to add in 30-30-30 rule

Self-check:

Qu. What are the typical weather patterns at your location during the season you want to burn?

Qu. When might be a good time of the day for you to burn to meet your burn objectives and the likely weather conditions?

Learning outcome:

Knowing what your typical weather conditions are like in your area and the appropriate times of the day to conduct your burn.