Fire weather: The science behind the signs
Grant Pearce
New Zealand Tree
Grower February 2009
At this time of the year people often take more notice of the fire
danger signs that appear along New Zealand’s rural highways. These
half-grapefruit dials indicating low, moderate or high fire
danger appear relatively simple. But the ratings are determined by
complex science aimed at effectively managing the fire risk associated
with different weather conditions.
The link between weather and fire risk has been the focus of much
research in recent years. A key element of this research has been the
development of a fire climatology database that collates data from
weather stations throughout the country. These daily readings give a
picture of New Zealand’s fire climate − in other words, the average
weather based on past history.
This database has provided the basis for a range of studies, including −
- Comparisons of fire season severity
- Identification of distinct fire climate regions
- Weather patterns that contribute to severe fire seasons in
different
parts of the country.
The database also helps understand the possible effects of future
climate change on fire danger.
The New Zealand Fire Danger Rating System
The New Zealand Fire Danger Rating System (NZFDRS) has been developed
to help support fire management decision-making. Examples include
setting burn restrictions, issuing fire permits and notifying the
public of prevailing fire danger. The NZFDRS analyses the weather,
vegetation and topography that affect ease of ignition, rate of fire
spread and intensity. The system is based on the Canadian Forest Fire
Danger Rating System that was introduced in New Zealand in 1980.
 At the core
of the NZFDRS is the fire weather index system. This
provides ratings of relative ignition potential and fire behaviour
based on the information collected daily by the weather station
network. These values are used to calculate fuel moisture codes and
fire behaviour indices. From these calculations come the ratings that
appear on the fire danger signs.
The fire danger classes are generally simplified into low, moderate,
high, very high and extreme ratings, which vary depending on the
dominant vegetation types in an area. These ratings provide an
indication of the increasing difficulty of fire suppression as fire
intensity increases. The very high and extreme fire danger classes
represent the conditions under which it will be difficult, if not
impossible, to control fires with conventional fire fighting techniques.
New Zealand’s fire climate
Fire risk varies considerably for different parts of New Zealand due to
the wide range of prevailing weather conditions. In general, the
eastern and northern parts of both islands tend to have the most severe
fire climates, as they are most prone to foehn wind and drought
conditions. Unlike many other parts of the world where there are more
stable continental climates, problem fire weather can also occur at
almost any time of the year.
To improve knowledge on the fire climate of New Zealand, Scion recently
completed an analysis of the long term average and extreme values of
weather and fire weather index system components for 127 station
locations. Stations in the Marlborough and Canterbury regions
demonstrated the highest values of the three fire climate severity
measures. At the other end of the scale, the West Coast, Taranaki and
Waikato regions had the least severe fire climates.
For individual station locations, three stations in Marlborough –
Awatere Valley, Woodbourne Aero and Molesworth – had the most severe
fire climates. Christchurch Aero in Canterbury and Castle Point in
Wairarapa were the other two stations in the top five. The seven
stations with the least severe fire climates included Opouteke in
Northland, Marco in Taranaki, Athol in the Waikato, Waimarino Forest in
Wanganui/Manawatu, and all three stations – Westport, Hokitika Aero and
Haast – from the South Island’s West Coast.
The highest ranked stations had more than 40 days each year on average
of very high and extreme forest fire danger, and the lowest less than
one day a year. As you would expect, the stations with the lowest risk
are generally characterised by the highest annual rainfalls.
Effect of climate change
A growing body of international evidence, recently summarised by the
international Intergovernmental Panel on Climate Change (IPCC),
suggests that future fire activity is expected to increase as a result
of global warming and associated climate change. Scion did a study to
determine the likely changes in fire danger under different
possibilities of climate change for New Zealand.
Results indicate that New Zealand is likely to experience more severe
fire weather and fire danger, especially in the Bay of Plenty, east of
both islands and the central Wellington and Nelson regions. In several
cases such as Gisborne, Napier and Christchurch, the average seasonal
severity ratings increased by more than 25 per cent to 65 per cent, and
the number of days of very high to extreme forest fire danger increased
by more than 20 days a year by 2080.
This study will help New Zealand rural fire authorities to manage and
prepare for fire risk now and in the future.
Grant Pearce works for
the Scion
Rural Fire Research Group
(top)
|
|
