Fire and Rescue Service Wildfire Operational Guidance

Listen

8B2 The Effects of Weather

Introduction

8B2.01 Weather is a key wildfire factor and has a significant impact on the fuel complex and the broader wildfire environment. Weather influences fire intensities, rates of spread and levels of risk and it is vital that fire service personnel understand how the prevailing weather conditions can change wildfire behaviour throughout the day, even if the fuel type remains the same.

8B2.02 The key factors that bring about changes within the local daily weather cycle are:

  • Changes in temperature which heats fuel and influences the moisture content of the air.
  • The effect of precipitation and other forms of moisture such as dew.
  • Local air movements such as katabatic and anabatic winds.

8B2.03 Whenever weather conditions are supportive, it is likely that Fire and Rescue Services will be called upon to respond to large numbers of wildfire events. It is a misconception to believe that wildfire is reliant on extreme weather conditions, that the threat of wildfire is limited to the summer months, or indeed that it requires a prolonged period of dry weather to create suitable conditions for wildfires to occur. A relatively short period of supportive weather can increase the risk of wildfire significantly and vegetation fires can occur at any time of the year.

The Effect of Weather on Fire Behaviour

8B2.04 Not only does the weather have an effect on the combustibility of fuel, it also has an influence on the combustion process itself. Weather influences fire behaviour by:

  • Raising and lowering the temperature of fuels and the air.
  • Increasing and decreasing the moisture content of the air.
  • Changing the moisture content of fuels, particularly dead ones.
  • Supporting or limiting the development of strong convection plumes.
  • Curing (drying of) fuel.
  • Changing the direction, strength and type of prevailing and local winds.
  • Changing the level of stability in the atmosphere.

8B2.05 During a wildfire event, changes to weather conditions, in both the short and longer term, create conditions that will result in either the fire situation improving or becoming worse. Understanding and interpreting these weather changes, and what they mean to personnel on the fire ground is of paramount importance to the effective management of a wildfire incident, and in particular, can have a substantial influence on the safety of personnel.

8B2.06 The aim of this section is to give an understanding of the influences that weather can exert on the wildfire environment. Whilst it is not the intention to give in depth detail on the complexities of the broader global climate, firefighters should have a basic understanding of how the global climate functions and its influence on local conditions.

Solar Radiation

8B2.07 The sun is the engine that powers the Earth's climate and weather systems. It transmits its energy in the form of solar radiation and it is this radiation which drives the global climate. When solar radiation reaches the Earth's atmosphere, it passes through the air having only a negligible effect on air temperature. It is only when it reaches the surface of the Earth and the solar radiation is converted to terrestrial radiation, that the surface is heated and the air temperature begins to rise. The air coming into contact with these heated surfaces is then warmed and it is this process which raises the air temperature throughout the day.

 Fig. B2.1 The effects of solar radiation on the atmosphere

Fig. B2.1 The effects of solar radiation on the atmosphere

Differential Heating

8B2.08 For a number of reasons, the effect of solar radiation on the surface of the Earth is not equal. This causes a variation in the solar radiation's ability to raise surface and air temperatures. This differential in heating causes an atmospheric imbalance which results in the formation of compensating weather patterns.

8B2.09 The sun's energy is concentrated at the Earth's equator, but at the poles the energy is spread over a greater surface area resulting in a reduction in per unit strength.

8B2.10 Higher temperatures over the equator and lower temperatures over the poles create a temperature imbalance across the surface of the Earth. The atmosphere responds by circulating the cold and warm air to compensate for the imbalance. This causes the formation of areas of low pressure known as 'depressions' or what are more commonly referred to as 'lows'.

Fig. B2.2 The sun’s intensity is concentrated at the equator

Fig. B2.2 The sun's intensity is concentrated at the equator

 Fig. B2.3 The concentration of the sun’s energy at the equator

Fig. B2.3 The concentration of the sun's energy at the equator

8B2.11 There are two main areas of rising air. These are in the Tropics and along the Polar Front, and it is here where the greatest average rainfall occurs. There are also two main compensating areas of descending air. These are in the Polar Regions and Sub-Tropical regions, these give rise to the sub-tropical and polar high pressure areas. At these locations there is little rainfall and subsequently the major deserts of the world are found in what is known as the sub-tropical high pressure belt.

Fig. B2.4 Atmospheric circulation. Descending air marks high-pressure areas; ascending air marks low-pressure areas.

Fig. B2.4 Atmospheric circulation. Descending air marks high-pressure areas; ascending air marks low-pressure areas.

Land and Sea Surfaces

8B2.12 Due to differential heating between land and sea, there is an interaction between the two surfaces that has an impact on the global climate. Land cools more than the sea in the winter and colder denser air over land surfaces leads to the formation of areas of high pressure. Conversely, in the summer the land warms more than the sea, and warmer, less dense air rises, leading to the formation of lower pressure systems.

Surface Heating

8B2.13 Direct heating from the sun has a major impact on the surface of the Earth. The amount of solar radiation absorbed by the Earth depends on a number of factors:

The time of the year

8B2.14 Longer days in the summer result in more exposure to solar radiation. This generally leads to higher temperatures and drier surface conditions. Shorter days result in less exposure to solar heating and longer nights means that the surface temperatures become lower.

The time of day

8B2.15 The sun is weakest in the early morning and late in the afternoon. The sun is generally strongest when it is at its highest trajectory. A lower angle results in less radiation being received by the surface and a corresponding reduction in the surface temperature.

Cloud cover

8B2.16 Cloud cover and air quality will absorb or reflect solar radiation before it reaches the surface.
On a humid day the moisture in the air will reduce the amount of solar radiation striking the surface. Pollution will also reduce the amount of sunlight reaching the surface.

 Fig. B2.5 Showing how some radiation is unable to reach the surface of the Earth as it is absorbed or reflected by cloud and pollutants

Fig. B2.5 Showing how some radiation is unable to reach the surface of the Earth as it is absorbed or reflected by cloud and pollutants

The shape of the surface

8B2.17 The shape of the Earth's surface is not flat and the shape of the topography arranged across it alters the amount of exposure time and strength of the solar radiation striking the surface.

The angle of the surface

8B2.18 The angle of the surface in relation to the sun's position throughout the day causes variations in surface temperature. This is particularly relevant to slopes, the direction and angle of slope can cause the fuel on them to be much warmer and dryer than those found on other parts of the landscape. In the UK, southerly-facing slopes receive a greater amount of surface heating.

The Effect of Surface Heating

8B2.19 It is the heated surface materials (rocks, vegetation, surface coverings) that warm the air that comes into contact with them. This heated air then becomes buoyant and is replaced by cooler heavier air; this process accelerates as the temperature of the surface increases during the day. This convection process distributes the heat upwards through the atmosphere sometimes to heights of many hundreds of metres. The strength of the solar radiation peaks at about noon when the sun is at its highest. As a result the surface continues to increase in temperature drying out the air until later in the afternoon.

8B2.20 As the air loses its moisture the fuel seeks to remain in balance with the air that surrounds it and also sheds water. This drying process increases the fuels combustibility. This process continues until the effect of the solar radiation begins to diminish and the air begins to cool.

8B2.21 Fuel will be at its warmest and driest about 2 hours after the sun has reached its zenith,
with fire behaviour peaking during the spring and summer between 1400-1600 hours.

Atmospheric Moisture Content - Relative Humidity

8B2.22 Relative Humidity (RH) is a measure of the amount of moisture in the atmosphere; this is represented as a percentage. For example when it is raining or foggy, relative humidity could be as high as 100%.

8B2.23 A correlation exists between temperature and relative humidity, it is highest around dawn when temperature is low, and is lowest later in the afternoon when temperatures are likely to be higher. When RH is high, fuel is unable to dry, when humidity levels fall, moisture content within the fuel also reduces. The effect of RH on fire behaviour is significant and is a good indicator of likely fire severity.

8B2.24 When RH falls to critical levels (below 40%) then extreme fire behaviour becomes probable. Fine dead fuels (such as dead grass and litter) respond very quickly to changes in relative humidity.

8B2.25 Later in the day, as temperatures fall, humidity levels will rise producing cooler damper air nearer the surface resulting in lower fire intensity which can aid firefighting operations. Information on humidity should be collected throughout a wildfire event as it is a useful indicator of future fire behaviour.

Dew Point Temperature

8B2.26 The dew point temperature is the temperature to which air must be cooled before moisture condenses from the atmosphere. It is common to find dew forming on vegetation during the night while other surfaces remain dry. Dew is more easily formed on fine fuels because they have a large surface to volume area. This results in rapid cooling, and the formation of dew on the surface reducing the fuel's capacity to burn. Knowing when dew point may be reached can assist firefighting activities during certain times of the day, particularly early in the morning.

The Effect of Shade

8B2.27 Fuels that are in shade are not subject to direct sunlight resulting in them remaining cooler and damper than fuels that are exposed to the full effects of solar radiation. Shaded fuels are more reliant on the general air temperature and the effects of wind to dry them out. Therefore, the position of vegetation within a vertical fuel arrangement is important. Those fuels existing in the canopy generally get most sunlight and are therefore warmer and drier than those existing at lower levels. In denser vegetation which the sun fails to penetrate, fuels that exist at the lower levels, particularly those at ground level, may remain cooler and damper. This is particularly so in fuel that is in contact with the saturated ground which restricts the capacity of the fuel to shed moisture. For this reason there may be an increase in fire intensity when a fire moves from an area in shade to an area where the fuel is in sunlight.

Hot and Cold Fuels

8B2.28 Fuel arranged across the landscape is affected by variations to its exposure to solar radiation. This can be dependent on the following factors:

  • The length of exposure.
  • The time of day.
  • Fuel is in or out of shade.
  • Fuel is in or out of aspect.

8B2.29 All of these factors will cause variations in the temperature and moisture content present within the fuel bed. Fuels that are subjected to significant amounts of solar radiation will become hotter than those that are not. These are descriptively referred to as 'hot fuels' as their temperature can be increased significantly; hot fuels are therefore more supportive to the combustion processes.

8B2.30 Cold fuels are those that are not positioned favourably and receive less direct sunlight and therefore have a lower temperature, as a result the fuels are normally cooler than those growing in hotter exposed areas.

8B2.31 There may be a substantial difference in fire behaviour in fuels of different temperature, particularly in fires burning within a fuel containing large amounts of fine fuel. Understanding which fuels are hot and which ones are cold is usually best achieved through observations on the fire ground.

The Effect of Cloud

8B2.32 Clouds are formed from particles of water and can be at variable heights above the surface. If there is a cloud covering, less sunlight will reach the surface and temperatures are likely to be lower, as a result the moisture contained within the air and fuels may be higher.

Precipitation

8B2.33 Rainfall has an obvious impact on the wildfire environment and will raise the moisture content of the air, fuel and the soil. If there has been a lack of rainfall prior to the fire the fuel is likely to be much dryer and more responsive to preheating by solar radiation.

8B2.34 There are three main phases that should be considered:

Short term drying environment

A short term drying environment is one that lasts long enough to only dry out finer fuels, mainly those that are exposed to solar radiation or those effected by the circulation of warm air.

Extended drying environment

Prolonged periods without rainfall can result in an extended drying environment that lasts for a number of days/weeks, during which most fine fuels within the fuel arrangement will readily support fire development. As increasing amounts of coarse fuels dry, there will be an escalation in the amount of fuel available to burn; this increase in fuel loading has the potential to drastically affect fire behaviour and intensities.

Long term drying environment (drought)

A long term drying environment, or drought, results in the ground becoming arid and baked. Fuel that exists at ground level, that would have otherwise remained saturated, dries and even fuels that exist below the surface become available to burn. Vegetation becomes dehydrated, parched or can even die off completely, leading to an abundance of available fuels and optimum wildfire conditions.

Wind

8B2.35 Wind plays a key role within the wildfire environment. Its strength and direction will affect fire spread and intensity. Wind can also assist in drying out fuels, a warm wind with little moisture content can accelerate the drying out process and make the fuel more receptive to the combustion process. Not all winds will have this affect and the general direction of the large scale winds is an important factor. Air blowing from continental Europe is usually the driest.

8B2.36 The diagram below gives a general guide of the potential characteristics of winds that are blowing from different directions.

 Fig B2.6 Wind direction can have a major impact on fire behaviour as the air temperatures and moisture content may differ

Fig B2.6 Wind direction can have a major impact on fire behaviour as the air temperatures and moisture content may differ

8B2.37 As well as the general wind direction, local conditions can influence the way air circulates across the landscape. This can result in variations to wind speed and its direction which can result in substantial changes to fire behaviour.

Local Winds

8B2.38 Local changes to wind are driven by the day-night cycle. This results in the surface of the Earth heating during the day and then cooling during the night. This process results in the natural movement of air within the local topography, this can have a significant impact on fire behaviour. This can either be in its intensity or direction or rate of spread.

8B2.39 Local wind types should not be confused with topographical winds which are described later in Section B3. It is important that firefighters understand the influence that local winds can have on a fire as they can often result in wind direction or strength changing suddenly.

8B2.40 The four main types of local winds that are likely to impact on wildfire in the UK are:

  • Anabatic or upslope winds
  • Katabatic or downslope winds
  • Land breezes
  • Sea breezes

Anabatic Wind (upslope wind)

8B2.41 Slopes that are subjected to exposure to solar radiation become heated. Prolonged periods of preheating results in a layer of air near to the land surface becoming very warm and dry. This increases the buoyancy of the air, with the result that the lighter air circulates upslope, producing a general movement of air at surface levels. Anabatic winds may form on what otherwise is a calm day resulting in increases in a fires rate of spread and intensity.

Fig. B2.7 Anabatic winds are formed on slopes that have been heated by the sun

Fig. B2.7 Anabatic winds are formed on slopes that have been heated by the sun

Katabatic Wind (downslope wind)

8B2.42 Katabatic winds are formed after the sun sets and the surface is no longer heated causing the surface and the materials on it to cool down. The air in contact with the surface also cools and becomes denser; this heavier air then flows down slope.

8B2.43 Air that is cooler and damper can impair fire development and as katabatic winds flow downwards, they will generally reduce fire intensity. Where the fire has previously been supported by an upslope wind, and then is subjected to the influence of a katabatic wind, this may force the fire to burn back into previously burnt fuel which can also lower the intensity and rate of spread.

Fig. B2.8 Katabatic winds can form on slopes that cool at night

Fig. B2.8 Katabatic winds can form on slopes that cool at night

Fig. B2.9 Downslope and upslope winds

Fig. B2.9 Downslope and upslope winds

A Land Breeze (night time wind)

8B2.44 A land breeze is a wind that blows from the land towards the sea at night. It is caused by the imbalance of temperature between the air above land surfaces and the air above the sea. When the land is no longer being heated by the sun the surface loses temperature very quickly and the air becomes colder and denser. The relatively warm air above the sea rises and the air covering the land moves to replace it.

A Sea Breeze (day time wind)

8B2.45 A sea breeze is a wind that moves from the sea onto the land and is caused by the imbalance between the temperature of the air covering the land and the sea. When the surface of the land is being heated by the sun it heats up relatively quickly. Water on the other hand has a high specific heat capacity; therefore it requires a larger amount of energy to raise its temperature. Much of the heat absorbed by the sea goes into the process of evaporation rather than raising the water temperature.

8B2.46 Once the air above the surface of the land becomes heated it starts to expand and becomes lighter, as it rises the denser, cooler air from the sea is drawn over the land to replace it.
Sea breezes are common during the summer and can be quite strong. They can start quite early in the day depending on how quickly the surface of the land is heated by the radiation from the sun.

(Similar effects can take place around large inland lochs and lakes)

8B2.47 The effect of a sea breeze on a wildfire can be significant, and potentially dangerous. They can have the following influences on fire behaviour:

  • Disturbs the movement of air on what otherwise would have been a calm day
  • Depending on direction it can strengthen an existing wind
  • Depending on direction it can reduce the strength of an existing wind
  • Can influence the direction of the general wind
  • Generally increase the intensity of a fire

Fig. B2.10 Showing the formation of sea breezes which can be common in the UK

Fig. B2.10 Showing the formation of sea breezes which can be common in the UK

Atmospheric Stability

8B2.48 Atmospheric stability is the resistance of the atmosphere to the vertical or upward movement of air that has been heated nearer to the surface. This resistance is in the form of a boundary layer that can fluctuate in height depending on the time of day and the temperature of the air at different heights within the atmosphere. The degree of stability or instability of the atmosphere will impact on fire spread, fire intensities and the movement of smoke.

Fig. B2.11 The boundary layer is influenced by changes in temperature throughout the day

Fig. B2.11 The boundary layer is influenced by changes in temperature throughout the day

Inversions

8B2.49 The temperature of the atmosphere usually falls with a rise in altitude, but when a layer
of air in the atmosphere is warmer than the air below, what is known as an inversion is formed.This creates a more stable fire environment by restricting the upward motion of the lower atmosphere and the fire plume.

A Stable Atmosphere

8B2.50 Temperature inversions are commonly formed during the night and early morning when the cooling surface lowers air temperatures. This cold air is denser than the warmer air above and because it is less buoyant remains close to the surface.

8B2.51 During a wildfire, a stable atmosphere tends to prevent the formation of a strong convection column. This restricts fire intensity and fire behaviour is usually predictable. Smoke is more likely to remain closer to the ground.

Stable atmospheric conditions may display the following visual indicators:

  • Low stratus type clouds
  • Smoke columns drift apart and do not rise to a great height
  • The vertical movement of air is limited
  • Hazy lower atmosphere
  • Possible fog layers particularly in low lying parts of the landscape
  • The winds are usually light and predictable

 Fig. B2.12 Observations to indicate a stable atmosphere

Fig. B2.12 Observations to indicate a stable atmosphere

An Unstable Atmosphere

8B2.52 As the surface temperatures near ground level increase during the day, the cooler air warms and the heat differential between it and the inversion above weakens. When the heated air near to the surface becomes warmer than the air above, its buoyancy increases and it starts to rise, cooler air is drawn in to replace it. This vertical movement of air mass or 'updraft', provides a fire with the means to develop a strong convection column which can result in an in-draft of air. This results in a potential increase in fire intensity that makes a fire less predictable and sometimes leads to its behaviour becoming erratic. Gusty winds make smoke movement more unpredictable. If atmospheric stability changes it may be necessary to alter suppression tactics.

Unstable atmospheric conditions may display the following visual indicators:

  • Cumulus type clouds which show noticeable vertical growth
  • Smoke columns develop vertically and can rise to a great height
  • Winds are gusty and unpredictable
  • Thunder storms can develop
  • There is a clear lower atmosphere

Fig. B2.13 Observations to indicate an unstable atmosphere

Fig. B2.13 Observations to indicate an unstable atmosphere

Collecting Weather Information

8B2.53 Firefighters should now be aware that weather is a major factor within the wildfire environment, and one that can bring about significant change to fire behaviour. Usually, weather brings slow and predictable change to fire behaviour, but sometimes the changes are more sudden and can result in extremely dangerous situations.

8B2.54 Globally, sudden and unexpected weather change is a major cause of firefighter fatalities, and it is of absolute importance that the current and predicted weather is closely monitored throughout the duration of a wildfire incident. Regular weather updates should be obtained and onsite intelligence regarding local weather variations should be gathered. This will play an important part in the risk assessment and decision making processes which underpin the tactical fire plan.

8B2.55 Detailed weather information should be included within operational briefings and all personnel should be made aware of any risks posed by the current or future weather conditions. Updated weather information should be communicated to all operational personnel on the fire ground to assist in the maintenance of situational awareness.

8B2.56 Equipment is available that can provide accurate onsite information. Small hand held weather units are particularly useful as these can be used to gather a wide range of weather intelligence including, air moisture content, altitude, barometric pressure, wind speed and wind direction.

Meteorological measurements must be made away from the influence of the fire and in clear terrain unaffected by nearby trees, other obstructions or factors which may result in false or inaccurate readings.

Met Office Support

8B2.57 Weather information provided by the Meteorological Office should form an essential and critical part of any wildfire incident command system. The Met Office can provide accurate information that will assist in defining the affect that weather will have during an incident. This can be provided by utilising a number of metrological services:

Photo B2.1 A wildfire specialist using a hand-held weather unit

Photo B2.1 A wildfire specialist using a hand-held weather unit

General Information

8B2.58 Information on the prevailing local weather conditions, can be provided by accessing the general weather forecast for a local area. The information given is for public use and can be obtained from the Met Office web site www.metoffice.gov.uk/

8B2.59 The pressure systems shown on a weather map are an indication of atmospheric stability. The presence of a high pressure system indicates that a stable atmosphere can be expected, a low pressure system indicates that the conditions will be unstable.

Specific Advice and Information

8B2.60 Recently the Met Office has improved its web-service so that it now provides a single source
of decision making information to the emergency response community. This source is known as Hazard Manager, the web-based resource for the emergency response community. This facility provides a single location where the Met Office holds all its environmental intelligence; it can be used to assist decision makers and allows visualisation and manipulation of different data types.

Specialist Advisors

8B2.61 Met Office Regional advisors should be used appropriately and are available to:

  • Assist emergency responders to assess the risk in their particular area from predicted
    or ongoing severe weather events.
  • Provide guidance on the use of Met Office services available to emergency responders.
  • Assist with weather related risk assessments as used within community risk registers.

8B2.62 If required, advisors can give support during an incident and their role may include:

  • Ensuring that the management team is aware of all the meteorological factors which could impact the incident.
  • Ensuring that all meteorological information is consistent and that all responders within the Command and Control structure use the correct information.
  • Interpreting any information for the responders if necessary.
  • Sourcing other scientific advice available from the Met Office.
  • Acting as a point of contact between the Met Office and the emergency responders.
  • If required and appropriate, arranging for routine forecasts and other information to be supplied to aid in the recovery phase of the incident.

Met Office Fire Severity Index

8B2.63 The Met Office Fire Severity Index (FSI) produces an assessment of the current day and gives a forecast of the coming five days' fire severity. The forecast uses information such as time of year, wind speed, temperature and rainfall. The index values are from 1 to 5 which represent an increasing degree of likely fire severity:

1 Very Low

2 Low

3 Moderate

4 High

5 Exceptional

8B2.64 The FSI maps display an index on a 10km x 10km colour coded grid square system. The Met Office FSI was primarily developed following the introduction of the Countryside and Rights of Way Act, 2000 (CROW Act). It assists in the management of land, and provides a guide for consideration as to when public access should be restricted during periods when there is an exceptional fire risk.

8B2.65 The FSI is a useful tool for fire and rescue services and land management agencies as it predicts the potential current and future fire risk. Services should access this information regularly and instigate appropriate action relevant to the risk levels indicated by the index.

8B2.66 Currently the Fire Severity Index is not available in Scotland or Northern Ireland. At the time of writing, arrangements to introduce a comparable system for Scotland are under review.

Climate Change

8B2.67 It is widely acknowledged that climate change is occurring and at a rate that many consider to be accelerating. Scientists, including those from Met Office, have carried out a great deal of research into what might be causing the changes to the climate. There is evidence to suggest that increasing amounts of greenhouse gases within the atmosphere are contributing to climate change and global warming. This warming may cause relatively large and long term change, not just to the UK climate, but to the wider natural environment.

8B2.68 Wildfire is influenced by the fuel that is available, the pre and prevailing weather conditions and the topography. If the global climate is changing then it is sensible to accept that this will affect our national weather patterns. Should local conditions change then this will also bring variations to the fuel types and the condition of vegetation.

8B2.69 If there are changes to the UK climate which result in warmer wetter winters and hotter drier springs and summers, then FRSs should be prepared for an increase in the scale and number of wildfires, as conditions become more supportive of a natural increase in fuel loading.

Risk Monitoring

8B2.70 The effects of climate change on the UK is uncertain but evidence and trends suggest that at least in the short term, temperatures are increasing. This coupled with a suggested decrease in the amount of rainfall during the spring and summer months, could have a substantial impact on the UK wildfire environment.

8B2.71 It would be beneficial if fire and rescue services gather information on the weather and its consequential impact on the number and frequency of wildfire incidents. This information will be invaluable in developing response projections. This data can also be fed into other scientific research projects including the development of a more response orientated FSI system.

Case Study

8B2.72 During the spring of 2011, FRSs across the UK responded to a large number of wildfire events. The diagrams below give an overview of the prevailing conditions during this period and can provide some explanation for the increase in the number of occurrences.

8B2.73 The diagrams show that March was very dry and warm while April had very high mean temperatures. The poor physical condition of the vegetation following the rather harsh winter and unseasonably warm spring provided substantial amounts of wildfire fuels.

Fig. B2.14 The mean temperatures during the early spring of 2011 were much higher than the average

Fig. B2.14 The mean temperatures during the early spring of 2011 were much higher than the average

Fig. B2.15 Low rainfall during March 2011 contributed to a rise in fires across the country

Fig. B2.15 Low rainfall during March 2011 contributed to a rise in fires across the country

 Fig. B2.16 Low rainfall and high temperatures gave supportive conditions to vegetation fires during the spring of 2011

Fig. B2.16 Low rainfall and high temperatures gave supportive conditions to vegetation fires during the spring of 2011

Fig. B2.17 Continued low amounts of rainfall led to increased activity during April and May 2011

Fig. B2.17 Continued low amounts of rainfall led to increased activity during April and May 2011

This final diagram shows how clear the skies were during April 2011 . This would have a significant impact on the amount of solar heating fuels received and would affect its combustability and increase the intensity of any fire.

Fig. B2.18 Clear skies during April 2011 had a significant impact on the moisture content of fine fuels

Fig. B2.18 Clear skies during April 2011 had a significant impact on the moisture content of fine fuels

8B2.74 FRS risk managers should assess whether sufficient consideration is being given to weather and climate, and its effect on their services. Consideration should also be given as to whether the vast amount of weather data that is already available is being used to its full potential.

8B2.75 Weather and climate assessments can be used by FRSs to judge the potential impact that weather patterns and climate change will have on their organisations.

8B2.76 UK FRSs should begin to assess through their integrated risk management plans and community risk registers, how a changing climate may place different challenges and demands on their services.

8B2 Key Considerations

  • Weather conditions that support a drying environment will progressively increase fuel availability and fire intensity. It is therefore important to understand how supportive the weather conditions have been in the days leading up to the event.
  • The cumulative effects of the daily weather cycle (on the drying process and on fire behaviour) will peak during mid to late afternoon.
  • During extended or prolonged drying environments, it should be anticipated that increases in fire intensity will occur much earlier in the day.
  • Wind strength and direction has the predominant effect on fire spread and intensity.
  • A fire that is burning in fuel which has been preheated by solar radiation will burn with more intensity.
  • An unstable atmosphere will make fire behaviour more dangerous and erratic.
  • The approach of a cold front can create unstable atmospheric conditions.
  • The levels of moisture in the air should be monitored; relative humidity readings can be used to explain current, and indicate future, fire behaviour.
  • The collective effect of high wind (above 30km/h), high temperature (above 30°C) and low relative humidity (≤30%), known as the 30-30-30 rule, can result in extreme fire behaviour and intensity.
  • Obtain frequent weather forecasts and specific updates on predicted air humidity, temperature and wind speed.
  • At prolonged incidents advantage should be taken of periods when weather is less supportive to fire development, such as early morning, in the evening or during the hours of darkness.