What is the temperature?

(Wang Zhengfei, Pan)

Various characteristic weather that is beneficial to the occurrence of forest fires and affects their behavior and control. Weather, forest combustible and fire source are the three major elements of forest fire. Forest fire weather reflects special weather conditions such as drought, high temperature and strong wind, so when discussing forest fire weather, we must consider the fire weather elements and the formed fire weather pattern.

Fire weather elements

All weather factors have effects on fire risk, but the most direct effects are as follows: ① Precipitation and precipitation interval: the shape, size and configuration pattern of dead combustible materials are different, and the time for absorbing and keeping moist is also different. The degree of dryness and wetness depends not only on precipitation, but also on the length of precipitation interval. Small combustible materials, such as hay, fallen leaves, pine needles and loose litter layers, quickly absorb and lose water, and may even become highly combustible after a few hours. However, the water content of medium combustible materials changes more slowly than the former, such as twigs, branches and surface humus layers. , and slowly exchange water with the outside world, have a certain draught capacity. This combustible affects whether the forest land can burn and spread in a certain area and the difficulty of forest fire control. Their water content determines the difficulty of fire fighting. As for coarse and dense combustible materials, such as big logs, deep humus and peat layer, the moisture changes slowly, and its degree of dryness and wetness reflects the long-term accumulation of precipitation. ② Temperature: it is the main factor to stimulate the flammability of combustible materials. The water content of combustible depends on the evaporation temperature. The higher the temperature, the stronger the evaporation power and the faster the combustibles dry. The influence of temperature is opposite to that of precipitation. At high temperature, the amount of heat that needs to be ignited becomes smaller and it is easy to catch fire. Violent fires often occur at high temperatures. Temperature is closely related to the accumulation and decrease of medium and coarse combustible materials, which is not only a monthly but also a seasonal factor of fire risk. ③ Relative humidity: it is a function of temperature. With the increase of temperature, the relative humidity immediately decreases, which accelerates the drying of combustible materials. Generally speaking, the relative humidity is in equilibrium with the water content of combustible materials. When the air humidity is low, the water content of combustible materials decreases, and vice versa, especially small combustible materials. Taking 50% air relative humidity as the critical line, it is easy to get angry below 50%; When the relative humidity exceeds 50%, the water of combustible basically stops evaporating, on the contrary, it absorbs water from the air. ④ Wind: Wind speed and direction have great influence on fire and fire behavior. On the one hand, the wind accelerates the evaporation of water, on the other hand, it transports oxygen for the spreading fire. At the same time, the advection of the wind can also make the fire spread along the wind direction. When the wind speed is 4 m/s, the fire spread twice as fast as when there is no wind. When the wind speed increases to 8 m/s, the fire spread speed can be increased by 4 times. However, when the wind force exceeds 6, the heat of small fire sources is easily blown away by the wind, and the burning heat energy can not be accumulated into a heat source, but has a flameout effect. ⑤ Air pressure: Although it has no direct influence on fire burning and spreading, air pressure is an important indicator of air mass and cyclone or anticyclone. At a specific time or place, there are obvious indications for future weather changes, especially for the prediction of temperature and wind power that directly affect the occurrence and development of fires, which depends on the nature of air mass and the distribution of air pressure. ⑥ Upper atmosphere conditions: According to the vertical distribution of temperature and the conditions of high-altitude wind, we can judge the stability and instability of the ground atmosphere, the existence of inversion layer and the development trend of local low-altitude jet, which is of great significance to whether the forest fire that has already occurred will lead to Fei Huo, fire storm or raging fire. ⑦ Atmospheric visibility: it is an important factor in finding fire. Whenever the forest fire danger is high and the visibility is poor, it will cause obstacles to the fire fighting command. This is a difficult problem for the meteorological department. It depends on the firemen's own experience to judge which situation is a burning smoke screen and which situation belongs to smog or smoke or flying dust.

The most important type of fire weather

The main types of fire weather in China are as follows: ① Dry air mass type: cold dry air mass from polar continent in summer or autumn, which gradually degenerates and heats up at mid-latitude, but remains dry and stagnates for a long time, resulting in strong evaporation of surface water and easy ignition of ground cover. According to the statistics of forest areas in northeast China, under the control of high pressure ridge, this kind of forest fire accounts for 32% of 104 times. ② Cyclone activity type: This type is more prominent in the northeast forest region of China. When Lake Baikal or Mongolian cyclone enters the Daxinganling forest region, it forms a long cold front, which makes almost all the southern Daxinganling forest region, Xiaoxing 'anling forest region, Changbai Mountain and other forest regions into the warm zone. The weather at this time is characterized by strong southwest wind and high temperature, and sometimes the relative humidity is less than 10%. At the same time, a cold front crosses the border with strong lightning, but the precipitation is very small (rarely more than 5 mm). Therefore, such dry cyclones provide favorable conditions for forest lighting and spreading, which is easy to cause forest fires, especially when cyclones pass by, which is easy to cause catastrophic fires. According to the statistics of 1970 ~ 198 1 year, six major fires in Daxinganling forest region belong to this type; In 104 fire, 35% was caused by Baikal or Mongolian cyclone. ③ Subtropical high type: in summer, a strong high pressure often appears in the western Pacific Ocean, that is, the so-called subtropical high sometimes spreads westward, controlling the whole central and southern China, especially the forest areas in Fujian, Jiangxi, Hubei, Hunan, eastern Sichuan, northeastern Guizhou and other places, and sometimes it is sunny and hot continuously in July and August, so this period has become a frequent period of forest fires in these areas. For example, in the history of Chongqing, the number of forest fires in July reached 48% of the whole year. ④ Plateau gale type: the high altitude of southwest plateau is controlled by westerly jet in dry season, and the low altitude and ground are affected by Mongolian cold high. In early spring, the temperature rose rapidly, and dry weather appeared, especially intermittent plateau gale, which further promoted the appearance of particularly dry weather in dry season and increased the risk of forest burning and spreading. When the polar vortex on the 500 hectopascal altitude map is biased to the western hemisphere, the circulation in the mid-latitude is zonal, and the southwest airflow prevails at high altitude, there will be better weather, which will easily increase the temperature, reduce the humidity and increase the fire risk. The map at 500 hectopascals shows a strong European ridge of high pressure, a stepped trough of low pressure on the east side of Urals, and a small trough crossing the border on the map at 700 hectopascals, which indicates that there will be a gale process in Yunnan-Guizhou Plateau. Therefore, this process is most likely to spread the forest fires that have already started.

philology

A.A.Brown and K.P.Davis, Forest Fire Control and Use, 2nd Edition. , McGraw-Hill Book Company, new york, 1973. Fire forecast.

(fire risk forecast)

(Duan Xiuying, Pan Zaichen)

According to weather factors, combustible conditions and fire sources, the occurrence and development of forest fires are predicted. Usually expressed by fire risk grade (or index). All kinds of fire risk prediction methods have their prescribed grade tables, including fire risk indicators, fire risk grades and management measures. Fire prediction is an important technical measure, which can not only predict whether forest fires will occur, but also quantitatively predict the spread range and intensity of forest fires that have already occurred, providing scientific basis for forest fire prevention, fire fighting and forest fire management.

Fire risk prediction started with single factor prediction at first, and then gradually developed from single factor to multi-factor. 1928 Canadian J.G.Wright used relative humidity to forecast fire weather, 1936 American H.T.Gisborne put forward multi-factor forecasting method, 1944 Soviet Union Nesterov (вгнннн). After 1950, more and more countries have studied and used fire risk forecasting, and China has studied methods such as double indicators. Following 1970, the United States and Canada successively put forward a national fire risk rating system suitable for national regional forecasting. In the early 1980s, China's fire forecast has begun to develop into a comprehensive forecast of forest fire occurrence and forest fire behavior.

General method of fire risk prediction

There are many such methods, and the typical ones are as follows: ① Comprehensive index method: according to the comprehensive influence of air saturation deficit, temperature and rainfall in rainless period, the forest combustibility is predicted and the corresponding index is determined. It is determined by the sum of the products of temperature and saturation deficit within a few days after rain, and the calculation formula is as follows:

Where t is the comprehensive index; Ti and di are the temperature (0℃) and water vapor saturation deficit (mb) on the first day after precipitation, respectively. When calculating the index, the temperature and saturation deficit of 13 must be measured every day and corrected according to the daily precipitation. If the precipitation on that day exceeds 2mm, the accumulated comprehensive index value before will be cancelled; When the precipitation is more than 5mm, subtract 1/4 from the comprehensive index value within 5 days after the rainfall, and then add it up to get the comprehensive index of the fire weather level. ② Effective Humidity Method: The flammability of forest combustible depends on water content, while the water content of combustible is affected by air humidity, and they always tend to be balanced. Therefore, the flammability can be estimated by judging the influence of air humidity on the water content of combustible materials. The calculation formula is as follows:

Effective humidity (%) = (1-a) (a0h0+a1h1+a2h2 ...+Anhn)

Where a is the coefficient, and the general value is 0.5; H0, h 1, h2, HN are the average relative humidity of the current day, the previous day, the previous two days and the previous n days respectively. The relationship between effective humidity and flammability is shown in the following table. ③ Double index method: the fire index is determined by daily minimum humidity and maximum temperature, and the fire spread index is determined by effective humidity and maximum wind speed. According to the fire index (I) and the spread index (S), the forest fire risk grade can be determined. The calculation formula is as follows:

I = a 1e-b 1H+a2tb 2+c 1

S=A3e-B3R+A4VB4+C2

Where h is the minimum humidity (%); T is the highest temperature (℃); R is the effective humidity (%); V is the maximum wind speed (m/s); C 1 and C2 are statistical constants. According to the observed values of the above meteorological elements at 13 of the day, the ignition index and spread index of the day can be obtained by calculating the retrieval map. Both indexes range from 0 to 100. Danger levels range from "incombustible" to "strongly combustible" and from "small spread" to "strong spread", all of which are divided into five levels.

Relationship table between actual humidity and combustion

American National Fire Risk Rating System

The NFDRS fire risk rating system consists of four indicators, namely: ① Man-made fire occurrence index (MCDI), that is, the numerical estimation of possible man-made fire hazards, which is determined according to man-made fire sources and fire components (IC) in a certain fire risk rating; (2) Lightning fire occurrence index (LOI): that is, the numerical estimation of possible lightning fire risk, which is obtained according to ignition composition (IC) and lightning risk (or thunderstorm activity); (3) Combustion index (BI): calculated from spreading component (SC) and energy release component (ERC), which is the main content of national fire risk rating, is interrelated with forest fire intensity and flame length, and reflects the behavior characteristics of forest fire; (4) Fire Load Index (FLI): the final index of the national fire risk rating, indicating the basic preparation of the fire department for possible forest fires in the control area.

There are 20 kinds of combustible materials classified by the national fire risk classification system in the United States, and more fire risk factors need to be input. Forest fire weather station not only needs to measure air temperature and humidity, wind speed, cloud cover, 24-hour maximum and minimum humidity and relative humidity of 13 ~ 15, but also needs to record precipitation duration, water content of combustible humidity bar and lightning activity. Through the forest fire information retrieval system (AFFIRRS), the fire risk grade can be processed by computer or calculated by looking up a table.

Canadian forest fire weather index

The index system consists of three humidity codes and two intermediate indexes (initial spread index and adjusted deciduous layer humidity code) to form a fire weather index (see figure). The three humidity codes reflect the humidity changes of three types of combustible materials: the humidity code of fine combustible materials (FFMC) reflects the dry and wet changes of surface litter layer under the influence of weather factors; The moisture code (DMC) of lower deciduous layer reflects the moisture change of lower deciduous layer (combustible layer with loose organic structure). Drought code (DC) reflects the change of water content in deep deciduous layer (combustible substance composed of fine organic matter). In the calculation, these three humidity codes are not only included in the weather elements of the day, but also substituted into the code values of yesterday. On the basis of three code values, wind speed is added to form two intermediate indexes, which are combined into the final index-forest fire meteorological index (FWI).

Canadian forest fire weather index system chart

In addition to the above fire forecasting methods, Australia, France, GDR and Japan also have their own fire forecasting methods.

philology

A.A.Brown and K.P.Davis, Forest Fire Control and Use, 2nd Edition; New york McGraw-Hill Book Company, 1973.