FIRE WEATHER

FIRE WEATHER

FOREST DEFENSE REFERENCE

by Mr. Ara L. Marderosian

Sequoia Forest Alliance

P.O Box 988

Weldon, CA 93283-0988 U.S.A.

(760)378-4574 (phone/fax)

FIRE WEATHER . . . A Guide For Application Of Meteorological Information To Forest Fire Control Operations, by Mark J. Schroeder, Weather Bureau, Environmental Sciences Administration, U.S. Commerce Department and Charles C. Buck, Forest Service, U.S. Department of Agriculture

U.S. Government Printing Office : 0-244 :923, first printed in May 1970

Reviewed and approved for reprinting August 1977

Stock No. 001-000-0193-0 / Catalog No. A 1.76:360

A SUMMARY OF FOREST SERVICE DISCLOSURES IN FIRE WEATHER

by Mr. Ara L. Marderosian

This joint U. S. Department of Commerce and U. S. Department of Agriculture document, FIRE WEATHER, is a 229 page detailed, scientific explanation, which describes the closed canopy forest as one which provides a variety of benefits that decrease the risk of forest fires and states that all features of the environment that affect heating and cooling are significant.

The forest canopy of dense timber stands shades the ground and the forest fuels from elevated temperatures from solar radiation. The forest canopy radiates out the heat accumulated from solar radiation. The forest canopy provides moisture by transpiration through the leaves to the air and forest fuels, which decreases the possibility of forest fires. Transpiration from an area of dense vegetation can contribute up to eight times as much moisture to the atmosphere as can an equal area of bare ground. The forest canopy slows down wind movement and fire progress, due to its large friction area. A forest with a dense understory is an effective barrier to downslope winds.

The two most important weather, or weather-related, elements affecting wildland fire behavior are wind and fuel moisture. Wind affects wildfire in many ways. It carries away moisture-laden air and hastens the drying of forest fuels. Logs under a forest canopy remain more moist (approximately 25% more moist) through the season than those exposed to the sun and wind. The flow beneath a dense canopy is affected only slightly by thermal turbulence, except where holes let the sun strike bare ground or litter on the forest floor, causing local heating. Convective winds have their origin in local temperature differences. The nature and strength of convective winds vary with many other factors. Since they are temperature-dependent, all features of the environment that affect heating and cooling are significant. Even small openings in a moderate to dense timber stand may become warm air pockets during the day. These openings often act as natural chimneys and may accelerate the rate of burning of surface fires. Temperature of forest fuels, and of the air around and above them, is one of the key factors in determining how wildland fires start and spread.

Logging and logging roads open the forest canopy and increase the temperature of the air, the ground and the forest fuels, which accelerate the rate of burning of surface fires. Logging and logging roads open the forest canopy and lower humidity of forest fuels, which increases the flammability of forest fuels and critically influences the behavior of wildland fires. Logging and logging roads open the forest canopy and may cause rapid and intense fire spread.

In summary logging and logging roads change the fire prevention characteristics of the closed canopy forest and increase the chance of wildland fire.

QUOTES FROM FIRE WEATHER:

PREFACE

"The environment is in control in wildland fire fighting." "Outguessing mother nature in order to win control is an extremely difficult task." 1

INTRODUCTION

"When fire weather is combined with the two other factors influencing fire behavior - topography and fuel - a basis for judgment is formed." 2

BASIC PRINCIPLES

"These 'change of state' transformations [between liquid (water) and gas (water vapor)] account for much of the energy in weather phenomena." 3

"Continued heating (of water) will cause the temperature of the liquid water to rise until it reaches the boiling point, 212oF." 4

"The water will begin to change to vapor and the temperature will remain at 212oF, until all of the water is changed to vapor." 5

"Conduction is the transfer of heat by molecular activity." 6

"Some substances, such as copper, are good heat conductors." 7

"Other substances like glass, wood, paper and water are poor conductors." 8

"Forest litter is also a poor conductor." 9

"Most gasses, including air, are poor conductors; for example dead spaces are used in the walls of buildings as insulation to prevent rapid heat exchanges." 10

TEMPERATURE

"Temperature of forest fuels, and of the air around and above them, is one of the key factors in determining how wildland fires start and spread." 11

"Temperature indirectly affects the way fires burn, through its influence on other factors that control fire spread and rate of combustion (e.g., wind, fuel moisture, and atmospheric stability)."12

"Most of the warming takes place by conduction and convection from the heated surface of the earth and from longwave radiation from the surface." 13

"But in fire weather, we are also concerned with small scale patterns - those that change from hour to hour, from one slope facet to another, from one forest type to another, from a closed canopy to a forest opening, etc." 14

"In these patterns, temperature variations are often the controlling factor." 15

"large-scale weather patterns are commonly identified by sampling the weather at regular weather observation stations. Small-scale patterns and their variations, however, cannot be defined from measurements made at the usual widely spaced fixed stations." 16

"Many factors affect the air temperature; these include the type of ground surface, nearby trees, the local topography, and the height above the ground." 17

"The causes of these temperature changes are many and varied. However, three important processes underlie all causes: (1) Heating and cooling of the earth's surface by radiation, (2) exchanging of heat between the earth's surface and the air above it, and (3) conversion of thermal energy in the atmosphere to other forms of energy, and vise versa." 18

"Shading and scattering by any means, such as clouds, smoke or haze in the air, and objects such as trees reduce the solar radiation reaching the ground surface." 19

"All vegetation creates some shade, but the variation in type and density cause local differences in surface temperatures." 20

"Both liquid water droplets in clouds and in the atmosphere directly affect temperature. They both absorb some incoming radiation and clouds reflect some of the solar radiation." 21

"However, even when a certain amount of radiation strikes a surface, there are several properties of a substance itself which affect its resulting temperature." 22

"Tree crowns, grass, plowed land and sand are all good radiators." 23

"Incoming radiant energy striking a good conductor, such as metal, is rapidly transmitted as heat through the material, raising the temperature of the metal to a uniform level. The same radiant energy applied to a poor conductor tends to concentrate heat near the surface, raising the surface temperature higher than that in the interior. Wood, for example, is a poor conductor, and heat applied to it concentrates at the surface and only slowly penetrates to warm the interior. Leaf litter is another poor conductor. Common rocks, damp soil, and water although not as efficient conductors of heat as metals, are much better conductors than wood, other organic fuels or dry soils." 24

"Air is a very poor conductor, so porous substances such as duff or litter with many air spaces will bar the passage of surface heat to the soil below." 25

" To summarize, the surfaces of poor conductors get hotter during the day and cooler at night than the surfaces of good conductors." 26

"Litter surfaces composed of dry leaves, needles, and grass have low heat capacities, and as mentioned above, are also poor heat conductors. For these two reasons, direct solar radiation often heats litter surfaces to temperatures far above temperatures of overlying air without heating the soil below." 27

"Since water has a high specific heat and is a fairly good conductor, the surface temperatures of substances are greatly influenced by the presence of moisture. Moist surfaces, when compared with dry surfaces, will not reach as high temperatures in the day or as low temperatures at night."28

"The presence of moisture is also important because of the heat used in evaporation and released in condensation. We have seen that while 1 B.t.u. will raise the temperature of 1 pound of water 1oF, nearly 1,000 are required to evaporate one pound of water under normal conditions of pressure and temperature. Thus, when water is evaporated from a surface, cooling takes place at the surface with a corresponding reduction in the surface temperature." 29

"In all situations, vegetation moderates air temperature within the vegetative layer for several reasons. First, it intercepts both incoming and outgoing radiation and therefore has a marked effect on ground temperature; Second, green foliage does not warm up as much as ground or dry litter; and Third, leaf surfaces exchange heat with air through a deeper, less restrictive boundary layer. These effects result in less pronounced temperature changes with height above the ground."30

"The crowns of trees in a heavy forest form a nearly continuous cover and the canopy thus becomes, in effect, the air contact surface. The highest day time temperatures are found near the crown top, and the temperature will decrease gradually between this level and the ground. Maximum air temperatures near the crowns may be 18o to 20o warmer than air near the ground. Above the tree crowns the temperature decreases fairly rapidly with height, although never as rapidly as over bare ground. This is because the temperatures of the tree crown surfaces in contact with the air are lower than the bare ground, and because the air circulation around these surfaces is better." 31

Less dense vegetation will permit more solar radiation to penetrate to the ground than will a dense cover. The degree of partial ground shading provided by less dense vegetation determines the air temperature distribution between the ground surface and the canopy top. It will range between that found over bare ground and that found under a closed canopy." 32

"Air temperatures at the standard 4 1/2-foot height within the forest in the afternoon are likely to be 5o to 8o cooler than the temperatures in nearby cleared areas. Openings in a moderate to dense timber stand may become warm air pockets during the day. These openings often act as natural chimneys and may accelerate the rate of burning of surface fires which are close enough to be influenced by these 'chimneys'." 33

"Night temperatures in dense timber stands tend to be lowest near the top of the crown where the principal radiation takes place. Some cool air from the crowns sinks down to the ground surface, and there is some additional cooling at the surface by radiation to the cooling crowns. Sparse timber or other vegetation will merely decrease the strength of the inversion just above the ground surface." 34

ATMOSPHERIC MOISTURE

"Atmospheric moisture is a key element in fire weather. It has direct effects on flammability of forest fuels and, by its relationship to other weather factors, it has indirect effects on other aspects of fire behavior" 35

"We have already seen that moisture influences all surface temperatures, including surface fuel temperatures, by controlling radiation in its vapor state and by reflecting and radiating when it is condensed into clouds." 36

"Water vapor in the air comes almost entirely from three sources: Evaporation from any moist surface or body of water, evaporation from soil, and transpiration from plants." 37

"Plants have large surfaces for transpiration; occasionally they have as much as 40 square yards for each square yard of ground area. Transpiration from an area of dense vegetation can contribute up to eight times as much moisture to the atmosphere as can an equal area of bare ground." 38

"Relative humidity is most important as a fire-weather factor in the layer near the ground, where it influences both fuels and fire behavior." 39

"The relative humidity that affects fuels on the forest floor is often quite different from that in the instrument shelter, particularly in unshaded areas where soil and surface fuels exposed to the sun are heated intensely, and warm the air surrounding them. This very warm air may have a dew point nearly the same or slightly higher than the air in the instrument shelter, but because it is much warmer, it has a much lower relative humidity." 40

"Vegetation moderates surface temperatures and contributes to air moisture through transpiration and evaporation - both factors that affect local relative humidity. A continuous forest canopy has the added effect of decreasing surface wind speeds and the mixing that takes place with air movement." 41

"The differences in humidity between forest stands and open areas generally vary with the density of the crown canopy. Under a closed canopy, humidity is normally higher than outside (the closed canopy) during the day, and lower at night. The higher humidities are even more pronounced when there is a green understory." 42

"While temperature and moisture distribution in the layer of air near the ground are important in fire weather because of their influence on fuel moisture, the distribution of temperature and moisture aloft can critically influence the behavior of wildland fires." 43

ATMOSPHERIC STABILITY

"Most commonly considered in evaluating fire danger are surface winds with their attendant temperatures and humidities, as experienced in everyday living." 44

Less obvious, but equally important, are vertical motions that influence wildfire in many ways.

Also, in many indirect ways, atmospheric stability will affect fire behavior. For example, winds tend to be turbulent and gusty when the atmosphere is unstable, and this type of air flow causes fires to behave erratically." 45

"Subsidence occurs in large scale vertical circulation as air from high-pressure areas replaces that carried aloft in adjacent low-pressure systems. This often brings very dry air from high altitudes to low levels. If this reaches the surface, going wildfires tend to burn briskly, often as briskly at night as during the day." 46

"The amount of air heating depends on orientation, inclination and shape of topography, and on the type and distribution of ground cover." 47

"During condensation in saturated air, heat is released and warms the air and may produce instability; during evaporation, heat is absorbed and may increase stability." 48

GENERAL CIRCULATION

"So far we have considered straight flow and curved flow at levels high enough in the atmosphere so that the force of friction could be disregarded. But, when we consider airflow near the ground, we must account for the friction force. The effect of friction on air flow is to retard the movement. Therefore, friction is a force acting in a direction opposite to the airflow." 49

"The speed of the airflow is always lower with friction than without friction." 50

GENERAL WINDS

"The two most important weather, or weather-related, elements affecting wildland fire behavior are wind and fuel moisture." 51

"Wind affects wildfire in many ways. It carries away moisture-laden air and hastens the drying of forest fuels." 52

"The amount of reduction in speed and change of direction depends upon the roughness of the earth's surface. It follows then that the effect of friction is least over smooth water and greatest over mountainous topography." 53

"The leaf canopy in a forest is very effective in slowing down wind movements because of its large friction area." 54

"The flow beneath a dense canopy is affected only slightly by thermal turbulence, except where holes let the sun strike bare ground or liter on the forest floor." 55

CONVECTIVE WINDS

"Hence, convective winds here refer to all winds -up, down, or horizontal - that have their origin in local temperature differences." 56

"The nature and strength of convective winds vary with many other factors. Since they are temperature-dependent, all features of the environment that affect heating and cooling are significant. Among the more important are season, diurnal changes, cloud cover, nature of the terrain and its cover such as water, vegetation, or bare ground, and the moisture and temperature structure of the overlying atmosphere." 57

"Strong surface heating produces the most varied and complex convective wind systems." 58

"Orientation of the topography is an important factor governing slope- and valley-wind strength and diurnal timing." 59

"The vegetative cover on slopes will also affect slope winds and, in turn, valley winds. Bare slopes and grassy slopes will heat up more readily than slopes covered with brush or trees. Upslope winds will therefore be lighter on the brush- or tree-covered slopes. In fact, on densely forested

slopes the upslope wind may move above the treetops, while at the surface there may be very shallow downslope flow because of the shade provided by the canopy." 60

"Downslope winds at night on densely forested slopes are affected by the presence or absence of a dense understory. Where there is an open space between the tree canopy and the surface, the downslope flow will be confined to the trunk region while calm prevails in the canopy region. A forest with a dense understory is an effective barrier to downslope winds. Here, the flow is diverted around dense areas, or confined to stream channels, roadways or other openings cut through the forest." 61

WEATHER AND FUEL MOISTURE

"Fuel moisture content limits fire propagation. When moisture content is high, fires are difficult to ignite, and burn poorly if at all. With little moisture in the fuel, fires start easily, and wind and other driving causes may cause rapid and intense fire spread."62

"Water in living plants plays a major role in all plant life processes. In the leaves,. . . some water is transpired through leaf pores to become water vapor in the atmosphere." 63

"Logs under a forest canopy remain more moist (approximately 25% more moist) through the season than those exposed to the sun and wind." 64

FOOTNOTES

1 FIRE WEATHER . . . A Guide For Application Of Meteorological Information To Forest Fire Control Operations, by Mark J. Schroeder, Weather Bureau, Environmental Sciences Administration, U.S. Commerce Department and Charles C. Buck, Forest Service, U.S. Department of Agriculture, PREFACE, page IV

2 FIRE WEATHER, INTRODUCTION, page V

3 FIRE WEATHER, BASIC PRINCIPLES, Chapter 1, page 7

4 FIRE WEATHER, BASIC PRINCIPLES, Chapter 1, page 7

5 FIRE WEATHER, BASIC PRINCIPLES, Chapter 1, page 7, 8

6 FIRE WEATHER, BASIC PRINCIPLES, Chapter 1, page 8

7 FIRE WEATHER, BASIC PRINCIPLES, Chapter 1, page 8

8 FIRE WEATHER, BASIC PRINCIPLES, Chapter 1, page 8

9 FIRE WEATHER, BASIC PRINCIPLES, Chapter 1, page 8

10 FIRE WEATHER, BASIC PRINCIPLES, Chapter 1, page 8

11 FIRE WEATHER, TEMPERATURE, Chapter 2, page 19

12 FIRE WEATHER, TEMPERATURE, Chapter 2, page 19

13 FIRE WEATHER, TEMPERATURE, Chapter 2, page 20

14 FIRE WEATHER, TEMPERATURE, Chapter 2, page 20

15 FIRE WEATHER, TEMPERATURE, Chapter 2, page 20

16 FIRE WEATHER, TEMPERATURE, Chapter 2, page 20

17 FIRE WEATHER, TEMPERATURE, Chapter 2, page 21

18 FIRE WEATHER, TEMPERATURE, Chapter 2, page 21

19 FIRE WEATHER, TEMPERATURE, Chapter 2, page 22

20 FIRE WEATHER, TEMPERATURE, Chapter 2, page 22

21 FIRE WEATHER, TEMPERATURE, Chapter 2, page 23

22 FIRE WEATHER, TEMPERATURE, Chapter 2, page 23

23 FIRE WEATHER, TEMPERATURE, Chapter 2, page 23

24 FIRE WEATHER, TEMPERATURE, Chapter 2, page 23, 24

25 FIRE WEATHER, TEMPERATURE, Chapter 2, page 24

26 FIRE WEATHER, TEMPERATURE, Chapter 2, page 24

27 FIRE WEATHER, TEMPERATURE, Chapter 2, page 24

28 FIRE WEATHER, TEMPERATURE, Chapter 2, page 24

29 FIRE WEATHER, TEMPERATURE, Chapter 2, page 24, 25

30 FIRE WEATHER, TEMPERATURE, Chapter 2, page 30

31 FIRE WEATHER, TEMPERATURE, Chapter 2, page 30

32 FIRE WEATHER, TEMPERATURE, Chapter 2, page 30 , 31

33 FIRE WEATHER, TEMPERATURE, Chapter 2, page 31

34 FIRE WEATHER, TEMPERATURE, Chapter 2, page 31

35 FIRE WEATHER, ATMOSPHERIC MOISTURE, Chapter 4, page 33

36 FIRE WEATHER, ATMOSPHERIC MOISTURE, Chapter 4, page 33

37 FIRE WEATHER, ATMOSPHERIC MOISTURE, Chapter 4, page 39

38 FIRE WEATHER, ATMOSPHERIC MOISTURE, Chapter 4, page 41

39 FIRE WEATHER, ATMOSPHERIC MOISTURE, Chapter 2, page 43

40 FIRE WEATHER, ATMOSPHERIC MOISTURE, Chapter 4, page 45

41 FIRE WEATHER, ATMOSPHERIC MOISTURE, Chapter 4, page 46

42 FIRE WEATHER, ATMOSPHERIC MOISTURE, Chapter 4, page 46, 47

43 FIRE WEATHER, ATMOSPHERIC MOISTURE, Chapter 4, page 48

44 FIRE WEATHER, ATMOSPHERIC MOISTURE, Chapter 4, page 49

45 FIRE WEATHER, ATMOSPHERIC MOISTURE, Chapter 4, page 49

46 FIRE WEATHER, ATMOSPHERIC MOISTURE, Chapter 4, page 49

47 FIRE WEATHER, ATMOSPHERIC MOISTURE, Chapter 4, page 59

48 FIRE WEATHER, ATMOSPHERIC MOISTURE, Chapter 4, page 67

49 FIRE WEATHER, GENERAL CIRCULATION, Chapter 5, page 77

50 FIRE WEATHER, GENERAL CIRCULATION, Chapter 5, page 78

51 FIRE WEATHER, GENERAL WINDS, Chapter 6, page 85

52 FIRE WEATHER, GENERAL WINDS, Chapter 6, page 85

53 FIRE WEATHER, GENERAL WINDS, Chapter 6, page 88

54 FIRE WEATHER, GENERAL WINDS, Chapter 6, page 104

55 FIRE WEATHER, GENERAL WINDS, Chapter 6, page 105

56 FIRE WEATHER, GENERAL WINDS, Chapter 6, page 108

57 FIRE WEATHER, GENERAL WINDS, Chapter 6, page 108

58 FIRE WEATHER, GENERAL WINDS, Chapter 6, page 108

59 FIRE WEATHER, GENERAL WINDS, Chapter 6, page 118

60 FIRE WEATHER, GENERAL WINDS, Chapter 6, page 118, 119

61 FIRE WEATHER, GENERAL WINDS, Chapter 6, page 119

62 FIRE WEATHER, WEATHER AND FUEL MOISTURE, Chapter 11, page 180

63 FIRE WEATHER, WEATHER AND FUEL MOISTURE, Chapter 11, page 182

64 FIRE WEATHER, WEATHER AND FUEL MOISTURE, Chapter 11, figure page 191