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

"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)." 1

"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." 2

"In these patterns, temperature variations are often the controlling factor." 3

"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." 4

"Many factors affect the air temperature; these include the type of ground surface, nearby trees, the local topography, and the height above the ground." 5

"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." 6

"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." 7

"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'." 8

"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." 9

"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" 10

"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." 11

"Relative humidity is most important as a fire-weather factor in the layer near the ground, where it influences both fuels and fire behavior." 12

"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." 13

"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." 14

"Most commonly considered in evaluating fire danger are surface winds with their attendant temperatures and humidities, as experienced in everyday living." 15

"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." 16

"The amount of air heating depends on orientation, inclination and shape of topography, and on the type and distribution of ground cover." 17

"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." 18

"The two most important weather, or weather-related, elements affecting wildland fire behavior are wind and fuel moisture." 19

"Wind affects wildfire in many ways. It carries away moisture-laden air and hastens the drying of forest fuels." 20

"The leaf canopy in a forest is very effective in slowing down wind movements because of its large friction area." 21

"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." 22

"Hence, convective winds here refer to all winds -up, down, or horizontal - that have their origin in local temperature differences." 23

"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.

"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." 24

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." 25

"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." 26

"Logs under a forest canopy remain more moist through the season than those exposed to the sun and wind." 27

FOOTNOTES:

1 FIRE WEATHER, TEMPERATURE, Chapter 2, page 19

2 FIRE WEATHER, TEMPERATURE, Chapter 2, page 20

3 FIRE WEATHER, TEMPERATURE, Chapter 2, page 20

4 FIRE WEATHER, TEMPERATURE, Chapter 2, page 20

5 FIRE WEATHER, TEMPERATURE, Chapter 2, page 21

6 FIRE WEATHER, TEMPERATURE, Chapter 2, page 21

7 FIRE WEATHER, TEMPERATURE, Chapter 2, page 30

8 FIRE WEATHER, TEMPERATURE, Chapter 2, page 31

9 FIRE WEATHER, TEMPERATURE, Chapter 2, page 31

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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