E('OMX4Y OF FlXE IN GRASSLANDS
213
point, and ttic materials do nut provide R continuouk range of values
and so give only ntepwinc. eqtirnates of tho maximum temperature
attained. What is wior(3 importtant, them is no indication of the duration
of high temperatures, and duration strongly conditions the biologic
effectivity of any temperature Ievcl.
The second clms of apparatus involves the use of thermistors or
thermocouples (Breuer, 1965), t h e latter being somewhat superior since
they measure t f , wider rrtiige of temperature and the smaller size of their
sensing elements permits a more precise determination of temperature
stratification. Either of these electric sensors may be connected to
automatic recorders so that the duration of temperature at each level
is then known. A special problem involved here are errors resulting
from the conductnncc of heat along the wires either toward or away
from the measuring junction, but this can be largely circumvented by
positioning tho terminal 10 cni or so of the wires (if they are no more
than 0.3 mm in diameter) so they will be heated to the same temperature as the measuring junction (Vaartajit, 1949). It is theoreticrtlly impossible to meltsure the temperature of the ctctual surface of minor81
eoil even with a thermocouple, but the very fine wires that can be used
may be inserttd just beneath the surface layer of grains to give the
closest approximation of surface temperatures that is possible.
Another approach to evaluating heat liberation consists of setting
out one-gallon cans, painted flat black, provided with a 1 cnia hole in
the lid, each containing 3 liters of water (Beaufait, 196G). In the interpret,ation of weight loss resulting from the evaporation of water I ~ B a
fire front passea, it h a been pointed out that 80 cal are required to
bring each gram of water a t 20°C up to the boiling point, then 540 cal
are required to convert it to vapor. In theory 1860 kcal would be
needed to boil off a11 3 litere of water.
111. THE CIIARACTER OF GRASS FIRES
Graw fires we characterized by a rather narrow zone of flames
advancing acrom a finely divided and rather homogeneously dispersed
fuel. The fine texture of such fuel makes i t more sensitive to weather
change than ie true of woody aerial stems. Also the former fuel type
burns more rapidly and completely as the fire front pusses (McArthur,
1966; Mortan, 1964).
The spread of a grass fire accelerates quickly to a maximum rate,
sometimes requiring as few as 5 min. Heyward (1938), studying fires in
gram beneath open pine forcnt , found that temperature condibions were
no different in firm of 3 x 3 m and those 18 hectares in extent, 80 that for
are= 3 x 3 m or larger, size is not, important in experimental burning.
213
point, and ttic materials do nut provide R continuouk range of values
and so give only ntepwinc. eqtirnates of tho maximum temperature
attained. What is wior(3 importtant, them is no indication of the duration
of high temperatures, and duration strongly conditions the biologic
effectivity of any temperature Ievcl.
The second clms of apparatus involves the use of thermistors or
thermocouples (Breuer, 1965), t h e latter being somewhat superior since
they measure t f , wider rrtiige of temperature and the smaller size of their
sensing elements permits a more precise determination of temperature
stratification. Either of these electric sensors may be connected to
automatic recorders so that the duration of temperature at each level
is then known. A special problem involved here are errors resulting
from the conductnncc of heat along the wires either toward or away
from the measuring junction, but this can be largely circumvented by
positioning tho terminal 10 cni or so of the wires (if they are no more
than 0.3 mm in diameter) so they will be heated to the same temperature as the measuring junction (Vaartajit, 1949). It is theoreticrtlly impossible to meltsure the temperature of the ctctual surface of minor81
eoil even with a thermocouple, but the very fine wires that can be used
may be inserttd just beneath the surface layer of grains to give the
closest approximation of surface temperatures that is possible.
Another approach to evaluating heat liberation consists of setting
out one-gallon cans, painted flat black, provided with a 1 cnia hole in
the lid, each containing 3 liters of water (Beaufait, 196G). In the interpret,ation of weight loss resulting from the evaporation of water I ~ B a
fire front passea, it h a been pointed out that 80 cal are required to
bring each gram of water a t 20°C up to the boiling point, then 540 cal
are required to convert it to vapor. In theory 1860 kcal would be
needed to boil off a11 3 litere of water.
111. THE CIIARACTER OF GRASS FIRES
Graw fires we characterized by a rather narrow zone of flames
advancing acrom a finely divided and rather homogeneously dispersed
fuel. The fine texture of such fuel makes i t more sensitive to weather
change than ie true of woody aerial stems. Also the former fuel type
burns more rapidly and completely as the fire front pusses (McArthur,
1966; Mortan, 1964).
The spread of a grass fire accelerates quickly to a maximum rate,
sometimes requiring as few as 5 min. Heyward (1938), studying fires in
gram beneath open pine forcnt , found that temperature condibions were
no different in firm of 3 x 3 m and those 18 hectares in extent, 80 that for
are= 3 x 3 m or larger, size is not, important in experimental burning.