Weather conditions associated with understory prescribed burning in south Georgia / by James T. Paul, Curtis S. Barnes and James C. Turner, Jr

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tsHL GEORGIA FOREST RESEARCH PAPER

46
Nov., 1983

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WEATHER CONDITIONS ASSOCIATED WITH UNDERSTORY PRESCRIBED BURNING IN SOUTH GEORGIA

by James T. Paul, Curtis S. Barnes and James C. Turner, Jr. Received
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MAY 3 1988

RESEARCH DIVISION

DOCUMENTS UGA LIBRARIES

GEORGIA FORESTRY COMMISSION

The Authors

JAMES T. PAUL is Research Meteorolo-
gist and Project leader with the Southeastern Forest Experiment Station, Ma-
con, Georgia. He received BS (1960), MS
(1966), and PhD (1973) degrees from the University of Georgia. He was an Air
Force meteorology student at the University of Texas from 1960-61 and became aviation forecaster, Hunter Air Force Base, Savannah, Georgia, 1961-63. He began work with the Southern Fire Labora-
tory, Macon, in 1967. From 1973-75, he was head of Project THEO, a joint Forest Service-Navy research program on fog and smoke. As Project Leader of Forestry Weather Data Systems, he is responsible
for the design, implementation, and operation of the Forestry Weather Interpretations System (FWIS).

CURTIS S. BARNES, formerly Associate
Chief of Forest Protection of the Georgia Forestry Commission, was a graduate of
the University of Georgia. He joined the Commission in 1949 as ranger of Dodge County, was made Assistant District Forester of Americus District in 1950, and District Forester of Newnan District in 1951. He was transferred to the Macon headquarters in 1955 to assume the position of Assistant Fire Chief, named Regional Forester in 1960, and was Associate Chief of Forest Protection from 1969 until his retirement in December
1981.

JAMES C. TURNER, JR. is retired Chief
of Forest Protection, Georgia Forestry Commission, Macon. Since joining the Commission in 1947, he had held a number of positions in various divisions. He received his Bachelor of Science in Forestry from the University of Georgia in 1947.

ACKNOWLEDGMENT
This work was sponsored by the Georgia Forestry Commission and could not have been completed without their personnel and financial contribution.

WEATHER
The Uncontrollable
Limiting Factor
in a Prescribed Burning Program
by James T. Paul, Curtis S. Barnes and James C. Turner, Jr.

Fire provides the southern forester with an economical and effective tool for use in wildlife habitat enhancement, removal of hazardous accumulations of fuels, disease control, seedbed preparation, and species management. Almost 500,000 acres were burned in Georgia
during 1972 for these varied management objectives (Hough and Turner 1974). Timing of a burn (hour of ignition, sea-
son), firing technique (headfire, backfire,
etc.), manpower and equipment, placement of firelines, and weather are elements a forest manager must consider for
a successful burn. All these elements, except weather, can be partially or wholly manipulated by the forester. Consequently, weather usually is the uncontrollable limiting factor in a prescribed burning program. Typically a forester plans the burn (selects firing technique, plows fireline, etc.), then waits for the unique set

of weather variables suitable for conducting his burn to meet a specific management objective. Sometimes it is a long
vigil.
Meteorology has progressed in this country from occasional observation and
comment by medical doctors, a military
expedition, or curious citizens to a hightechnology science involving electronic sensors, high-speed digital computers, satellite observations, and mathematical models used to estimate the state of the atmosphere out to 72 hours. Curiously, the use of weather data in planning and executing prescribed burning has changed
relatively little in the past 20 to 40 years. Perhaps this is partially due to an incomplete knowledge of how the various weather elements interact with the fuel complex to generate reproducible rates of spread, flame height, residence time, and

fireline intensity. Also, foresters frequent-
ly do not have a clear understanding of sources of weather data, which weather elements are subject to unexpected change on the short time scales relevant
to prescribed fire, and how these impor-
tant weather/fire variables vary over relatavely short distances. For example, a manager might conduct a successful burn
in one county, but in the adjoining county, weather conditions could produce a fire too cool or too hot.
In this study, data are presented that describe spatial and diurnal variability of the weather elements important for prescribed burning, the frequency of major weather systems that are responsible for rapid change of these variables, and the application of data in the Forestry Weather Interpretations System (FWIS) to the prescribed burning problem.

Other compensating factors include a higher windspeed, which dissipates heat from the fire, or a higher fuel moisture, which produces a lower intensity fire. Experienced burners frequently burn with air temperatures up to 60F with appropriate attention to the compensating factors. Conversely, a burn in a young plantation might require an air temperature of no more than 40 F to avoid damaging scorch. Air temperature is relatively easy to forecast, but a major departure from the forecast can occur if timing is off on the passage of a frontal system or when the percentage of sky covered by clouds is different from what is expected.

3.5

35

3.5

Figure 1. --Average February rainfall in inches, south Georgia, 1954 to 1963.

WEATHER ELEMENTS THAT INFLUENCE PRESCRIBED BURNING
The National Weather Service (NWS) provides Georgia forestry forecasts which include information on rainfall type (rain, rain shower, thunderstorm, etc.), the expected amount and duration, and the
probability of occurrence for today,
tonight, and tomorrow. NWS also pro-
vides estimates of today's maximum, tonight's minimum, and tomorrow's maximum temperature; today's minimum, tonight's maximum, and tomorrow's minimum relative humidity; and highest wind for today, tonight, and tomorrow.

may be important for prescribed burning.
For example, during February (19541963), the average monthly rainfall varied almost 2 inches over south Georgia (figure 1). Averaging over a long period smooths the variability associated with individual storms; consequently, on a given day, variable fuel moisture over fairly short distances should be expected even though there was a general rain across the state. The exact timing of a rainfall event is difficult and is probably the most com-
mon error in precipitation forecasts.
However, attention to the latest available forecast should avert most problems that foresters experience with precipitation
timing.

Rainfall
The rainfall history at a burn site is the single most important weather element influencing total fuel moisture. Lack of
rainfall, especially over long periods, or
recent heavy rains may result in fire intensity inconsistent with the management
objective.
Rainfall during October through May
is usually associated with frontal systems that produce a relatively uniform spatial
distribution when compared to spotty summertime showers. However, even the
lower spatial variability of frontal rain

Temperature
Air temperature contributes to the rate at which fuels dry, but more directly, it influences needle scorch. In general, the higher the air temperature, the greater the scorch potential. Mobley et al. (1978)
recommend an air temperature in the range of 30 - 50F. There are modifying
factors that permit burning at higher temperatures with no great increase in the
scorch potential. For example, when burning under a mature stand the crowns
are high and heat generated by the fire has a better opportunity to dissipate.

Relative humidity
Next to precipitation, relative humidity is the major factor influencing fine fuel moisture. If the humidity is low, finer fuels (such as the upper layer of pine needles and grass) will burn within a few hours after rain. Since the finer fuels are largely responsible for rate of spread, it is of obvious importance to the manager.
Rapid changes in humidity may occur with frontal passage (cold, warm, occluded, or sea breeze) and when the air becomes unstable with resultant vertical
mixing. Relative humidity is also relatively
easy to forecast with major departures (forecast vs. observed) occurring with timing and intensity errors on frontal systems. The guideline for relative humidity is 30-50 percent (Mobley et al. 1978). Burning at less than 30 percent is risky because of the higher fire intensity and potential for spotting usually associated with these lower humidities. Burning at humidities appreciably higher than 50
percent may result in a missed management objective.
Wind
Once the fire is ignited, wind has the potential to create more problems than any other weather element. Wind at a specific time and place is relatively difficult to forecast. Accuracy of wind-direc-
tion forecasts increases with increasing
windspeed. Less than 5-7 mph (measured
at 20 feet in the open) surface friction and unequal surface heating of different soils and vegetation types interact to increase the variability of wind direction over a short period. At the low speeds, a predominant direction can be specified, but it should be understood that observed
wind direction may visit all points of the
compass within a 5- to 10-minute period. Many burns are planned as either headfires or backfires, and a deviation of more

1

than 45 degrees in wind direction may defeat the management objective through
its influence on fire intensity, rate of spread, and the increased potential for
control problems. An example of a wind-
direction shift is shown in figures 2 and 3 for an experimental burn near Waycross,
Georgia, during the spring of 1982. Figure 2 is typical of a backfire where the wind bends the flame into the burned area. Figure 3 is the same fire about 5 minutes after a wind shift occurred.
Mobley et al. (1978) recommend a windspeed of 2 to 10 mph in the stand, or 5 to 18 mph in the open. Low windspeeds
contribute to needle scorch since heat tends to rise vertically instead of being dissipated horizontally as with stronger
winds. The most common cause for fore-
cast error in wind (more than 5 to 7 mph) is missed timing on frontal passages and
failure to anticipate degree of pressure system intensification.

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Figure 2. -An experimental backfire near Waycross, Georgia, on February 10, 1982. Photo
courtesy F ire Science Research Work Unit, Macon, Georgia.

MAJOR WEATHER SYSTEMS AND PRESCRIBED BURNING

Major changes in individual weather
elements occur with the movement of

weather systems. Those systems that

commonly produce major changes, espe-

cially those with the potential to change

during the course of the burn, will be dis-

cussed.
A typical frontal system (figure 4) has

cold, warm, and occluded fronts as boun-

daries between warm, cool, and cold sec-

A tor air masses.

cold front is cold air

moving over an area formerly occupied
by warm air (the temperature drops with passage), and a warm front replaces cool

air (the temperature increases with pass-
age). An occluded front occurs when the faster moving cold front forces the warm

air sector aloft. The actual frontal zone is

not a sharp, narrow band but is typically

a diffuse zone where rapid changes in weather occur. The cold frontal zone is

usually 10 to 50 miles wide, while the
zone associated with warm fronts may be

more than 100 miles wide. Occluded fronts may have a wide band of weather,

but those that pass through Georgia usu-

ally are very similar to cold fronts.
Warm fronts that influence Georgia

weather usually move from the Gulf of

Mexico, traveling in a northerly direction.

Cold fronts approach Georgia from the

west, northwest, north, and occasionally

from the northeast. Occluded fronts usu-

ally approach from the west. The follow-

ing discussion will focus on cold fronts

because the best burning conditions usu-

ally occur after a cold frontal passage.

Figure 3. --An experimental backfire near Waycross, Georgia, on February 10, 1982, after a wind shift. Photo courtesy F ire Science Research Work Unit, Macon, Georgia.

OCCLUDED FRONT

Cool Sector
-- 1 1 ) Wind direction Southeast to Northeast -- (2) Windspeed Light to moderate -- (.i) Temperature Cool -- (41 Relative humidity High -- (5) Weather Continuous ram, drizzle, with
occasional showers
(6) Stability --Stable

Cold Sector
-- ( 1 1 Wind direction West to North -- (2) Windspeed Strong, typically gusting -- (3) Temperature Cold -- (41 Relative humidity Low (5) Weather - Clear sky -- (6) Stability Stable except where midday
heating may produce low-level pockets
of unstable air

Warm Sector
-- 1 ) Wind direction East to Southwest -- (2i Windspeed Light to moderate (ill Temperature -- Warm -- I4i Relative humidity Moderate to high -- (5) Weather Intermittent rain, drizzle,
showers, occasionally organized line
thunderstorms
-- )Bi Stability Usually unstable, expecit
during daylight hours

COLD FRONT
Figure 4.-Diagramatic frontal system with a description of weather commonly associated with each sector. Area inside the hatched line indicates where precipitation is most probable.

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NOV

DEC.

JAN.

FEB.

MAR.

APR.

MAY

MONTH

Figure 5. --Average number of cold frontal passages for south Georgia, October through May, 1968, 1969, 1978.. 1979.

Cold front
The passage of cold fronts in south Georgia from October through May averaged six fronts per month or one about every 4 or 5 days (figure 5). The average number of frontal passages in November and February was five, while May averag-
ed only four. Conditions suitable for prescribed burning usually occur 2 or 3 days after a rain-producing cold frontal system
has passed. The probability that a second
front will pass within 7 to 8 days after

initial frontal passage is about 85 percent (figure 6). There is also about a 30 percent probability that a second front will pass within 3 days of the initial front. The high frequency of these systems is a major limitation on the probable number of days with good prescribed burning
conditions in Georgia.
The results of a cold frontal passage on an active prescribed burn can be devastating. Typically, wind shifts from a south-

westerly direction to northwest and speed
increases to 10 to 15 mph with gust po-
tential of 30 to 35 mph. Humidity might drop from the high 90s to less than 30 percent within a few hours. Temperature might drop from the high 70s to 40 or lower with a strong front. The frontal zone itself is an area of rapid change where various physical forces are adjusting to a new equilibrium as the front moves forward. Consequently, wind direction may shift or spin around to all points of the compass until the front is well past the burn site.

Sea breeze front

Figure 6. -Probability (%) of a cold frontal passage occurring within a specific day (1-14) given a
passage on day for the period October through May in south Georgia.

The Georgia coastline from the Savan-
nah River in the north to the St. Mary's River in the south is periodically influenced by a heat-driven sea breeze front. Figure 7 is a diagramatic representation of a sea breeze front, showing typical weather
behind and ahead of the front. An Atlan-
tic Coast sea breeze front is unlikely to
occur and move inland when there is a
strong westerly wind. Favorable condi-
tions for formation and movement are a weak wind and pressure field over land and high land temperatures when compared to the ocean. As the hot air over
land rises, it is replaced with cooler air flowing from the ocean.
During the 1960s and 1970s, the U. S. Forest Service operated an automated weather collection system in the coastal strip in cooperation with the U.S. Navy (Paul and Williams 1971, Williams 1973). Figures 9, 10 and 11 are based on a portion of these data collected at the Harris Neck Wildlife Refuge (see figure 8 for location). The frequency of sea breeze fron-

Ahead of Front
-- (1) Wind direction Variable
Northwest to Southwest
-- (2) Windspeed Light -- (3) Temperature Warm to hot -- (4) Relative humidity Low to
moderate
-- (5) Weather Scattered to broken
clouds, occasional rain shower
-- (6) Stability Usually slightly
unstable, especially in vicinity of front
The area between dashed line and sea breeze front will likely have broken clouds with a higher probability of showers and thunderstorms

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Behind Front
-- (1) Wind direction Typically about
1 10 or perpendicular to coastline
-- (2) Windspeed Low to moderate -- (3) Temperature Cool -- (4) Relative humidity High -- (5) Weather Usually clear to
scattered clouds
-- (6) Stability Usually stable

Figure 7.-A diagramatic sea breeze front with descriptions of conditions ahead and behind front.

HOURLY WEATHER OBSERVATIONS
. GEORGIA FIRE WEATHER STATIONS
HARRIS NECK WILDLIFE REFUGE

Figure 8. --Location map of study area.

ST. SIMONS ISLAND JACKSONVILLE

tal systems reached a maximum in May,
largely because land/sea temperature dif-
ferences are greater during this month (figure 9). The minimum number of passages occurred in December, while the average for all months was eight. Favor-
able conditions for a sea breeze occur-
rence are likely to be persistent on consecutive days. This is reflected by the 40
percent probability of sea breeze front
tomorrow if one has occurred today (fig-
ure 10).

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Most sea breeze passages occurred between 1200 and 1500 e.s.t. with no occurrences before 0800 or after 1800 e.s.t. (figure 11). If a sea breeze front has not passed Harris Neck by 1500 e.s.t., the chances of one passing after 1500 e.s.t. become progressively smaller. Data presented by Williams (1973) on passage time and inland penetrations of the sea breeze front were reanalyzed for this study. There is a seemingly logical inconsistency in the data because more sea breeze fronts pass a point 10 to 15 miles inland than at locations to 10 miles from the ocean (figure 12). According to Williams, not all sea breeze fronts that form on a given day retreat to the ocean or dissipate during the late afternoon and nighttime hours. Consequently, on the succeeding days, the front begins its inland movement from its current position (or forms at an inland location) and never passes the more seaward points. As a result, some coastline zones may be influenced by flow from the ocean for 24 to 72 hours with attendant higher fuel moisture, higher winds, and lower temperatures. If a sea breeze front passes in the 10- to 15-mile coastal zone, then figure 12 can be used to roughly estimate the depth of inland penetration and the likely arrival time of the front.
The sea breeze influences prescribed burning in the following ways:
1. With passage, the wind may shift 45 to 180 degrees and will usually
increase in speed.
2. Humidity will increase and temperature drops behind the front, and
the burn may not be as hot as
planned.
3. The zone immediately ahead of the
front is typically unstable. If a stand is being burned under potentially high fire-danger conditions, the approach and passage of a sea breeze front could result in excessive scorch, unacceptable mortality, or an escaped fire.
4. The sea breeze is a weak frontal system (figure 7) and, to the observer on the ground, has the appearance of surging back and forth over
a given location before final pass-

OCT.

NOV.

DEC

JAN.

FEB

MAR.

APR

MAY

MONTH

Figure 9. --Average number of sea breeze frontal passages at Harris Neck National Wildlife Refuge for a 3-year period 1967-1970 (Williams 1973).

age. The result of this surging action on burning would be similar
to that discussed in 3 above, but might generate additional problems due to multiple passages.
SPATIAL VARIABILITY OF
PRESCRIBED BURNING WEATHER
Weather can vary over relatively short distances even in the absence of fronts or other major meteorological systems. November is typically a dry month with clear skies and light or calm winds. With
these conditions, local site factors (vege-
tative type, soil series, etc.) are more obvious in their influence on weather elements, such as the daily range of temperature. With no wind, how energy from the

sun is absorbed and reradiated determines air temperature at a specific location. Over water, the daily temperature range would be small compared to what might be observed over an asphalt surface. Forested sites fall somewhere between these extremes. With increasing windspeed, the influence of site factors begins to disappear and vanishes at some higher windspeed due to rapid mixing of the air near the surface.
By plotting temperature range, one might expect some hint of spatial variability in temperature due to site (figure 13). The zone of rapid change along the coast-
line reflects the differences in how energy
from the sun is absorbed and reradiated over land and water. If there was no wind, there would be a single sharp line dividing the temperature over land vs.

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DAYS BETWEEN SEA BREEZE PASSAGES

Figure 10.-Probability (%) of a sea breeze frontal passage occurring within X days (1-14) given a passage on day 0.

8

that over water. Frontal systems and

other weather systems mix the air over

land and water, and convert the dividing

line to a more diffuse zone. The magni-

tude of the temperature range (about

18-20F) is less near the coast due to

the moderating influence of the ocean.
A second area of fairly rapid change

in temperature range was found north of

Valdosta. This is in the agriculture belt of

Georgia, and the vegetation is mixed for-

ests and agriculture crops. This suggests

that the differing vegetative types over

short distances at least contribute to the

observed range. The vegetation between

Valdosta and Brunswick is largely pine

forests, and the spatial change of temper-

ature range is small in this area.
NWS observational stations are usually

located at airports where large open
spaces are common (figure 14). Forestry

fire weather stations (figure 15) are typi-

cally located in smaller clearings and are

more likely to reflect local site condi-

tions. Data from Georgia Forestry Com-

mission stations in Turner, Mitchell, and

Lowndes Counties (see figure 8 for loca-

NWS tion) were chosen instead of

sta-

tions for the portion of this study relating

to space differences in order to show
maximum differences that might occur

over relatively small distances. These data

(figures 16-19) are useful to illustrate the

spatial variability of weather. For

example, if the 1300 e.s.t. observation at

a fire weather station is:

Temperature Relative humidity Windspeed 1-hour timelag fuei
moisture-17

70 F 40 percent
10mph
6 percent

the probability that similar values will occur within a 50-mile radius of the observing station is:
Temperature (figure 16): Within -- 5
or less, 80 percent of the time Relative humidity (figure 17): Within
-- 10 percent or less, 80 percent of
the time

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HOUR OF DAY (e.s.t.)

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Figure 1 1 .--Number of sea breeze frontal passages at Harris Neck Wildlife Refuge by hour for October through May (Paul and Williams 1971).

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DISTANCE FROM COASTLINE (MILES)

Figure 12. -Probability of sea breeze frontal passage at inland stations given a passage at either 10 or 15 miles. Representative passage times (e.s.t.) are plotted on the curve (Williams 1973).

mph, 80 percent of the time Thour timelag fuel moisture (figure
19): Within 2 to 3 percent or less, 80 percent of the time.
Figures 16-19 are valid only for the area
and months used to develop the curves. However, they highlight three major points important for prescribed fire that should be generally true for any compar-
able-size area or time period: 1. Spatial variability of weather data is usually low for most days.
-- Fuel moisture was calculated from
temperature, relative humidity, and cloud
cover.

On some days the variability is high (temperature -- 15F, relative humidity +-- 30 percent, windspeed +--
10 mph, and fuel moisture -- 15
percent.
If a prescribed burn is conducted on a high variability day, the burn-
er may experience different weath-
er at the burn site from what existed at a central office. If he proceeds without an onsite observation (such as with a belt weather kit), the results of the burn could be dramatically different from that expected, or result in an escaped fire.

DIURNAL VARIABILITY OF PRESCRIBED BURNING WEATHER
Changes in weather over a 24-hour period at a given location usually exceed spatial variability within a 50-mile radius of an observation point. The days available for prescribed burning might be increased if the manager could tailor his burn time to take advantage of the 24hour variability in weather.
The diurnal curves for temperature (figure 20), relative humidity (figure 21), windspeed (figure 22), and 1-hour timelag fuel moisture (figure 23) are based on 10

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Figure 14.-National Weather Service instrument site at Cochran Field, Macon, Georgia.
Figure 13.--Average November temperature range in degrees F, south Georgia, 1954 to 1963.
Figure 15. -Georgia Forestry Commission fire weather station at Turner County, Georgia. 10

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TEMPERATURE DIFFERENCE (F.,sign ignored)

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RELATIVE HUMIDITY DIFFERENCE

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Figure 16. --Probability of air temperature varying less than value
specified on X axis within 50 miles of an observation, based on 2 years of 1300 e.s.t. observations.

Figure 17. --Probability of relative humidity varying less than the
value specified on the X axis within 50 miles of an observing station, based on 2 years of 1300 e.s.t. observations.

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DIFFERENCE IN WINDSPEED

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DIFFERENCE IN I-H0UR TIMELAG FUEL

MOISTURE (%, sign ignored)

Figure 18. -Probability of windspeed varying less than the value
specified on the X axis within 50 miles of an observing station, based on 2 years of 1300 l.s.t. observations.

Figure

19. -Probability of 1-hour timelag fuel moisture varying less
than the value specified on the X axis within 50 miles of an observing station, based on 2 years of 1300 e.s.t.
observations.

11

cases of 3 successive days of data after frontal precipitation. The upper and lower limits (Mobley et al. 1978) are shown on each figure. The number of hours within a prescription limit, the beginning hour, and ending hour can be approximated by using these figures if an observation and forecast are available that
contain as a minimum:

1. Observed a. Temperature b. Relative humidity c. Windspeed d. Cloudy or sunny

2. Forecast
a. Maximum temperature b. Minimum relative humidity c. Maximum wind

Assume a 1000 e.s.t. observation was
A available (point in the figures)

Temperature Relative humidity Windspeed Sunny

35 F 50 percent
5 mph

and a forecast (point B in the figures)

down. Where wind is critical, these hours should be avoided or otherwise accounted for in the burn plan. The climatological curve and the sample plot (dashed line, figure 23) shows 1-hour timelag fuel moisture slightly below the preferred range from about 1100 to 1700 e.s.t.
FREQUENCY OF WEATHER VARIABLES USED IN PRESCRIBED
BURNING

Foresters frequently use the 1300 e.s.t. observation in conjunction with a weather forecast in making decisions for prescribed burning because this is usually the only observation available. The preferred range of weather variables (Mobley et al. 1978) for prescribed burning is:

Temperature

20-50F

Relative humidity

30-50 percent

Windspeed (20 ft. open) 5-20 mph

Fuel moisture

7-20 percent

Frequency of occurrence of these weather variables was computed with
data for October through May when these
weather variables were within the preferred range in Turner, Mitchell, and

Lowndes Counties. Temperature was the single most limiting variable, occurring in the preferred range only about 20 percent of the time (Table 1). The upper limit of temperature can be raised to about 60 if the stand has a low scorch potential. Relative humidity was the least limiting factor at Turner and Mitchell, and was with-
in the preferred range almost half the time. In general, the overall frequency of individual variables was about the same for Turner and Mitchell Counties, with
Lowndes exhibiting a somewhat different
overall pattern.
Average February maximum tempera-
tures range from the low 60s near Savannah to just over 70 near the Florida border (figure 24) As expected, temperature increases toward the southern part of the state. Stands less susceptible to scorch as a result of higher ambient temperature could be burned on the days with higher temperatures. Because the desired temperatures occur on only 20 percent of the days during the burning season, a manager should plan to fully utilize these days when they do occur. This can be done effectively by matching climatology with burning objectives and the resources available to conduct the burn.

Maximum temperature Minimum relative
humidity
Maximum windspeed

51 F
35 percent
10 mph

By plotting the appropriate observation (point A) and the forecast (point B) on figures 20-23, and by constructing a dash-
ed line through points A and B roughly
parallel to the climatological curve, one can estimate when the weather variables will be within prescription. For example, temperature would be within limits all day except for a short period in midafternoon, when it would be 51F. This small excursion outside the preferred range would not be of importance for most burns. Relative humidity would be in range from 1000 to about 1900 e.s.t.
When burning in an area where a hot fire
could produce lethal temperatures, consideration should be given to adjusting the time of burn away from the relative
humidity minimum at 1400. Windspeed does not follow a smooth
diurnal curve, but figure 22 does repre-
sent what is commonly observed, i.e.,
after a cold frontal passage, windspeed drops sharply after sundown and picks up
T after sunup. For a or 2-hour period be-
fore and after sunup and sunset, the wind field is undergoing adjustment to reflect changing surface-heating conditions. Typically, both windspeed and direction can be quite variable near sunup and sun-

Variable

Preferred
Range

Percent frequency within preferred range

Turner County

Temperature (F)

20-50

22

Relative humidity (%)

30-50

48

Windspeed (mph)

5- 18

29

Fuel moisture (%)

7-20

32

Lowndes County

Temperature (F)

20-50

17

Relative humidity (%)

30-50

35

Windspeed (mph)

5- 18

48

Fuel moisture (%)

7-20

27

Mitchell County

Temperature (F)

20-50

20

Relative humidity (%)

30-50

48

Windspeed (mph)

5- 18

37

Fuel moisture (%)

7-20

30

Table 1.-- Frequency of occurrence of individual weather variables used in prescribed burns for the months October through May, 1978-1979, in three Georgia
counties.

12

70i-

60-

^50-

UPPER LIMIT

25

%40
I
ki 20 10

A
LOWER LIMIT

^-^.
20

UPPER LIMIT

1

1

1

1

i

i

i

i

i

i

i

i

8

10

12

14

20 22

HOUR OF DAY (est)

Figure 20. --Average diurnal temperature curve for 3 days after precipi-
tation occurred. Data are averaged for three south Georgia
NWS stations. The upper and lower limits for prescribed
burning are shown with an expected curve constructed using a 1000 e.s.t. observation (A) and the forecast maximum (B).

15
10
LOWER LIMIT \.

90r-

J

LJ

I

8

10

12

14

16

18 20 22

HOUR OF DAY (est.

Figure 23. -Average diurnal 1-hour timelag fuel moisture curve for 3
days after precipitation occurred. Data are averaged for three
south Georgia NWS stations. The upper and lower limits for
prescribed burning are shown with an expected curve constructed using a 1000 e.s.t. observation (A) and the forecast maximum (B).

8

10

12

14

HOUR OF DAY (e.s.t.)

Figure 21 .-Average diurnal relative humidity curve for 3 days after pre-

cipitation occurred. Data are averaged for three south Geor-

NWS gia

stations. The upper and lower limits for prescribed

burning are shown with an expected curve constructed using

a 1000 e.s.t. observation (A) and forecast minimum (B).

10

12

14

16

HOUR OF DAY (est.)

Figure 22. --Average diurnal windspeed curve for 3 eays after precipita-
tion occurred. Date are averaged for three south Georgia
NWS stations. The upper and lower limits for prescribed
burning are shown with an expected curve constructed using a 1000 e.s.t. observation (A) and the forecast maximum (B).

Figure 24. -Average February maximum temperature in degrees F,
south Georgia, 1954 to 1963.

13

The Forestry Weather Interpretation System
As An Aid To Prescribed Burning

The Forestry Weather Interpretations System (FWIS) was designed to provide the forest manager with current weather (updated hourly in some cases) localized
to his operational site, with interpretation
as to how it might apply to his management problem. It is a computerized sys-
tem, but no previous training in computer science or meteorology is required for effective use of the system.
The system is resident on a computer at the University of Georgia. FWIS was developed as a cooperative endeavor between the Georgia Forestry Commission,
the U.S. Forest Service, the National Weather Service, and the University of
Georgia Office of Computing Activities. It was developed to meet the expressed needs of forestry and designed in close
consultation with operational foresters.
The result is an easy-to-use, "user friendly," system (Paul and Clayton 1978). In
lieu of directly accessing the system, a burner can request this service from any
Georgia Forestry Commission District Office. The District Offices are equipped with terminals, and the professional staff can access the system and advise the burner of weather conditions appropriate
for his location.
To access the system one must have:
1. An office telephone

2. A computer terminal (purchase
price starts at about $1,000)
3. A valid user number at the Univer-
sity of Georgia
FWIS can be used for (system product names are shown below in bold type):
1. Planning: PRESMOK, RXBURN
2. Monitoring weather for near-term planning:
a. REGION.-An overview of
weather in the South and its gen-
eral implication for forestry
b. Hourly maps (MAP) of tempera-
ture, humidity, wind, cloud cover, and current weather c. FDFCST.-District forescasts for
today -to night- tomorrow
d. GAMAP.-A plot of existing weather at NWS office locations

ley et al. 1978) and an estimate of the smoke management problem 21
Perhaps the greatest utility of FWIS to
a burner is as an aid in "surprise prevention." For example, if on the day of the burn, the weather estimated at the burn
site by OBSI does not agree with the forecast, it would be advisable to seek clarification with NWS. Disagreement between forecast and observation is an expression
of our current imperfect understanding of weather. This should alert the forester to a potential problem, which can be resolved by consultation with a knowledgeable
meteorologist who will have the latest
information on which to base a forecast
revision if required.
***
SUMMARY

3. Day of burn:
a. OBSI. --Observational data from
NWS stations can be used to esti-
mate weather at a burn site b. FDFCST.-District forecast data
for today-tonight-tomorrow
c. FORCST. -Interpolated forecast
for today-tonight-tomorrow d. RXBURN.-Evaluation of ob-
served and forecast variables being in the preferred range (Mob-

Once the burn is ignited, success is largely dependent on weather at the burn site. Exactly how the various weather elements interact with each other and the fuel complex being burned is imperfectly known. Consequently, the forester, of necessity, will burn many times with a large measure of uncertainty that his management objective will be met. By careful planning and attention to current and expected weather, this uncertainty can be minimized.

2/ RXBURN combines the information found in OBSI, FDFCST, and FORCST.

14

3 ElDfi QMSSM 2M31
REFERENCES CITED
1. Hough, Walter A.; Turner, James C, Jr. Open burning on Georgia's forest and agricultural land in 1972. Ga. For. Res. Pap. No. 76. Macon, GA: Georgia Forest Research Council; 1974. 5 p.
2. Mobley, Hugh E.; Jackson, Robert S.; Balmer, William E. (and others). A guide for prescribed fire in southern forests.
Atlanta, GA: U.S. Department of Agriculture, Forest Service, Southeastern State and Private Forestry; 1978. 40 p.
3. Paul, James T.; Williams, Dansy T. Project THEO Report 1969-1970. Report to the Naval Air Systems Command, De-
partment of the Navy on work performed under order IPR No. 19-0-8002, IPR No. 19-1-8016 and others. Macon, GA: Forest Meteorology Work Unit, Southern Forest Fire Laboratory; 1971. 97 p. 4. Paul, James T.; Clayton, Joe, comp. User manual: forestry weather interpretations system (FWIS). Asheville, NC: U.S. Department of Agriculture, Forest Service, Southeastern Forest Experiment Station and Atlanta, GA: Southeastern Area
State and Private Forestry, in cooperation with U.S. National Weather Service, NOAA; 1978. 83 p. 5. Williams, Dansy T. Project THEO Report 1971-72. Report to the Naval Air Systems Command, Department of the Navy,
on work performed under order IPR No. 19-2-8006, IPR No. 19-2-8007 and others. Macon, GA: Forest Meteorology Work Unit, Southern Forest Fire Laboratory; 1973. 35 p. plus appendices.
15

GEORGIA
FORESTRY
John W. Mixon, Director J. Fred Allen, Chief of Research