HomeMy WebLinkAboutAPA924~ 7S"tJI33~, ... 1971 '
USDA Forest Service R~earch~P~er PNW•126
SOt33
o~,,
FIB I
SBISII
CLIIItiC ··.
ZDIIS u 1411L41D 4L4-14
· william 1. trigg
NOAA-National Weather Service
U.S. Department of Commerce
Pacific Northwest Forest and Range Experiment Station
Forest Service U.S. Department of Agriculture Portland_~ Oregon ·
-~-
The author is fire weather meteorologist with the National Weather Service,
National Oceanic and Atmospheric Administration, U.S. Department of
Commerce, Fairbanks, Alaska.
ABSTRACT
Calculated values of precipitation effectiveness
index and temperature efficiency index for 48 weather
observation stations on the Alaska mainland are used to
delineate areas that have different climatic subclassifi-
cations during the wildfire season of April through Sep-
tember. The paper outlines procedures, provides maps
showing step-by-step analysis along with the resulting
areal boundaries, and suggests possible uses of the in-
formation ..
Keywords: Climatology, fire prevention.
ACKNOWLEDGMENTS
This study was produced in cooperation with the Forestry Sciences Labora-
tory, Pacific Northwest Forest and Range Experiment Station, Forest Service,
U.S. Department of Agriculture. Many helpful ideas were received from
Mr. Albert L. Comiskey, Fire Weather Supervisor, National Weather Service,
Anchorage, who also cooperated in the analysis of the maps.
INTRODUCTION
The fire weather forecaster's area of interest in the climate of
Alaska is the Alaska mainland--that part of the State lying between the
Canadian border on the east and the Bering Sea on the west and between
the Brooks Range on the north and the Gulf of Alaska on the south. A
major problem in making forecasts for this area, however, is that the
only classifications of climate hitherto available have shown broad
categories, each containing wide ranges of precipitation and temperature.
Furthermore, all the classification systems have been based on annual
values of these parameters. The climatological regions of mainland
Alaska were described by Watson!_/ as four zones:
1. A zone dominated by maritime influences, which would include the
north coast of the Gulf of Alaska.
2. A zone of transition between maritime and continental climatic
influences.
a. The Copper River basin.
b. Cook Inlet.
c. West-central area--southern Seward Peninsula and the deltas
of the Yukon and Kuskokwim Rivers.
3. A zone dominated by continental climatic influences, referred to
as the interior bas in.
4. A zone of dominant arctic influences, referred to as the Arctic
drainage.
Other climatological systems, among them those of Fitton,?:_/ U.S.
Department of Agriculture, 'i_/ and U.S. Army Air Force,!/ display
similar classification patterns. Thornthwaite~/ categorized the climate
of mainland Alaska into two types--taiga and tundra, both moisture
!_/ D C. Watson. Climate of the States -Alaska. Weather Bur.,
U.S. Dep. Cammer. Climatography of the United States No. 60-49, 8 p.,
1959.
~/ Edith M. Fitton. The climates of Alaska. Mon. Weather Rev.
58(3): 85-103, illus., 1930.
'i_/ U.S. Department of Agriculture. C~imate and man, yearbook of
the Department of Agriculture. Washington, D. C. 1, 248 p., illus. , 1941.
4/ -U.S. Army Air Force. Climatic atlas for Alaska. Weather
Inform. B r. Rep. 444, 229 p. , 1943.
?_/ C. Warren Thornthwaite. The climates of North America according
to a new classification. Geogr. Rev. 21: 63 3-655, ill us. , 1 931.
deficient--on the bas is of calculated values of precipitation effectiveness
index ( PEI) and temperature efficiency index (TEI), which integrate
temperature and precipitation. All these classification systems are
acceptable for general climatic cons ide rat ions but lack sufficient detail to
be of value for forecasting for a single-season activity such as fire control.
These broad climatic regimes in Alaska can be subdivided into smaller
areas of reasonably homogeneous climate. The objective of this report,
therefore, is to outline more specifically the fire-season climatic zones
of the Alaska mainland.
SCOPE
The scope of this report is that afforded by available climatic data.
These data consist of observations of temperature and precipitation from
48 weather observation stations, as listed in table 1 and shown on figure 1,
operated by the Bureau of Land Management, the Federal Aviation
Administration, the U.S. Air Force, the National Weather Service, and the
Alaska State Highway Department, for the months of April through Septem-
ber, 1956 through 1965. The restriction limiting the stations to mainland
Alaska serves to exclude the Arctic Slope, the Aleutian Island chain, and
the Alaska Panhandle southeast of Yakutat, but encompasses the portions
of the State that are chiefly affected by wildfires.
This report is intended to provide a "first look" at some of the
differences that exist in what can be called fire weather climate classifi-
cations.
METHODS
Thornthwaite' s PEI and TEl were selected as the basis of the climatic
subclassification because large quantities of data were involved; and since
the calculation of PEl and TEl is entirely numerical, computer techniques
were easily applied. Minor modifications were made to Thornthwaite' s
procedures. Maximum daily temperature was used in this study because
it was the only data available. This use of maximum daily temperature does
not change the relationships between the various climatic subclassifications
which are only relative. Thornthwaite' s precipitation index is
12
I=L_llS
n=l
where P is the normal monthly precipitation in inches and T is the
normal monthly temperature in degrees F. (T < 28.4°F. = 28.4°F.).
(1)
For the purpose of this study, Thornthwaite's formulas 1 and 3 were
modified. PEI and TEl as used in this study are average values for the
2
Table 1.--Statian val-ues of the preaipitatian effeativeness index and the
temperatUPe effiaienay index for the stations used in the report
Station number,!/
2/ name, and agency-
1. Allakaket, NWS Climat
2. Anchorage, NWS
3. Aniak, NWS
4. Bethel, NWS
5. Bettles, FAA
6. Big Delta, FAA
7. Cape Newenham, USAF
8. Cape Romanzof, USAF
9. Chitina, NWS Climat
10. Circle Hot Springs, NWS Climat
11. Dillingham, NWS
12. Eagle, NWS Climat
13. Eielson Air Force Base, USAF
14. Fairbanks, NWS
15. Farewell, NWS
16. Flat, NWS Climat
17. Fort Yukon, NWS
18. Galena, USAF
19. Gulkana, NWS
20. Holy Cross, NWS Climat
21. Homer, NWS
22. Hughes, NWS Climat
23. Iliamna, NWS
24. Kenai, FAA
25. King Salmon, NWS
26. Kotzebue, NWS
27. Lake Minchumina, NWS
28. Manley Hot Springs, NWS Climat
29. McGrath, NWS
30. Moses Point, FAA
31. Nenana, FAA
32. Nome, NWS
33. Northway, FAA
34. Palmer, FAA
35. Paxson, NWS Climat
36. Puntilla, NWS Climat
37. Sheep Mountain, NWS Climat
38. Slana, NWS Climat
39. Sleetmute, BLM
40. Sparrevohn, USAF
41. Sunnnit, NWS
42. Talkeetna, FAA
43. Tanacross, BLM
44. Tanana, FAA
45. Tok, NWS Climat
46. Trim's Camp, ASHD
47. Unalakleet, FAA
48. Utopia Creek, USAF
!/
Keyed to location of station on figure 1.
];/
Station
precipitation
effectiveness
index (PEI)
15.5
17.6
22.1
23.7
14.5
15.5
43.3
43.7
7.5
12.4
26.7
11.7
15.2
11.8
25.2
14.7
7.7
19.4
12.7
20.5
21.2
18.7
33.9
24.2
20.3
15.2
20.0
16.7
19.7
24.2
14.8
22.0
13.6
14.8
30.0
15.2
17.0
21.0
20.6
29.2
28.6
32.4
10.6
19.3
7.4
41.8
27.3
18.7
Station
temperature
efficiency
index (TEl)
20.8
21.0
20.3
16.3
21.6
20.8
13.0
11.2
23.5
23.4
21.2
21.2
23.6
22.8
17.6
22.7
20.8
19.3
22.0
21.3
18.1
19.8
17.8
18.4
20.5
13.3
19.7
25.0
20.0
16.4
21.9
14.2
21.1
23.7
18.2
17.8
19.7
24.9
20.6
17.3
14.4
22.3
25.3
20.7
24.5
17.5
14.6
19.8
Source of data: NWS Climat = National Weather Service Climatological Station
NWS = National weather Service Observation Station
BLM Bureau of Land Management Fire Weather Station
USAF U.S. Air Force Weather Station
FAA Federal Aviation Administration Station
ASHD Alaska State Highway Department Station
Data base,
number of
months (M)
54
60
48
60
60
60
43
57
46
60
57
54
57
60
60
30
58
60
60
60
60
54
60
60
60
60
60
59
60
60
60
60
58
53
26
56
52
46
35
35
60
60
46
60
53
55
60
60
3
~
Figure 1. -Location of weather stations used as data source for this study.
6-month fire season of April-September covering the period of record as
shown in table l.
PEl=[~
M
(2)
10
115 ( P' )9
T'-10 n 6
where P' is the total precipitation for each month, T' is the mean maxi-
' 0 0 ) mum daily temperature for each month in degrees F. (T (28.4 F. = 28.4 F.
and M is the number of months of data.
Thornthwaite' s temperature efficiency index is:
where T = normal monthly temperature values in degrees F.
(T<32°F. = 32°F.). Formula 3 was also modified for the purpose of
the study. The modified version of the formula is:
M
L
n=1
6 TEl=
M
where T' is the mean maximum daily temperature for each month in
degrees F. (T' <32°F. = 32°F.) and M is the number of months of
data used.
( 3)
( 4)
Maximum daily temperature, which usually occurs about 1400 local 6 I
standard time, is used to calculate the values of PEl and TEL Landsberg-
gives the approximate optimum length of record for observations of
temperature for plains and mountainous terrain in tropical regions as
15 to 20 years; for humidity, the period is 5 to l 0 years; and for rainfall,
40 to 50 years. In the area of Alaska, no stations being cons ide red had
50 years of data as of 1965, and only four stations had data for 40 years.
A total of l 7 stations had data for 25 years. In order to obtain as dense
a network of data as possible, the time period for the report was fixed
at l 0 years. This represents a compromise. High network density with
short-term records was considered more important than the need for the
smoothing effect produced by use of low network density with long-term
records.
6/ -H. E. Landsberg and W. C. Jacobs. Applied climatology. In
Compendium of meteorology, p. 976-992. Baltimore: Waverly Press,
Inc. , l 960.
5
A computer program was written to calculate the PEl and TEl for
each month of the fire season period of April through September, 1956
through 1965, for each of the 48 stations. The monthly values were added
to produce 10 seasonal values, from which a seasonal average was calcu-
lated to produce PEl and TEl values for the station (table l). The mean
and the standard deviation of PEl and TEl values for the 48 stations were
then calculated. For TEl, the mean is 19. 8 and the standard deviation
is 3. 3. For PEl, the mean is 20. 4 and the standard deviation is 8. 6.
Station values of PEl and TEl were then plotted separately on two maps
and isolines drawn (figs. 2 and 3) for the following intervals: at the mean,
at one standard deviation above the mean, and at one standard deviation
below the mean. Thus each map appeared with four classes of areas as
described in table 2.
Table 2.--Names and class Zirrrits of precipitation effectiveness index
and temperature efficiency index in climatic zones
Item PEI Class TEI Class
name limits name limits
>(x + cr) Wet >29.0 Hot >23'.1
x to (x + a) Moist 20.4-29.0 Warm 19.8-23.1
x to (x a) Dry 11.8-20.3 Cool 16.5-19.7
<(x -a) Arid <11.8 Cold <16.5
For the PEl, the classes were: an arid area encompassing those
regions where the value of PEl fell beyond one standard deviation below
the mean, a dry area where the PEl fell between the mean and one stand-
ard deviation below the mean, a moist area where the PEl fell between
the mean and one standard deviation above the mean, and a wet area where
the PEl was beyond one standard deviation above the mean. In a similar
manner, the TEl map was divided into cold, cool, warm, and hot areas.
Mean and standard deviation of the station values were used to provide
cutoff points for segregating the different areas in order to make the end
product of this investigation, the bounding of areas of different climate,
as objective as possible. It was soon recognized that much of the map
analysis and the final decisions as to exact placement of the boundary lines
between areas with different climatic regimes were bound to be subjective.
The simplest example of this is the question of which side of a range of
hills a line should be placed dividing two areas of radically different
6
Figure 2. -Map of Alaska with isoline analysis of PEl. Isolines are drawn for the mean and
for one standard deviation above and below the mean.
7
8
Figure 3.-Map of Alaska with isoline analysis of TEl. Isolines are drawn for the mean and
for one standard deviation above and below the mean.
temperature--in the absence of data, how do you tell? Use of the mean
and the standard deviation gave a consistent and reasonably objective
foundation on which the analysis could be based, however, and narrowed
the amount of subjectivity that had to be tolerated.
The next procedure was to superimpose one map above the other and
draw all the lines from both maps onto a third composite map (fig. 4), so
that any portion of the State could be referred to as one of the 16 combi-
nations of TEI and PEI (fig. 5). An exact rendering of this composite map
produced over mainland Alaska thus outlined 44 areas, for each of which
it could be said that the climatic conditions varied from those of the
surroundings. Some of these areas were enormous, but many were small
and frequently were sharply elongated where the isolines on the PEI and
TEI maps were essentially parallel to each other (fig. 4).
The next step was to consolidate these small and irregularly shaped
areas with other areas nearby, both to reduce the total number of regions
to a manageable level and to adjust the sizes of the regions to realistic
values (fig. 6). This is the point where the greatest amount of subjectiv-
ity was encountered. Where it was possible, this adjustment was based
on topography. Rarely, a station categorized as being in a particular
climatic zone was shifted to an adjoining one. Unless the presence of
two or more data points made it obviously incorrect, divisions between
areas were alined parallel to mountain ranges and perpendicular to the
direction of river valleys. This feature is particularly evident in the
treatment of the areas which appear in figure 4, along the coastal ranges
of the northern Gulf of Alaska and along the arc of the Alaska Range, and
which are consolidated into fewer areas in figure 6.
The presence of numerous data points which define separate climatic
regimes within the Knik, Matanuska, and Susitna River valleys south of
the Alaska Range is also apparent from a comparison of figure 4 with
figure 6. Within the range of the subclassifications derived by use of the
mean and the standard deviation of the station values, the climate of the
Palmer area differs from that around Anchorage, and both are different
from that around Talkeetna, and so on.
The 44 original regions were eventually reduced to 25. This number,
however, includes areas which could not, by any stretch of the imagination,
be considered sensitive to wildfire such as the mountainous areas on the
north coast of the Gulf of Alaska and the high sections of the Alaska
Range. When these areas are removed from consideration, 20 separate
climatic subclassifications still remain.
9
Figure 4.-Map showing composite PEl-TEl climatic zones produced when map in figure 2 is
overlaid on that of figure 3.
10
Figure 5. -Names and identifying numbers for composite PEl-TEl climatic zones.
PEl CLASS
TEl
CLASS ARID DRY MOIST WET
HOT Hot-arid Hot-dry Hot-moist Hot-wet
1 3 6 10
WARM Warm-arid Warm-dry Warm-moist Warm-wet
2 5 9 13
COOL Cool-arid Cool-dry Cool-moist Cool-wet
4 8 12 15
COLD Cold-arid Cold-dry Cold-moist Cold-wet
7 11 14 16
DISCUSSION
This scheme of climatic subclassification is expected to provide fire
control supervisors and fire weather forecasters with a first approximation
of the size and shape of the areas for which particular weather stations'
observations may be generalized and considered representative. In addi-
tion, the technique described of integrating temperature and precipitation
values allows a comprehensive, graphic description of integrated param-
eters in the form of climatic zones. This same technique can be used to
derive climatic zones based on longer or shorter periods of time. The
zones as outlined in this paper, derived over the 6-month fire season,
have value as a wildfire management planning tool.
This paper basically designates the maximum and minimum fire
danger areas on a seasonal basis. It further points out where inadequate
data exist as well as extreme or peculiar climatic variations of particular
areas. Further work of a more detailed nature is probably desirable, but
this depends on the needs and capabilities of both fire weather and fire
control personnel.
11
Figure 6.-Map showing composite PEl-TEl climatic zones produced when map in figure 2 is
overlaid on that of figure 3-after consolidation.
GPO 795-371
12
••••• lo * 1-f ••••• * " ••••• It •• t •• W • t. ••• IJ • lo ••• t to It •• " •••• a •• ·-• it ti ol f ill f •• 4 ••• " II " " •• t t t t ••• t * " lo lo ••• ·-" tl ·• • . -..... " ................. -· .. " ...... " ......... .
T William M.
Fire season climatic ~ones of mainland Alaska.
USDA Forest Serv. • Papq PNW -126, 12 p.,
illus. Pacific Northwest Forest and Range Ex-
periment Station, Portland, Oregon.
Calculated values of precipitation effectiveness index
and temperature efficiency index for 48 weather observation
stations on the Alaska mainland delineate areas that have dif-
ferent climatic subclassifications during the wildfire season
of April through September. Uses are suggested.
Keywords.: Climatology, fire prevention •
. . .. ·-·-~-· ......... " ... ' .......... -·:··· ................. Ill ••••••••••• t ••••• Ill :
Trigg, William M.
1971. Fire season climatic zones of mainland Alaska.
USDA Forest Serv. Res. Pap. PNW-126, 12 p.,
illus. Pacific Northwest Fore Range Ex-
periment Station, Portland, Oregon.
Calculated values of precipitation effectiveness index
and temperature efficiency index for 48 weather observation
stations on the Alaska mainland delineate areas that have dif-
ferent climatic subclassifications during the wildfire season
of April through September. Uses are suggested.
Keywords: Climatology. fire prevention.
. .
Trigg, William M.
1971. Fire season climatic zones of mainland Alaska.
USDA Forest Serv. Res. Pap~ PNW -126, 12 p.,
illus. Pacific Northwest Forest and Range Ex-
periment Station, Portland, Oregon.
Calculated val<les of precipitation effecti\>'eness
and temperature efficiency index for 48 weather observl!Ltion
stations on the Alaska mainland delineate areas that have dif-
climatic subclassifications the wildfire season
through September. Uses
Keywords: Climatology, fire prevention,
···················••••t~••••·····-···························-········ .
Trigg, William
1971. Fire season climatic zones of mainland Alaska.
USDA Forest Serv. Res. Pap. PNW;;;J26, 12 p. •
illus. Pacific Northwest Forest and Range Ex-
periment Station, Portland, Oregon.
Calculated values of precipitation effectiveness index
and temperature efficiency index for 48 weather observation
stations on the mainland delineate areas that have dif-
ferent climatic subclassifications during the wildfire season
of April through September, Uses are suggested.
Keywords: Climatology, fire prevention.
.
. .
...... " •• ·-· ..... <II ............. '" ••••••••••••••••••• Ill •• '." ........... " ••••••••••••••••••••• " .............................. -.................... •'• .... -•••
The m1ss1on of the PACIFIC NORTHWEST FOR EST
AND RANGE EXPERIMENT STATION is to provide the
knowledge, technology, and alternatives for present and
future protection, management, and use of forest, range, and
related environments.
Within this overall mission, the Station conducts and
stimulates research to facilitate and to accelerate progress
toward the following goals:
1. Providing safe and efficient technology for inventory,
protection, and use of resources.
2. Development and evaluation of alternative methods
and levels of resource management.
3. Achievement of optimum sustained resource produc-
tivity consistent with maintaining a high quality forest
environment.
The area of research encompasses Oregon, Washington,
Alaska, and, in some cases, California, Hawaii, the Western
States, and the Nation. Results of the research will be made
available promptly. Project headquarters are at:
College, Alaska Portland, Oregon
Juneau, Alaska Roseburg, Oregon
Bend, Oregon Olympia, Washington
Corvallis, Oregon Seattle, Washington
La Grande, Oregon Wenatchee, Washington
The FOREST SERVICE of the U.S. Department of Agriculture
is dedicated to the principle of multiple use management of the
Nation's forest resources for sustained yields of wood, water,
forage, wildlife, and recreation. Through forestry research,
cooperation with the States and private forest owners, and
management of the National Forests and National Grasslands, it
strives -as directed by Congress -to provide increasingly
g\"eater service to a growing Nation.