04 MONTHLY WEATHER REVIEW. MARCH, 1917
SS-/. 57gb 7 ( 73) SECTION IL-GENERAL METEOROLOGY.
EAIE I10 TEE 034ITED STATES.
By ALFRED J. HENRY, Profemor of Meteorology.
[Dated: Weather Bureau; approved by Chief of Bureau, Mar. 18,1917.]
The occurrence of hail at Weather Bureau stations is made a part of the record of precipitation, the principal object sou ht bein to complete the record of reci ita- tion in wEatever form it may hare occurredp. PBhile the printed record does not distmguish between a damng- ing and a harmless hailstorm, yet such distinction is
made in the manuscript records and a very brief account of damaging hail is kept under the caption Meteorologi- cal notes. These latter have not been printed and it is not possible, therefore, to compile statistics of damag- ing hailstorms except by the expenditure of tr large amount of time and labor in examining tlie nianuscript records. The compilation of hail statistics on which this paper is based was undertaken because of the ininie- diate necessit for a knowledgc of the approximate frequency of tail throughout the United States. The printed records will be found in the series of annual reports of the Chief of Weather Bureau (Annual Meteoro- logical Summaries). A previous attempt made in 1898 (MONTHLY WEATHER REVIEW, 1898, 26: 546), was not wholl successful became the data then available were incomp s ete. The phenomenon of hail is even more local in its
dietributton than precipitation, hence one of the great dif6$t&a in. determining its frequency over areas of
co- 8128.
DiFector A. Angot, of the French meteorolo ical serv-
ice, hae 8tateda that about 1,000 stations woul c f be neces-
srtry for an area of 4,050 square miles in order to study the distribution of hail in sufficient detail. Such distribution of stations, if uniformly made, would require one station fm eveq 4 square miles, or in a State t.he size of Iowa, for example, nearly 14,000 stations. The total number of records used in the preparation of
the present paper and included in Table 1 is 167 for an
area of 3,026,789 s uare d e s , or, if uniformly dia- tributed, one station or every 18,124 s uure m*, or one station for an area t i e size of New $ampshire and Ver- mont. The actual distribution for certain regions is, however, better than the above average. Thus, for example, Iowa, With an area of 56,147 square miles, has six stations within its borders and one separated from it by the Missouri River only, a total of seven, or one for each 8,021 square
miles. five of these stations, however, are massed in the river valleys which bound the State on the east and the West. reSD0CtiVdV.
f i e &ual fcequency of hail in Iowa, as shown in Table- 1 , is: St.tfonr EIdEtorIM.
Sioux Cig ..................................................... 3.3 Omaha( ebr.) ................................................ 2.8
Des Moinea .................................................... 3.8 Charles City .................................................... 2.8
Dubuque ...................................................... 2.8
.................................................... 2.9
P%%? ..................................................... 2.4
1 Aa?anpmW by RW LI chutr a v 4 4 (0 x~v-82, InclurivA ~aca-thlrBEvRw#,:Y%# 1014 4 1 w h l m a 1 .h l n ~*
The above figures mean simply that, on the av hail falls about three times each season. While T t Q figures give no information as to the intensity of the storms, it is a matter of common experxence that many hailstorms are harmless and that damaging hailstorms are not numerous, at least in many portions of the country. One hail insurance company of Iowa states that in the last 24 years it has paid a total of $1,860,633.04 in hail losses, or an average of $i7,536.40 per year. Naturally tlie losses in some years are greater than in others. The losses sustained by this one compan for 1915 were
$225,219 and in 1916 were 8135,204. &om these figurea there is no way of arriving at the number of acres damaged or the csteiit of the damage, whether total or only partial in each case. What is needed is an accurate statement of the number and extent of damaging hailstorms in each quarter section of the State for at least 10 years. The
mafi clear from &e fi res of Table 1 that the frequency of hail as a rneteorof&ical phenomenon varies some- times within the limits of a single State; thus the fie+ uency of hail is greater in western Kansas and Nebraska %an in the eastern portions of the same States, and according to Section Director George M. Chappel, of Des Moines, Iowa, mutual hail insurance companies of t,hat State charge a higher rate for hail risks in northern than in southern counties. The average cost of $1,000
hail insurance for a period of 5 ears according to a
prominent company was for no d ern countiea 818.10
and for southern counties $17.70. This difference is not great, and mor0 detailed data might easil change the
frequency in northern than in southern countiee, but it would not be surprising to find B diminution of storms in an eastward direction, since the region of infrequent storms lies east of the Mississippi River. In Kansas hail insur- ance costa as much as 10 per cent in the extreme north- western counties and about 4 per cent in extreme eastern
counties. Geographic distribuEion.
The chart of annual hail frequency shows that the region of most frequent occurrence, four or more storme per year, is in southeastern Wyoming and e a s t d
therefrom, includin the western porbons of KUM, Nebraska, and 0 d ahoma. Adjoinin this region of
tude of the roblem is thus apparent.
ratio, since there is seemingly no reason 7 or a greatez
especially to t%e eastward, the
practically all of South Dakota, 3f ebraska, Kansas, the
of Missouri, all of Co P orado, and the southeastern portion
per annum decreases to three. the region of the occuz~ence of at
on an avera e per mum includes
western and central ortions of Iowa, northwestern third
of Wyoming. These two districts having three and four storms per annum may be considered as the chid hail regions of the United States. East of the Missisaippi the
annual average is two storms or less. per annum. A second region of hail frequency compnses a portion of
southwestern Montana and southern Idaho and the mountain districta of northern New Mexico and northern Arizona. (See fig. 5, XLV-33.) The oocux~~ce of bail in
k c m , 1917. MONTHLY WEATHER REVIEW. 95
the foregoing named regions is a henomenon of late Spring
coast from San Francisco northward, particularly at the mouth of the Columbia River. Hail also occurs in Winter, but rather infrequently in the East Gulf States,
&mme northeast Texas, northern Louisiana, northern Ikitaippi, northern Alabama, northern Georgia, and southwmtem North Carolina. In eneral, there is an
k c e of hail at all seasons along t8e Gulf and Atlantir
coasts.
and Summer. Winter hail is P requent dong the Pacific
seas& a i s t t a u h .
Hail in the United States is, in generd, a phenomenon of the warm season, the only notable exception being do the immediate Pacific coast from San Francisco noxward. On that strip of coast hail occurs chiefly from November to March, a season that is substantially
th0 same as that of the rains in that part of the United
states.
Accordin to District Forecasters Beals and Willson o l
Portland, 8 re ., and San Francisco, Cal., respectively,
the hail of d n t e r and Sprin on the Pacific coast is soft hail or graupel. "rue fail, of sufficient size to injure crops, rarely occurs. Mr. Beals is able to recall the occurrence of but a smgle destructive hailstorm in Oregon in the 17 years of his residence in that State, and, likemse,
Mr. Willson recalls a damaging hailstorm in California in September, 1916. In both States hail is associated with thunderstorms. The latter are not uncommon in Or on
fornia, although they occur with some frequency on the her mountains beyond the level of agricultural lands.
% ail also occum infrequentl , it is t,rue, in the Gulf States during the cold season; d e region of most frequent occurrence is, however, some distance inland from the mast, via, in the hilly regions of northern Alabama, northern Georgia, and southwestern North Carolina, particularly. Winter hail in the United States occurs in connection with the movement of LOWS from the Pacific inland, and a ain when LOWS of whatever place of origin move across t B e Gulf States. (See fig. 4, XLV-31.) In the warm season the occurrence of hail is very closely associated with thunderstorm and tornadic phenomena. The writer has pointed out elsewhere that tornadoes occur in Spring a short distance inland from the Gulf coast and that as the season progresses the region of greatest tornadic activity seems to spread northward over the Great Plains States. A development and move- ment closely paralleling that of tornadoes is observed in hail storms, the last-named are most frequent in Okla- homa in March, for example. As the season advances the region of greatest frequency is found to the northward in Kansas, and the month of greatest frequency is May in- stead of March. I n western Kansas and southeastern Wyoming the month of greatest frequency is June.
(See fig. 1, 2. XLV-2930.)
and Washington, but are very rare in the lowlands of F ali-
-
Damage to crops by Wad.
Praotically no damage to agricultural crops b hail
age is possible in the Gulf States both b reason of the
at the time of greatest frequency. In Kansas, Nebraska, South Dakota, western Iowa and northwestern Missouri, hail falls a t a time when destruction of crops is possible.
is possible in the Pacific Coast States and only sma 19 dam-
infrequency of the phenomenon and the a % sence of crops
.
&trib.uEi.on. of hail in gensml.
Hailstorms over both land and water occur most fre- quently in temperate latitudes, the belt of greatest fre- quem bein between the 35trh and the 60th parallels in
especially over the low ands. In rctic and Antarctic regions while hail occurs more frequently than was once supposed, lack of precise observations makes it somewhat conjectural whet.her the hail reported is graupel or true hail. "lie geographic distribution of hail and its destructive effects on crops have been studied in Europe in great detail. TTilrtternberg.-Dr. Anton Biihler, of Zurich, has dis- cussed in great detail the damage by hail in Wiirttem- be during the 60 years 1828-1887.' The number of ha7days during the 60 years avera ed 13 and ranged from
1879. The smallest superficid area damawed in any one year was 1,617 hectares (4,020 acres) an3 the greatest was 32,133.7 hectares (70,4113 acres).
Indiin.-M t,hough records of hail frequency and dam- w e in India are not available for so long a period as in some European countries, et much valuable information has already been collecte B and is summarized in Indian Meteorological Memoiiu, volume 6, from which t,he matter below has been abstracted.'
Hailstom occur in India almost exclusively during the dry or northeast monsoon. In the first half of this period or durin the cold weather, they are restricted to northwestern and central fndia and occur in connection with and during the p m g e of cyclonic storme acrm India. The prevailin winds are of continental origin and the air ia hence comparatively &, more especial1 in Raj utma and Central India
monsoon, or the hobweather month, March, April, and May, storm usually occur under different conditions from those which give rise to hailstorms in the cold weather and ala0 occur chiefly m areas
where thw storms are of rare occurrence in the receding cool months. FIailstorms in the hobweather months inVariab$ accompany thunder- tdmm or, aa it would be more correct to state, they are severe local thunderstorms, the preci itation in part of the area occurriq rn hail. Thunderstorm and haigtorma hence occur chiefly at +e perrod when convective action is most vigorous. The conditions whch accompany and a pear to be essential to their formation are: High temperature, large %mal ran of temperature. The large ascensional movement necessary for thegrmation of hailstones appears to be provided either: (1) By exaggerated hobweather conditions in the open plains, giving rise to unusually vigoroue convectional movement. (2) By a strong, dry land current advancin seaward and Paeeing under a sea current and forcm the latter u p w d (3) BY air movement from &e plains a c m link of hills. In the h a m Valley the Aassm Hills, and the Cachar hailatorme sp- pear to be the result of forced ascent of sea winds in hot weather, blow- ing a c m the Bengal mt into east and central Bengal and across the haam Hills into ham. In Bengal they appear to bedue chiefly to $e second action, the dry, westerly winds of the Gangetic Plama workmg under and forcing up- ward the Boutherl sea winds prevailin in that area at the time. Hailstorms rare& occur in India muti of latitude 16O.
cold-weather hailslorma in India.
The general inference suggested from the atatktica of hail fre uency
In&a and Rajputana, area *ugh which the prmary depramna of the cold-weather storm almost invanably pasa and that they are much less frequent in the northwestern rovinces to the north of their g e n d line of movement and the ce.ntrafprovinces to the muth of it.
3 'A
both 2 -1 emisp eres. The are infre uent in the Tropics,
a maximum of 28 days in 1852 to % ut 4 days in 1867 and
where hailstorm chiefly occur. &uring tEe second half of
is robably correct, viz, that hailstormfl are moat. frequent in 8 .entral
~
: Die Hagelbeschkl ngen in Wtirttembq. Stutt rut 1690
4 Hailstarms in Indpdurhg the 15891597. 3. Ihslrior, &tadogieal Reparter to the Government d Indla (In&=. mern., v. 6.)
96 MONTHLY WEATHER REVIEW. . MARCH, 1917
Hailstorms of the eecond class, viz, hobweather or summer thunder-
THEORIES OF HAIL.
Man theories of the formation of hail have been ad-
discussed from a number of slightly different view oints. vance 2 within the last 100 years and the subject has been
There -. appears to be unanimity of opinion upon t 1 le fol-
l o n g :
(1) Hailstorms (in temperate latitudes at least) are almost invariablv associated with t,hunderstorms in which a displa i f electrical action is a prominent fea-
does not appear to be any relation such as cause and effect between the two phenomena. In the United States the region of greatest thunder- storm frequency does not coincide mth the region of eatest hail activity. This fact is strikii ly illustrated
and with thunderstornis at two oints representin spectively, the region of greatest {ail frequency an the reatest thunderstorm frequency. In this connection see fV. H. Alexanders paper in this REVIEW, 1915,43:328,
for tables and charts of thunderstorm frequency. The two oints selected are: For hail, Cheyenne, Wyo., and
In the table below will be found the average number of days with hail and thunderstorms, respectively, for these two places.
Average number of days with hail and zcrlth thuderstonns.
Montgomery, Ala.
ture. While t i is association is an intimate one there
& a comparison of the average number of 3 ays with liail
f re-
for t % understorms, Montgomery, Ala.
Cheyenne, Wyo.
e-t 1
10.2 0.1 12-5 0.4
1
7.5 1.4 1 -9 3.2 1 .0 1.4 111.1 1.5 14.9 1.2 10.6 0.2 10 0 18 I 4::
B 8 a . .................. 1 1 1 R 1:5 1:4 1:lO 1:I 1:/ 1:3 .......... 133
At Mon omer , Ala., the eriod of maximum hail is
maximum in June; but the latter may e more apparent than real, due probably to the locd distribution of hail. Statistics for other stations in Alabama (Table 1) show that there is practically no hail dong the Gulf coast after
Ma but that with distance inland from the coast and the
State, as at Birmingham, the season of maximum htd falls in April and May. At Jacksonville, Fla., the chief hail maximum falls in June, thus tending to confirm the secondary maximum in June at Montgomery, Ala. The period of maximum thunderstorm activity at
Montgomery, Ala., falls in the summer, June, Jul , and August, whde hail ceases after July. The ratio of i ail to
%
in the mont Y l g s Fe ruary, Marc\, and A ril, with a second
dig t t-increase of elevation, in the northern part of the
thunderstorms is greatest in April, 1 : 11, and least in July, 1:114.
The ratios  h d to thunderstorms given above for the two stations Mont omery and Cheyenne show that the
than in Mont omery and it also may be interpreted as emphasizing t E e infrequency of hail with thunderstorms in the Gulf States. The director of the French meteorolo ical service in a
hail can not be seen in the electric mamfestations that accompany it. The view that all hailstornis are nierely intense thun- derstorms in which a art of the redpitation is in the
cussin@ the hailstornis of India-has much to commend it.
(2 ) %hat a prere uisite to the formation of hail is an
water or small balls or bunches of mout snow upward into the colder air strata there to be frozen. There are several lines of w ument in favor of the orign of hail in a
(a) The constitution of hailstones-concentric layers of ice or of ice and snow of different texture-ppints to repeated condensations and freezings suggesting the existence of a strong ascensional current. (b) The evidence of cumdo-nimbus cloud! also strongly points toward the esistence of a strong verbcal uplift and the measured height of these clouds In the warm season provide further confirmation, if confirmahqn is necessary, of the fact that their summits extend well into the region of ice clouds.
(c) The tops of these clouds must therefore be chilled
by expansion, by radiation, by e m oration into the dry
Whether the cloud garticles will freeze on p.assmg through the isotherm of 0 C. or whether they d pass into ~t
state of subcooling is of come a matter of specula- tion; but in any event it seems clear that the origm of hail is to be found connected with thechanges that are
Prof. William
in which the ascendin currents are so strong and reach up so hi h that rain o s are carried. up into the cold
%ail is enerally associated with tornadoes, yet the
for thc formation of a tornado are always present in a hailstorm. Rather it should be reco nized that just as
nomena, so t % ere must be degrees of iiitensity in the conditions that produce hail. It is a fact worthy of mention that the re on of greatest thunderstorm fre-
region of greatest hail frequency. Hail occurs with greatest frequency in southeastern Wyoming and over the Plains region immediately to the eastward, also in elevated regions of New Mexico and Arizona. It is con- ceivable that an ascending air current starfing from the elevated regions of New Mexico and Arizona will not
only be cooler initially, but will also sooner reach the isotherm of 0C. than will an ascending current starting, say, in Florida or the Southeastern States where thunder- stornis during the summer months are of almost daily
ratio hail to thun d erstorm is much greater in Cheyenne
recent paper states that the cause of t 1 e production of
solid form-enunciate B first, I be P ieve, by Eliot in dis-
ascending current o 1 sufiicient strength to carry drops of
strong ascensiona f current, VIZ:
air, and by their contact mth t E e colder-air sffata.
Ferre takin% f ield the view that a hailstorm is simply a tornado lace within great cumulo-nimbi.
re ions a % ove and there f p rozen into hail.
writer muc a doubts whether the fundamental conditions
there are de rees of intensity in t I understory phe-
quency in the Unite f l States does not coincide with the
1 A: Angot in Compt. rend. Acad. agrie. de Francs, 2 (1916), NO. 31, pp. 912-913; trcnul. in t h ~~ Rrvmw, Dee., 1916, k:679.
M a B a q 1917. MONTHLY WEATHER REVIEW. 97
--
a 3 as a8
TABLE 1.-Average number of days with hail, 1906-1916-Continued.
Great Basin.
-
a 3 ae ae
occurrence. It also seems reasonable to suppose that an ascendin current that originates, sa., in the lee of
mergy to enetrate the colder u per strata than would
found at a greater elevation. Kite and balloon observa- tiom made by the Weather Bureau tit, Mount Weather, Va., show that the isotherm of 0C. is at its greatest alti- tude in the months of July, August, and September. bother reason for the absence of hail in the lower lati- tudea is the likelihood that even should hail occasionallJ- form there, it will be melted before reaching thc earth. Hail observed at great altitudes in the Tropics is ilzvari- ab1 small.
8, has been computed that ti hailstone 6 millimeters (nearlyaquarter of an inch) in diameter can not fall throu h the air at a speed greater than 6.43 meters per
kilometers-a not unusual altitudc--s little more than nine minutes would be consumed in fading to the ground. There is ample reason to believe that at air tern eratures which prevail during thunderstorms in lower Etitudes, an had which mi ht be formed would probably be m 3 ted before it reac % ed the surface of the earth! Elsewhere the writer has expressed the opinion that thunderstorms in the Southeastern States are less violent than in the Northern States. The freedom from hail in the South seems to confirm the opinion hereto- fore expressed. I have been assisted in the preparation of this aper by Mr. Bertrmd W. Bailey of the ltiver and Flood!bivi- aon.
TABLE 1.-Average number of d y s with hail, 1906-1915.
ppeinc Coast.
the Rocky lf ountains will require less imtial ascensioiial
be require i in regions where t I! e isotherm of OOC. is
won % . If then, the origin of the hail be placed a t 5
0
a i
a3 0.2
..... __ ... __ . -
0.4
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a4 a3
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Wwhiwton.
NorthH end.... Brttle .........
~~t o w h ~~l a n d . a 4 L 4 0 a i a 4 0.7
as as
as
a 3 a5 a3 a5
1.0
1.4 as a5 a5 a5
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1.2 0.4
a7
a7
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0.6
0.3 a i a 4 o o
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a3 a i
a 2
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a 2
a i 0.3 0.2
a i
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a i a i
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a i a i 0 0
o a i 0.1 0
a i o 0
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a 4
a 4 aa o
a i
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Willkt on... .... Devils Lake.. .. Bismnrck.. ....
0
0
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a4 a5
a i
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a i
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0.2
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Pwtlmd ....... BILke CIty ..... Raseburg......
cbiilomir.
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0 0
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Valmtine ...... NorthPlat te... Om aha ......... Lincoln ........
0.2 0 0 0
TOPICS ......... Concoda. ..... Iola ............ Wichita ........
Oklahoma.
OklahomaCfty.
aa 0
0 0
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s'
8 - 0
111
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a3
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Ne#&. I ;
Rmo ........... 0 I 0
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!- Arkona. I I
I '
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4.9
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Flagstan ....... i a i I 0
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Rocky Mountains.
-
LO 21 4.6 1 9
4.4 1.7 0.4
a9 a i a 4
l i s L I
-
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3.0 2.6
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28 3.5
4.5 3.0 8 6 3.4 3.7
61
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Y e l l o w i t o m 1 I I Park .........
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Lander ......... 0 I 0 0.2 Chey enne...... 0 : u a i
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Grand Junction Denver. ........ Pueblo.. ....... i 1 ai zi New Mczieo.
Santa Fe.. ..... a 1
........ I
Q l 0 0 0
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4.1 2 9 1.6
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.Irorih Dakota. I
Sotub Dakota. I
Nebrmka. I
8 Bulletin, Mount Westhex Obswvntory. 6: 179.
7 Bee slso In thb mnnection R. Russell 011
8 A. J. H e y . Locm of life In the United P ~~~&t %u g . Washington, 1901. 8'. p. W, 4. ( eathex Bureau bulletin No. 30.)
98 MONTHLY WEATHER REVIEW. MARCH, 191'1
5 g s l h
1.5 1.7
D.2
0.2 0.3 0.3
0.2
0.6 0.6
0.2 0.1
0.5 0.1 0.3 0.2
TABLE 1.-Average number of days with hail, 1906-1916%ontinued.
Ohlo Vplley.
--
0.3 0
0.3
0.3 0.1 0.3
o.a
0.7 0.4
0.2 0.4
0.4 0.2 0.2 0.3
TABLE l.-Avmzqe number c,f days with hail, 1906-1915-Continued.
1 % 4 8
0 0.1
0.1 0.1 0 0.1
Mlsslarlppl Valley.
d .I
--
0.2 0.1
0 0 0.2 0.1 0 0
-
t 4
-
0.2 0.5
0.4 0.4 0.5 0.3 0.4
0.5 0.6
0.3 0.4
0. a 0.6 0.7 0.4
-
' I
stations.
k
-
I d i a m .
1ndiSnaplls. ..I 0.1 Evansville .....I 0.1
OW.
Toledo. .... _. . . 0
Sandusk _..... 0 Clevelani. ...__ 0.1 Cplumbus.. . . -. 0
cmeinnati..... 0
West Virginia.
Parprsburg . . . 0.1
E l h i .. ._ -. . . . I o
-
ii -
0.4 0. a
0.3 0.3 0.2 0.3 0.2
0 0
aa 0.1
0. I 0.3
C ai
-
f j
%I Z R
0.1
0.3
0.1
a i
o ai ai a i
a 2 aa
ai a 2
a i a i
0.1
-
i
9 -
2.1 2.6
1.7 1.6 1.8 1.6 1.4
a. 4 2 3
2.2 1.7
1. e 2.0
2.0 2 2
-
a 7 a7
1.4
0.9
0.8 0.8
0.6 0.0
1.2 0.8
-
--
0
0 0
0 0
0 0
0 0 0
0 0 0
o
a i o ai o
a i o
o ai
o 0.1
0
-
I
k
-
0 0
0
a i
ai
0
0 0
0
0
0 ai
a i
ai
a3
a 3 a4
0
a 2 0
-
5 8
Z h
a 7 0.2 0.4
ae 0.9 0.6
1.2 0.0 0.8
o.a a 4 a3 0.1
0.9 0.5
0.6
0.7
a 7
a 4 a 3
-
f sl -
0
0
0
0 0.1 a 2 a 3 a 2 0.4
a 2 a 4 a 7 a 3
a5
a 3 a4
a i
a 3
ai
0
-
--
0.2 a 3 a 3
0.7 a 7 a 3 0.3 0.6 0
a 5 a 2 0.3
a i
a6 0.6
0.2 0.1
a 4
a5 a4
-
- 3 -
0.1 0 0.2
0.7 0.2 0.6
a6 0.9 a 4
0.6
0.5 0. R
ai
a9
0. a
a5
a( a4
1. I
0. B
-
Statlona.
dlfnncaota.
Duluth .._....-. Moorhead...... st. Paul.. .....
Ima.
-
6s
2 -
0.6 0.1
a i
a 4
a 3
0.3 0.2
0.1 0.3
aa a2 ai 0.1
0.2
C
C
ai
a4
0. I
ai -
9 k
-
0 0
0
-
1 4
-
0.1 a 5 a 2
0.2
0.2 0.1
a 3 a 3
a 3
a 3
ai
ai
0.1 0
0 0 0
0.1
0 0
-
B.1 D .l
0.1 0.2 0 0.1
0.2 0
0 0.1
0
0 0
o.a
-
5 Po
3 -
a 2 0
0
0 0.2 0.1
0 0.1
0.4
0
ai a a
0.4
0. I
a5 ai
ai
(
(
-
o "z a ---
0 0 0.2 0
0 .1 0 .1 0 0 0 0 0 0.1 0 0 . 0
0 0.1 0.1 0.1
0.3 0.1 0 0
0 0.1
0.1 0 0.1 0
o 0.2
-
d 9 -
2 1 1. a 1.4
3.3 2. s 2.8 3.S
2.9 2 4
2 0 2.3 2.5 1.2
4.2 3.5 2.8 2.7 2. a
2 5
a. 1
-
29 3. i 1.9 0.7 1.6 2.8 1.7 1.0
ae
1.4 1. e
2.3 1.0
2.3 1.4 1.4
1.4 1.4 0.2 0.2 0.0
3.1 0.3
0 0 0
0 o
0.2 0.1
0
0.1
0.1 0.2 0.2 0.6
o.a 0.3
0.2 0.3 0.1
0.1 a 3
0 0.2
0.3 0.3
0.4 0.2 0.3 0.2
hhtucky.
Loufsvllle .._.._ Lexington.....
0.1 0
0 0.1
o.a 0.1
0.2
ai
0 0
0
0 0
0.1
Ttnw8ee.
Nashville. - _._. Knoxville ______ Chattanooga.. - Memphis ......_
0 0.1 0 0.1
KaaPasClty .... cdmbia.. .... H d b al_....-. st. LauS ...... - Springfield .....
0
0.2
0.1
(1
aa
Arkmwos.
F&S ma.... LittleRock ....
a1 0.1
0.2
0.3
0.1
0.1
0.3
0.3 0.2
0 0 0
0.1
0.4
0 0 0
0.1 0 0
0.2 0
ai ai
0 .3 0 .1
0.2
0.1 0
0
0.3 0.1
o o 0.1 ai
0
0 0
0 0.1 0 0.1
0.1 0 0 0 0
0.1 0 0.1 0.1
Mains.
East rt .......
New Hanrpsliirs.
Concord.. .. . .. .
Vmnont.
Northfield ... . . .
hfadaachu8etIs.
Bmton __....._. Nantucket.. . . .
Bhodc I.bnd.
Providence.. ... Block Island., .
oonncdf&.
Hartford ._...... New Haven.. . -
PortEd ..-....
0.2 0
0.1
0
0
0
0
0
0
O 1
TaOr.
Amarillo __..... AWene ....__._ ElPaeo __...... DelIUo .._._... sm A n t d o ... F ~W arth.... Palestine ..... .. c CMSti.. Q z o n ......
I h U b h a .
shreveport..... New Orleans. ..
Mllaiadppi.
Vicksburg ...._. Meddtsn .......
AWWM.
Bhd@8m... Mon mery... m&'._.._....
Fbrfda.
Peneacola.... .. Jacksonville... .. Tam -....-.. Jupit2 ...._..- Keywest ......
0 0 0
0 0
ai a 2 0 0
0.1 0.2
0.2 0
0.1 0.1
ai
0 0 0 0 0
0.3 0.3 0.1 0.1 0.1 0.2 0.2
0
0.1 0 0.2 0.4 0.1
0.6
0.1 0.2 0.1 0.1
0.2 0.1 0.1 0 0.1 0.1 0.1
0.1
0.1 0.3 0.1 0.1 0.3
0 0
0.1
0.1 0.1 0 0
~
Nem Yark.
N W y ......... SFraeuse ....... -
Rochester ...... Buffalo ..._..... Binghsmm .... New York City.
Ncm Jmry.
AtlmtkCity ...
Pmnaylmnia.
.Erie ...__...._._ Pittsburgh.. . . - H m s b urg..... Benrnton ........ Philadelphia. . .
Marybnd.
Baltimore.. . . . -
Dislrlel of CO- lumbs.
Wnshingtm ....
rii-ginia.
n;smgo .........
0.1 0 0 0 0 0.1 0.1
0
0.1 0.1 0 0.1 0
0.3
0
0 0 0
ai
0
0 0 0
0
o
0.2 0.2 0.1 o
0.1 0 0 0 0 a 3 0.2 0.1 0 0 0.1
ai
0.2 0.3 0.3 0.2
0.3
0.3 0.2 0.1 0.2 0.1 0.1 0.1
a 4
0 0 .1 0.2 0.:
0 (
0.1 0.:
0 0.1
0.1 (
0.1 0.;
0 (
0.2 I
a 4 (
a 1 IO.! 0.11 (
0.1 ' 0.1
I
0.1
0.9
0.5 a 7
0.2 0.5 0 0.3 0.6 0.3
0.2 0.3
0.3
0.1 0.5 0.3 0.1
0.3 0.5 0.1 0.1 0.1 0.1
0.3 0.2
0.8
0.1 0.1 0.2 0.1
0.1 0.1 0.1 0.1
0
0 0 0
0 0
0 0 0 0 0
0 0 0
0 0 0 0
QreanBay ..... Lacrosaa...... y[adlaon .....-. MuWauk&.....
0 0 0 0
Alpene ._._.._.. GrmdHaven.. GlrendRapbds.. mtmit. ........
Port =%--.--- won....
0 0 0 0
0 O
-
s fl 8
e m -
L1 L1 I. 2
3.3 1 1 3.2
D. 5 a 2 0
0 a 2
0.1 a i
0.2 0.3 0
0 a 2
0 0.1
-
0 0 0.3 0
0.1 0 0
ai a i
0.1 0.1
0.3 0
0.1 0 0
0 0.1 0
0 a
.. ._
New England.
-
0. 1 D. 1
0.1
0.1
0 0.2
0.1 0.3
0.1 0.1
-
-
0 0
0
0
0 G
0 0
0 0.1
-
0.1
ai
0.1
0
0
0
0 0.1
0 0
-
11 0
0
0
0 0.1
0 0
0 0
__
0 0
0 0 0 0 0
0
0.1 0.1 0 0.1 0
0
0
u. 1 0 0 0
--
ai 0
0.2
0.1
0 0.1
0
0
0
0
-
0 0
0.1
0
0.1 0.2
0 0.2
0.2 0.1
-
0.1 0 0.6 0 0.2 0.1 0
0
0.3 0 0 0 0.1
0
0.1
0 0 0
0
-
aulf R a n .
a 2 0.1 0 0 0 0 0
0
0
a 2
a 2
a i
a i
0. a
0.1
0 0.1
0
0.1 0.1
0.1 0
0
0.1 a
C C
c c
(
.
0 0.1 0.1 0
0
0.1 0.1 0
a i
0.1 0
0.1 0
0 0 0
0. a
a a a
0
a5 0.6 0.3 0.1 0.5 0.8
0.3 a 3
a 2
0.2 0.5
0.6 0.7
0.4 0.4 0.6
0.4 0.2 0
0 ai
-
0.8 1.5
0.3 a 5
0.6 0.4 0
a 3
a5
0.1 0.3
0. a 0.1
0.4 0.1 0.4
0.3 0.4 0.1 0. I c
-
a5 0.2 0.3 0 0 0.4 0.1 0 0
0.1 0.1
0.2 0
0.2
0 a 3
0
0. e
0 0
a
-
a 2 a 2
ai ai ai
0.1
0
0 0
0 0.1
0 0
0.4 0.1 0.1
0 0 0 0 0
__
0.5 0.2 0 a 2
0.1 0.1
0
0
a 4 0.1 0. a 0 0.1 0.1
0 0
Mlddle Atlantlc States. -
0
0.1 0 0 0 0 0.1
0
0 0 0.1 0 0.1
0
0.1
u 0.1 9
0.1
-
0.1 0. a ai
0.1
a a
a i 0 0
-
0.2 0 0.1 0.1 0.1 0 0.1
0.2
0.2 0.3 0 0 0.1
0.1
0.2
0.1 0.2 0 0.3
-
-
0 0.1 0.1 0 0.1 0.1 0
0.1
0.3 0.6 0.6 0.3 0
0
0
0.5 0.1 0.1 0.2
--
-
D. 5 D. 3 D. 1 0.1 0.3 0.1 0.2
0.1
0 0.3 0.3 0.3 0.1
0
0.1
0.3 0.4 0 0.3
-
-
0.2 0.3 0.3 0.1 0.2 0.4 0.2
0.2
0.4 0.3 0.2 0.8 0.2
0.4
0.3
0. a 0 0 0.3
-
1.7 1.3 2.2 0. e 1.2 1.4 1.0
I. 1 0
1.3 0 1.1 0
1. a
0
3.3 0 3.1 0 0
0
0.1
u D. 1 0 0
-
0.2 0 @ C
E
0.3
0 0
ai
a
C
c 0.1
(
(
0 0.7 __
anat Lakes Region.
-
0
0.6 0.6 0.3
0 0.2 0.1 0 0. a 0.4 0.6 0.1 0.3
-
-
0.1 0 0 0.2
0 0 0 0
0.3 0.2 0.1 0 0
-
1.0
0 0.8
0 1.4
Richmid ..._. .I 0.1 Wythe vU..... 0 Lvnchburg.. . . . 0.1 N'wIolk ....._._. 0
0.1
0
a a
1.6 1.2 0.2 1.3
-
Ibhmx, 1917. MONTHLY WEATHER REVIEW. 99
d
*a
TABLE l.--doncrgs nu&u of &8 udh ha& 1906-1915-Continued.
South Atlantk States.
a 5 --
0 0.3 0.3
0.1
0
0.1 0.1
0.3
_.
~h k ...._... 0.1 0.a
AU~US~S. ...._.. I 8 1
0
mcixl. ... . . . . . . 0 BrMnnah.. . . . . 0
---
0.1 0 0.2 0 0 0 0 0 0
0.a o 0.1 0.1
0.2 0 0 0
0 0.1 0 0 0
-
-
i a -
0.1 0.1 0.5
0.3
0. 2 0
0. a 0 0.2 0
Ainth (Rmlina.
m w h ......... Ambvnb ....-.. chulott4 .....-. wllmhgb&...
5mtb Cbmlfna.
Cdumbh ....._. chsrlastam ......
am@.
o o.a 0 0 .1 0.2 0.1
0 0
0.1 0 0 0.1
-
k d -
0
0 0. a
0. a
0.3 0
0. a 0 0 0.1
._
0.1
0.1
0
0.1
0.1
0 0 0.5
0 0
0.1 0 0
0.3
O D o n
-.
ki
s 0
c.
-
0 0 0
0
n 0
0.3 0 0
0
-
-
0.0 1. I 1. i 0.6
1.2
n. 5
1.0
0.8
n. 1 1.n
3 c 7950 -LIQETNk AND FOBEST FIBES W CALIFOBNU.
By ANDREW H. PALMER, Obaerver.
[Daw U. 8. Weather Bureau a m , S m Fnmclsco, W., July 14,1016.l
The inau ration of the fire-weather warning service
opened another interesting field for investigation in meteorology.1 New problems have presented themselves for solution. The dficulties encountered to date have been largely the result of a lack of data, the absence of normals, and the want of precedent. Though forest fires doubtless occurred long before man appeared on the earth, a systematic record as to their causes extends over comparatiwly few years. In the United States the matter was not given serious attention until lSS;), when a table of forest-fire statistics was prepared as B
part of the Tenth Census. The investigation of the rela- tion of weather to orest fires is of even more recent data, while the fire-weather warning service was inaugurated in the Pacilic Coast States in 1913 on t!ie recornmenda- tion of District Forecaster E. A. Reals. With reference to their origin, forest fires may be di- vided into two grou 3, those caused b man and those
larger of the two groups, it is not the purpose of this paper to discuss them in detail. Those caused by nature may be subdivided into three groups, (1) those caused by spontaneour combustion, (2) those caused by vol- canic eruptions, and (3) those caused by lightning. Spontaneous combustion is a direct cause of forest fires only in rare iastances, and as an observed source there are few cases on record. However, of the many forest fires of unknown ori n it is believed that some, at least, were thus produce8 The exudation of oils and other mineral matter from the ground, or the close pack- ing of damp leaves and grass on the forest floor may at timw produce chemical reactions which might result in combustion. Forest fires caused by volcanic eruptions had not been recognized in the United States until May 19, 1915, when an eruption of Lassen Peak in northeast- ern California was accompanied by a blast of superheated gases which kindled two forest fires in that vicinity.z XY natural causes of forest fires spontaneous combustion
as a part o rl the work of the U. S. Weather Bureau has
caused by nature. b i l e those cause C l by man are the
and volcanic eruptions must therefore be conaidefed rare in the United States. The third natural cause, Eghb ning, and its relation to forest fires, is the subject of this discussion.
LIQETNING ANI) FOREST FIRES IN THE UNITED STATES.
On the national forests of the United States during the five-year period 191 1-1915, inclusive, &es were caused
as follows: Railroads, 14.4 per cent; campers, 15.6 per cent; brush burning, 7.9 per cent; lumbering, 1.8 per cent; lightning, 29.5 per cent; incendiary, 8.7 per cent; miscellaneous, 5.3 per cent; and unknown, 16.8 per cent. Lightning is a more important factor in causing forest *es than it is in causing fires in cities, the proportion -- being in the ratio of 7 to i. The relation of lightning to forest fires in the United States was studied in 1912 bv Mr. Fred G. Plummer. of the United States Forest Serkice. The more im ortant conclusions reached by Mr. Plummer may be brie i! y sum-
marked as follows: Trees are the objects most often struck by lightning, because: (a) They are the most numerous of all obecta; (6) as a gart of the ground, they extend upward and shorten t e distance to a cloud; (e) their spreadin
present the ideal form for conducting an electrical dis- char e to the earth. Any kind of tree is likely to be
locality will be the dominant species. The likelihood of a tree bsing struck by lightning is increased: (a) If it is taller than surrounding trees; (b ) if it is isolated; (c)
if it is on high ground; (d ) if it is well (deeply) rooted;
(e! if it is the best conductor at tho moment of the flash;
rain, transform it for thc time from a poor con % uctor wet by to that is, if temporary conditions, such as bein
a good one. Lightning ma bring about a forest fire by
forest fires caused by lightning robably start in the humus. Other things being equaf trees growing in dif- ferent soils differ slightly 111 siisceptibilit to hghtning stroke. One study gave these results: Eoam, 23 per cent; sand, 18 per cent; clay, 17 per cent; and others,
42 per cent. Zones of marked hazard from li htning- clue part1 to soil variations, partly to minera f deposita, and p a r t i to altitude-are recognized throughout the West. T e conductivity of wood is governed by its moisture content and its temperature. Electricity trav- erses wood more easily in the longitudinal direction of its fibers than across them. About 2 per cent of trees struck by lightning are ignited. While trees do not dif- fer greatly as to theii susceptibility to lightning, they do differ greatly as to inflammability.
branches in the air and spreading roots in the groun f
struc s c by lightning. The greatest number struck in any
igniting the tree itself or t P le humus at its base. Many
LIQHTNINQ AND FOREST FEES IN CALIFOENIA.
In California there are 18 national forests in which are included a total area of 19,575,000 acres. Not allof this land is timbered, but there are about 17,400,000 acrea of standing timber in the State. These 18 national fore&, which are separate and distinct from the national parks, are shown in outline in figure 1. As California is a large State containin 5 per cent of the total area of the coun-
topography, soil, and climate,. its trees include 125 o the 500 to 600 species growing in the United States. An 9
try, and as it h as within its borders a great variet of
Ibhmx, 1917. MONTHLY WEATHER REVIEW. 99
d
*a
TABLE l.--doncrgs nu&u of &8 udh ha& 1906-1915-Continued.
South Atlantk States.
a 5 --
0 0.3 0.3
0.1
0
0.1 0.1
0.3
_.
~h k ...._... 0.1 0.a
AU~US~S. ...._.. I 8 1
0
mcixl. ... . . . . . . 0 BrMnnah.. . . . . 0
---
0.1 0 0.2 0 0 0 0 0 0
0.a o 0.1 0.1
0.2 0 0 0
0 0.1 0 0 0
-
-
i a -
0.1 0.1 0.5
0.3
0. 2 0
0. a 0 0.2 0
Ainth (Rmlina.
m w h ......... Ambvnb ....-.. chulott4 .....-. wllmhgb&...
5mtb Cbmlfna.
Cdumbh ....._. chsrlastam ......
am@.
o o.a 0 0 .1 0.2 0.1
0 0
0.1 0 0 0.1
-
k d -
0
0 0. a
0. a
0.3 0
0. a 0 0 0.1
._
0.1
0.1
0
0.1
0.1
0 0 0.5
0 0
0.1 0 0
0.3
O D o n
-.
ki
s 0
c.
-
0 0 0
0
n 0
0.3 0 0
0
-
-
0.0 1. I 1. i 0.6
1.2
n. 5
1.0
0.8
n. 1 1.n
3 c 7950 -LIQETNk AND FOBEST FIBES W CALIFOBNU.
By ANDREW H. PALMER, Obaerver.
[Daw U. 8. Weather Bureau a m , S m Fnmclsco, W., July 14,1016.l
The inau ration of the fire-weather warning service
opened another interesting field for investigation in meteorology.1 New problems have presented themselves for solution. The dficulties encountered to date have been largely the result of a lack of data, the absence of normals, and the want of precedent. Though forest fires doubtless occurred long before man appeared on the earth, a systematic record as to their causes extends over comparatiwly few years. In the United States the matter was not given serious attention until lSS;), when a table of forest-fire statistics was prepared as B
part of the Tenth Census. The investigation of the rela- tion of weather to orest fires is of even more recent data, while the fire-weather warning service was inaugurated in the Pacilic Coast States in 1913 on t!ie recornmenda- tion of District Forecaster E. A. Reals. With reference to their origin, forest fires may be di- vided into two grou 3, those caused b man and those
larger of the two groups, it is not the purpose of this paper to discuss them in detail. Those caused by nature may be subdivided into three groups, (1) those caused by spontaneour combustion, (2) those caused by vol- canic eruptions, and (3) those caused by lightning. Spontaneous combustion is a direct cause of forest fires only in rare iastances, and as an observed source there are few cases on record. However, of the many forest fires of unknown ori n it is believed that some, at least, were thus produce8 The exudation of oils and other mineral matter from the ground, or the close pack- ing of damp leaves and grass on the forest floor may at timw produce chemical reactions which might result in combustion. Forest fires caused by volcanic eruptions had not been recognized in the United States until May 19, 1915, when an eruption of Lassen Peak in northeast- ern California was accompanied by a blast of superheated gases which kindled two forest fires in that vicinity.z XY natural causes of forest fires spontaneous combustion
as a part o rl the work of the U. S. Weather Bureau has
caused by nature. b i l e those cause C l by man are the
and volcanic eruptions must therefore be conaidefed rare in the United States. The third natural cause, Eghb ning, and its relation to forest fires, is the subject of this discussion.
LIQETNING ANI) FOREST FIRES IN THE UNITED STATES.
On the national forests of the United States during the five-year period 191 1-1915, inclusive, &es were caused
as follows: Railroads, 14.4 per cent; campers, 15.6 per cent; brush burning, 7.9 per cent; lumbering, 1.8 per cent; lightning, 29.5 per cent; incendiary, 8.7 per cent; miscellaneous, 5.3 per cent; and unknown, 16.8 per cent. Lightning is a more important factor in causing forest *es than it is in causing fires in cities, the proportion -- being in the ratio of 7 to i. The relation of lightning to forest fires in the United States was studied in 1912 bv Mr. Fred G. Plummer. of the United States Forest Serkice. The more im ortant conclusions reached by Mr. Plummer may be brie i! y sum-
marked as follows: Trees are the objects most often struck by lightning, because: (a) They are the most numerous of all obecta; (6) as a gart of the ground, they extend upward and shorten t e distance to a cloud; (e) their spreadin
present the ideal form for conducting an electrical dis- char e to the earth. Any kind of tree is likely to be
locality will be the dominant species. The likelihood of a tree bsing struck by lightning is increased: (a) If it is taller than surrounding trees; (b ) if it is isolated; (c)
if it is on high ground; (d ) if it is well (deeply) rooted;
(e! if it is the best conductor at tho moment of the flash;
rain, transform it for thc time from a poor con % uctor wet by to that is, if temporary conditions, such as bein
a good one. Lightning ma bring about a forest fire by
forest fires caused by lightning robably start in the humus. Other things being equaf trees growing in dif- ferent soils differ slightly 111 siisceptibilit to hghtning stroke. One study gave these results: Eoam, 23 per cent; sand, 18 per cent; clay, 17 per cent; and others,
42 per cent. Zones of marked hazard from li htning- clue part1 to soil variations, partly to minera f deposita, and p a r t i to altitude-are recognized throughout the West. T e conductivity of wood is governed by its moisture content and its temperature. Electricity trav- erses wood more easily in the longitudinal direction of its fibers than across them. About 2 per cent of trees struck by lightning are ignited. While trees do not dif- fer greatly as to theii susceptibility to lightning, they do differ greatly as to inflammability.
branches in the air and spreading roots in the groun f
struc s c by lightning. The greatest number struck in any
igniting the tree itself or t P le humus at its base. Many
LIQHTNINQ AND FOREST FEES IN CALIFOENIA.
In California there are 18 national forests in which are included a total area of 19,575,000 acres. Not allof this land is timbered, but there are about 17,400,000 acrea of standing timber in the State. These 18 national fore&, which are separate and distinct from the national parks, are shown in outline in figure 1. As California is a large State containin 5 per cent of the total area of the coun-
topography, soil, and climate,. its trees include 125 o the 500 to 600 species growing in the United States. An 9
try, and as it h as within its borders a great variet of
Ibhmx, 1917. MONTHLY WEATHER REVIEW. 99
d
*a
TABLE l.--doncrgs nu&u of &8 udh ha& 1906-1915-Continued.
South Atlantk States.
a 5 --
0 0.3 0.3
0.1
0
0.1 0.1
0.3
_.
~h k ...._... 0.1 0.a
AU~US~S. ...._.. I 8 1
0
mcixl. ... . . . . . . 0 BrMnnah.. . . . . 0
---
0.1 0 0.2 0 0 0 0 0 0
0.a o 0.1 0.1
0.2 0 0 0
0 0.1 0 0 0
-
-
i a -
0.1 0.1 0.5
0.3
0. 2 0
0. a 0 0.2 0
Ainth (Rmlina.
m w h ......... Ambvnb ....-.. chulott4 .....-. wllmhgb&...
5mtb Cbmlfna.
Cdumbh ....._. chsrlastam ......
am@.
o o.a 0 0 .1 0.2 0.1
0 0
0.1 0 0 0.1
-
k d -
0
0 0. a
0. a
0.3 0
0. a 0 0 0.1
._
0.1
0.1
0
0.1
0.1
0 0 0.5
0 0
0.1 0 0
0.3
O D o n
-.
ki
s 0
c.
-
0 0 0
0
n 0
0.3 0 0
0
-
-
0.0 1. I 1. i 0.6
1.2
n. 5
1.0
0.8
n. 1 1.n
3 c 7950 -LIQETNk AND FOBEST FIBES W CALIFOBNU.
By ANDREW H. PALMER, Obaerver.
[Daw U. 8. Weather Bureau a m , S m Fnmclsco, W., July 14,1016.l
The inau ration of the fire-weather warning service
opened another interesting field for investigation in meteorology.1 New problems have presented themselves for solution. The dficulties encountered to date have been largely the result of a lack of data, the absence of normals, and the want of precedent. Though forest fires doubtless occurred long before man appeared on the earth, a systematic record as to their causes extends over comparatiwly few years. In the United States the matter was not given serious attention until lSS;), when a table of forest-fire statistics was prepared as B
part of the Tenth Census. The investigation of the rela- tion of weather to orest fires is of even more recent data, while the fire-weather warning service was inaugurated in the Pacilic Coast States in 1913 on t!ie recornmenda- tion of District Forecaster E. A. Reals. With reference to their origin, forest fires may be di- vided into two grou 3, those caused b man and those
larger of the two groups, it is not the purpose of this paper to discuss them in detail. Those caused by nature may be subdivided into three groups, (1) those caused by spontaneour combustion, (2) those caused by vol- canic eruptions, and (3) those caused by lightning. Spontaneous combustion is a direct cause of forest fires only in rare iastances, and as an observed source there are few cases on record. However, of the many forest fires of unknown ori n it is believed that some, at least, were thus produce8 The exudation of oils and other mineral matter from the ground, or the close pack- ing of damp leaves and grass on the forest floor may at timw produce chemical reactions which might result in combustion. Forest fires caused by volcanic eruptions had not been recognized in the United States until May 19, 1915, when an eruption of Lassen Peak in northeast- ern California was accompanied by a blast of superheated gases which kindled two forest fires in that vicinity.z XY natural causes of forest fires spontaneous combustion
as a part o rl the work of the U. S. Weather Bureau has
caused by nature. b i l e those cause C l by man are the
and volcanic eruptions must therefore be conaidefed rare in the United States. The third natural cause, Eghb ning, and its relation to forest fires, is the subject of this discussion.
LIQETNING ANI) FOREST FIRES IN THE UNITED STATES.
On the national forests of the United States during the five-year period 191 1-1915, inclusive, &es were caused
as follows: Railroads, 14.4 per cent; campers, 15.6 per cent; brush burning, 7.9 per cent; lumbering, 1.8 per cent; lightning, 29.5 per cent; incendiary, 8.7 per cent; miscellaneous, 5.3 per cent; and unknown, 16.8 per cent. Lightning is a more important factor in causing forest *es than it is in causing fires in cities, the proportion -- being in the ratio of 7 to i. The relation of lightning to forest fires in the United States was studied in 1912 bv Mr. Fred G. Plummer. of the United States Forest Serkice. The more im ortant conclusions reached by Mr. Plummer may be brie i! y sum-
marked as follows: Trees are the objects most often struck by lightning, because: (a) They are the most numerous of all obecta; (6) as a gart of the ground, they extend upward and shorten t e distance to a cloud; (e) their spreadin
present the ideal form for conducting an electrical dis- char e to the earth. Any kind of tree is likely to be
locality will be the dominant species. The likelihood of a tree bsing struck by lightning is increased: (a) If it is taller than surrounding trees; (b ) if it is isolated; (c)
if it is on high ground; (d ) if it is well (deeply) rooted;
(e! if it is the best conductor at tho moment of the flash;
rain, transform it for thc time from a poor con % uctor wet by to that is, if temporary conditions, such as bein
a good one. Lightning ma bring about a forest fire by
forest fires caused by lightning robably start in the humus. Other things being equaf trees growing in dif- ferent soils differ slightly 111 siisceptibilit to hghtning stroke. One study gave these results: Eoam, 23 per cent; sand, 18 per cent; clay, 17 per cent; and others,
42 per cent. Zones of marked hazard from li htning- clue part1 to soil variations, partly to minera f deposita, and p a r t i to altitude-are recognized throughout the West. T e conductivity of wood is governed by its moisture content and its temperature. Electricity trav- erses wood more easily in the longitudinal direction of its fibers than across them. About 2 per cent of trees struck by lightning are ignited. While trees do not dif- fer greatly as to theii susceptibility to lightning, they do differ greatly as to inflammability.
branches in the air and spreading roots in the groun f
struc s c by lightning. The greatest number struck in any
igniting the tree itself or t P le humus at its base. Many
LIQHTNINQ AND FOREST FEES IN CALIFOENIA.
In California there are 18 national forests in which are included a total area of 19,575,000 acres. Not allof this land is timbered, but there are about 17,400,000 acrea of standing timber in the State. These 18 national fore&, which are separate and distinct from the national parks, are shown in outline in figure 1. As California is a large State containin 5 per cent of the total area of the coun-
topography, soil, and climate,. its trees include 125 o the 500 to 600 species growing in the United States. An 9
try, and as it h as within its borders a great variet of
Ibhmx, 1917. MONTHLY WEATHER REVIEW. 99
d
*a
TABLE l.--doncrgs nu&u of &8 udh ha& 1906-1915-Continued.
South Atlantk States.
a 5 --
0 0.3 0.3
0.1
0
0.1 0.1
0.3
_.
~h k ...._... 0.1 0.a
AU~US~S. ...._.. I 8 1
0
mcixl. ... . . . . . . 0 BrMnnah.. . . . . 0
---
0.1 0 0.2 0 0 0 0 0 0
0.a o 0.1 0.1
0.2 0 0 0
0 0.1 0 0 0
-
-
i a -
0.1 0.1 0.5
0.3
0. 2 0
0. a 0 0.2 0
Ainth (Rmlina.
m w h ......... Ambvnb ....-.. chulott4 .....-. wllmhgb&...
5mtb Cbmlfna.
Cdumbh ....._. chsrlastam ......
am@.
o o.a 0 0 .1 0.2 0.1
0 0
0.1 0 0 0.1
-
k d -
0
0 0. a
0. a
0.3 0
0. a 0 0 0.1
._
0.1
0.1
0
0.1
0.1
0 0 0.5
0 0
0.1 0 0
0.3
O D o n
-.
ki
s 0
c.
-
0 0 0
0
n 0
0.3 0 0
0
-
-
0.0 1. I 1. i 0.6
1.2
n. 5
1.0
0.8
n. 1 1.n
3 c 7950 -LIQETNk AND FOBEST FIBES W CALIFOBNU.
By ANDREW H. PALMER, Obaerver.
[Daw U. 8. Weather Bureau a m , S m Fnmclsco, W., July 14,1016.l
The inau ration of the fire-weather warning service
opened another interesting field for investigation in meteorology.1 New problems have presented themselves for solution. The dficulties encountered to date have been largely the result of a lack of data, the absence of normals, and the want of precedent. Though forest fires doubtless occurred long before man appeared on the earth, a systematic record as to their causes extends over comparatiwly few years. In the United States the matter was not given serious attention until lSS;), when a table of forest-fire statistics was prepared as B
part of the Tenth Census. The investigation of the rela- tion of weather to orest fires is of even more recent data, while the fire-weather warning service was inaugurated in the Pacilic Coast States in 1913 on t!ie recornmenda- tion of District Forecaster E. A. Reals. With reference to their origin, forest fires may be di- vided into two grou 3, those caused b man and those
larger of the two groups, it is not the purpose of this paper to discuss them in detail. Those caused by nature may be subdivided into three groups, (1) those caused by spontaneour combustion, (2) those caused by vol- canic eruptions, and (3) those caused by lightning. Spontaneous combustion is a direct cause of forest fires only in rare iastances, and as an observed source there are few cases on record. However, of the many forest fires of unknown ori n it is believed that some, at least, were thus produce8 The exudation of oils and other mineral matter from the ground, or the close pack- ing of damp leaves and grass on the forest floor may at timw produce chemical reactions which might result in combustion. Forest fires caused by volcanic eruptions had not been recognized in the United States until May 19, 1915, when an eruption of Lassen Peak in northeast- ern California was accompanied by a blast of superheated gases which kindled two forest fires in that vicinity.z XY natural causes of forest fires spontaneous combustion
as a part o rl the work of the U. S. Weather Bureau has
caused by nature. b i l e those cause C l by man are the
and volcanic eruptions must therefore be conaidefed rare in the United States. The third natural cause, Eghb ning, and its relation to forest fires, is the subject of this discussion.
LIQETNING ANI) FOREST FIRES IN THE UNITED STATES.
On the national forests of the United States during the five-year period 191 1-1915, inclusive, &es were caused
as follows: Railroads, 14.4 per cent; campers, 15.6 per cent; brush burning, 7.9 per cent; lumbering, 1.8 per cent; lightning, 29.5 per cent; incendiary, 8.7 per cent; miscellaneous, 5.3 per cent; and unknown, 16.8 per cent. Lightning is a more important factor in causing forest *es than it is in causing fires in cities, the proportion -- being in the ratio of 7 to i. The relation of lightning to forest fires in the United States was studied in 1912 bv Mr. Fred G. Plummer. of the United States Forest Serkice. The more im ortant conclusions reached by Mr. Plummer may be brie i! y sum-
marked as follows: Trees are the objects most often struck by lightning, because: (a) They are the most numerous of all obecta; (6) as a gart of the ground, they extend upward and shorten t e distance to a cloud; (e) their spreadin
present the ideal form for conducting an electrical dis- char e to the earth. Any kind of tree is likely to be
locality will be the dominant species. The likelihood of a tree bsing struck by lightning is increased: (a) If it is taller than surrounding trees; (b ) if it is isolated; (c)
if it is on high ground; (d ) if it is well (deeply) rooted;
(e! if it is the best conductor at tho moment of the flash;
rain, transform it for thc time from a poor con % uctor wet by to that is, if temporary conditions, such as bein
a good one. Lightning ma bring about a forest fire by
forest fires caused by lightning robably start in the humus. Other things being equaf trees growing in dif- ferent soils differ slightly 111 siisceptibilit to hghtning stroke. One study gave these results: Eoam, 23 per cent; sand, 18 per cent; clay, 17 per cent; and others,
42 per cent. Zones of marked hazard from li htning- clue part1 to soil variations, partly to minera f deposita, and p a r t i to altitude-are recognized throughout the West. T e conductivity of wood is governed by its moisture content and its temperature. Electricity trav- erses wood more easily in the longitudinal direction of its fibers than across them. About 2 per cent of trees struck by lightning are ignited. While trees do not dif- fer greatly as to theii susceptibility to lightning, they do differ greatly as to inflammability.
branches in the air and spreading roots in the groun f
struc s c by lightning. The greatest number struck in any
igniting the tree itself or t P le humus at its base. Many
LIQHTNINQ AND FOREST FEES IN CALIFOENIA.
In California there are 18 national forests in which are included a total area of 19,575,000 acres. Not allof this land is timbered, but there are about 17,400,000 acrea of standing timber in the State. These 18 national fore&, which are separate and distinct from the national parks, are shown in outline in figure 1. As California is a large State containin 5 per cent of the total area of the coun-
topography, soil, and climate,. its trees include 125 o the 500 to 600 species growing in the United States. An 9
try, and as it h as within its borders a great variet of
xLv-31. March. 1917.
xLv-31. March. 1917.