MONTHLY WEATHER REVIEW Editor, EDGAR W. WOOLARD
____._. ~-
CLOSED AUGUST 3, 1936 JUNE 1936 ISSUED SEPTEMBER 15,1936 VOL. 64, No. 6 W. B. No. 1185
DUSTSTORMS IN THE SOUTHWESTERN PLAINS AREA
By H. F. CHOTJN
[Weather Bureau, Denver, Colo., April 19361
Numerous articles, and photographs which apparently
substantiate them, have in the recent past predicted thn.t either large acreages of land in the dust-bowl area of the
southwestern plains must be abandoned, or else continued depression and failure must be faced by those trying to
combat the elements. This paper, while it will verify
the seriousness and severity of soil erosion due to wind in the area shown in figure 1, will also show that total abandonme,nt of the area is not imminent or necessary.
FIGURE 1.-Extent and intensity of duststorms over the southwestern plnlns during March 1936.
The topographic features of this section of the Great Plains ares conform to the underlying rock formations.
Valleys and undulating plains have been formed through erosion of the softer, lesser resisting beds. There are
many miles of comparatively level country, as well as
upland valleys and mesas, which comprise thousands of acres of fertile land that have been reclaimed by irrigation
enterprises and improved farming methods. The climate is very similar throughout the region.
The general climatic features are: A low relative humidity;
a large amount of sunshine; a light rahfall, confined largely to the warmer half of the year; a moderately high wind movement; and a large daily range in temperature.
The wind movement is lowest near the westem limits; and over the plains it increases toward the east. At Las
Animas, Colo., the average hourly wind movement is
8.5 miles; a t Dodge City, Kans., 10.2 miles; a t Santa Fe,
N. Ma., 7.8 miles, and a t Amarillo, Tes., 12 miles. The
velocities are greatest in spring, following the driest
s63e8-36-1
period of a normal year. The diurnal range in the wind movement is large, the velocities being generally higb in
the afternoon and dying down a t night. Chinook winds prevail td a great extent throughout the area, causin
large daily range in temperature. Occasional dry spe Is, and periods of abnormally heavy precipitation, persist
over periods of 2 or 3 years with resulting serious droughts
and great variations in crop yields. The question of whether there has been any progressive 3r retrogressive
change in the amount of precipitation since the Plains area was settled, has been the subject of much discussion.
The esamination of reliable precipitation records, same of which were begun more than 50 years ago, bears out the view that no appreciable permanent change has taken
place that will greatly afl’ect the normal. There has, however, been a gradual increase in temperature and a
gradual decrease in precipitation for the past several
years. Figure 3 shows the general rainfall trends a t two long-record stations, LRS Animas, Colo., and Dddge City,
Ilans., within the so-called dust bowl. This graph is based on the average annual aniount of precipitation for these two stations, smoothed by 5-year moving averages. That is, the entry for each year on the graph shows the
5-year average for the two stations, up to and including
that year. It will be noted that for the 5 years ending with 1894 the average was almost exactly the same as for
the 5 years ending with 1935. In other words, 4milar droughts occurred a t these two times.
The occupation of this Great Plains area for agricultural purposes began about 1885, when there was a migration of land seekers into the regan. The settlers made their
livelihood by crop farming, a t which they succeeded fairly
well for 3 or 4 years. Then came 3 “lean years”; 1894
was a year of camplete crop failure, due to drought. In some parts of the plains region as many as 90 percent
of the settlers left their farms. Following this recession in land settlement, a gradual healing over of the denuded
fields by native grania-buffalo sod took place. The
settlers who remained learned from experience how to cope more effectively with the unfavorable crop years and with
drought. The nest impetus to land settlement and crop farming was given by the high prices offered for agricul-
tural products during and immediately following the World
War. Promoters launched campaigns to get crop farmers instead of cattle raisers int.erested in the region, tractors and other labor-saving machinery were introduced, and
grain farming was undertaken on a rather large scale.
Reckless denuding by cultivation and overgrazing con- tinued on this land, where ramfall a t best is light. Ex-
H a
195
196 MONTHLY WEATHER REVIEW JUNE 1938
perienced cattlemen knew the danger of overgrazing the
plains, but the temptatmion of high prices led them to take the chance. I n the area where there is an urgent need for rain, shown in figure 2, an average of from 60 percent to less
than 50 percent of the normal rainfnll occurred during the
18-month period from September 1934 to hhrch 1, 1936.
PIOURE 2.-Percentage of normal precipitation during the 16 months from September 1934 to February 1935, inclusive.
Abnormally high temperatures continued from June 1933
to August 1935, with but temporary relief during short
periods. This prolonged periocl of warmth has never been
equaled in the recorded climatological history of the area. The digging of wells and installation of pumping plants
were resorted to in an effort to save small crops and obtain drinking water. The digging of t81iese wells showed that
the storms, sufficient static electricity was generated to render automobile ignition systems useless, and scores of
motorists were temporarily stranded ; when the storms abated, drivers found that their cars again functioned properly. The static electricity, produced by tlie friction
among minute dust particles in the air, was so intense that distinct electrical shocks were received when one
came in contact with any metal part of a car. Many resorted to the espedient of dragging chains and wires to
ground the static accumulation and prevent short circuits.
Cherry orchards in the rich agricultural section between Fort Collins and Loveland, Colo., suffered severely when
the freshly plowed soil was swept away, leaving the roots
of the trees bare. The topsoil was also blown from fields
devoted to winter wheat and alfalfa, so that in many instances reseeding was necessary.
In Colorado the damage was greatest in southeastern counties, e. g., Kiowa, Prowers, Baca, eastem Bent,
and Las Animas Counties, where the suffocating dust storms occurred frequently from March 12 to 25, 1935,
bringing death to 6 persons and serious illness to more than 100 others. Sickness continued in this section,
where lung congestion was reported to have been aggra- vated by tlie dust-laden air. In the more seriously
affected areas, the dust lay from a few inches to more
than 6 feet deep, and considerable livestock perished from starvation and suffocation. The following is quoted from n cooperative observer’s report: “The dust has
driIted into feed stacks and covered pastures until live- stock have suffered greatly; many cattle have perished
from eating so much dust. Dead stock lying along the
roads is a common sight.” During these “dusters” or “black blizzards”, the air
was so heavily dust-laden as to ninke breathing nnd living condit,ions generally uncomfortable, and residents
LMS HN/MHS, COf d HND DUDGE C/T): KRNJ. CUMB/NED
I I .I I I I 1 1 I 1 1 1 I I I
FIGUEE 3.-Reclpitation trends, La8 Anlmas, Colo., and Dodge City, Kana.
the subsoil moisture was depleted to a great depth. The
prolonged drought made it nearly impossible to secure
stands of grain; and where stands were secured, not enough
reserve moisture was available to meet the requirements
of the plants for survival to maturity. Increased insect
activity during this period also played an important part in the destruction of vegetation. A shortage of feed for
cattle, and the poor condition of pastures generally, prompted farmers to take advantage of every opportu-
nity to obtain feed; in desperate attempts to obtain for- age, Russian thistle and weeds were harvested so closely
that no protection was left to hold the soil, and great areas were exposed to the action of the wind.
During the spring months of 1935 loose topsoil from
thousands of farms in the drought-stricken area .?as carried off by winds of high velocity, reducing visibility and causing hazardous driving and flying conditions. In
of the affected areas hung wet blankets over their doors and windows and covered their faces with wet cloths.
Schools were closed temporarily in many localities as a measure of safety, and many ranch homes were deserted
by their occupants. Visibility became so limited a t
times that artificial light was used during midday, while a t other times the visibility was reduced to zero. During
the height of some of t’hese “blows”, it became so dark that business was temporarily forced to be suspended, even when artificial light was used. Aviators reported
dust present a t altitudes of from 15,000 to 20,000 feet), and brisk winds carried the dust clouds far into the
elevated regions east of the Continental Divide. During the early afternoon of April 14, 1935, the most severe duststorm of the series t,hnt started on March 1
began moving across extreme western Kansas and eastern
Colorado. The coppery-hued sky cast a brown shadow,
Monthly Weather Review. June 1936
t
-I -*
FIGURE 4.-Dust accumulation around farm buildings and farm machinery in the affected areas.
.
i
JUNE 1936 MONTHLY WEATHER REVIEW 197
giving the scene a weird appearance. The storm struck
Denver a t 2 p. m. and blotted out the sun as it moved southward. Sand drifts were piled up that stopped trains and automobiles, grounded airplanes and enveloped regions little affected by previous storms. Homes and
stores were filmed with a thick coating of the powdery
dust. Winds of gale force again were frequent over most of the area during the spring months of 1936. Crops, pastures,
and ranges, already suffering from a lack of moisture and
the duststorms of February, were further damaged by
severe duststorms. On March 1, 1936, light. dust began in east-central Colorado, and by the 4th the storm had
extended from the Black Forest region in Colorado over
the lower Arkansas River Valley into the Panhandles of Oklahoma and Texas. On the 10th moderate to heavy
dust occurred in estreme southeastern Colorado and in
the Oklahoma and Texas Panhandles, which reduced the visibility considerably in the lower part of the area. On
that date, north and northeast winds, with velocities of 30 to 35 m. p. h., bore dust up the South Platte River Val-
ley as far as the South Park region, reducing the visibility to ‘(poor’) over much of the mea. Southeasterly winds
bore clouds of dust which enveloped the entire region enst
of the 104th meridian in Colorado on the 11th. Dust
continued intense on the 13th) and during the following two days practically the entire region east of the moun-
tains was covered by the dust pall, Teclucing the visibility to from 500 yards to 1 mile in Colorado; to 4 miles in
Curry County, New Mexico; and to 25 feet a t Kenton, in the Oklahoma Panhandle, where daylight was turned
to darkness. In this region, the dust clouds turned from a dark blackish color a t the beginning to a yellowish red
tinge. A driving wind accentuated the storm. A maxi-
mum velocity of 33 m. p. h. was reached a t Pueblo on this date, and the air was filled with fine driving silt far into
the foothills sections, reducing the visibility to 10 feet a t
Monument, on the main arterial highway between Denver
and Pueblo, and making driving and flying conditions hazardous. The driving sand removed paint from auto-
mobiles, and pitted windshields.
Some areas of ungrazed grassland in the foothills were
completely covered with the fine dust during the storm, and then uncovered by brisk winds a few hours later. In
Baca County, Colo., the visibility was limited to one city block for 9 consecutive hours and to zero for 1 hour.
Duststorms continued intense through the remainder of
the month in extreme southeastern Colorado, southwest-
ern Kansas, and the twin Panhandles, keeping Baca
Count in semidarkness for periods of from 3 to 24 hours each Jay. During the height of some of these blows,
highway travel was paralyzed and electric-lighted show
windows were not visible from a distance of 40 feet; while
at times the ragin “dusters” cut the visibility to zero,
and pedestrians cofided with one another in attempting to
get about. Reports from extreme southeastern Colorado and ex-
treme southwestern Kansas on March 25 and 26 stated
that the storm was the most severe and of the longest duration of any ever experienced in that region. On these
dates the storm spread from extreme east-central New
Mexico, across the Panhandles of Texas and Oklahoma, and covered practically the entire State of Oklahoma,
reducing the visibility to 350 yards as far east as Vinita, and to 600 yards in the southeastern portion of that State.
The clouds of whirling si1 t appeared russet, blue, orange,
and black in succession. On the 31st, the storm covered the entire region south
of the fortieth parallel east of the mountains, far into
the upper reaches of the Arkansas River and westward
alon the Colorado-New Mexico border to the San Juan
history of those sections was reported. At Pueblo, the sky was turned to a saffron hue, and when the dust
clouds settled, they brought darkness to the city. The
clouds of whirling silt made living conditions generally uncomfortable in the affected areas. Veils of the coppery- hued silt were observed over western counties; and snow
flurries and rain, mingled with dust, brought mud showers
in northeastern localities, in the San Juan Mountain region, and as far north as Columbine, Colorado. A
phenomenal accumulation of muddy snow in the form of
balls, the diameter of silver dollars, fell over some north- central districts of Colorado.
The data in table 1 were compiled with a view of detect- ing any progressive or retrogressive changes in duststorms
of the area since they have become of proportions suffi- ciently great to affect human comfort. Data recorded by the Weather Bureau office a t Amarillo, Tex., the only
first order Weather Bureau station near the center of greatest activity, were chosen as the most reliable and
complete for this purpose. Two years ago, the topsoil that blew in these regions was coarsely granular in struc-
ture; but due to its having been shifted back and forth,
it is now as fine as face powder, and erodes a t a lower wind velocity. This condition is especially noticeable in the
Clovis, N. Med., area. It is likewise noticeable that the
wind can support and carry the topsoil to greater dis-
tances, as was evidenced during March 1936. Road maintenance problems were not major engineering dificulties; but much trouble was experienced by, and
most serious problems confronted, the highway depart-
ments during the dust periods. Snow fences running
parallel wibh and about 100 feet north of the east-west highways were, in many areas, completely covered with
dust drifts. These wooden picket fences were removed
with portable hoists, and the drifts leveled by team or tractor-drawn blades. Drifts frequently filled road
ditches and interfered with traffic on the highways. Fol- lowing an inspection trip by automobile through south- eastern Colorado, the writer found nearly a pint of sand
and dust had accumulated in the oil-bath air filter of his
car. Undoubtedly, automobiles not equipped with some sort of air filter, as a precaution against having dust enter
the motor, suffered bearing trouble. Northerly winds seem to be the most potent. in respect to erosion, as they are of slightly greater velocity, colder,
heavier, and have great’er carrying power near the ground. This is evident from the appearance and size of dust
drifts along fences and other obstructions and barriers.
Yet southerly winds often are quite strong, with violent gusts; their work of erosion is notable, and they tend to
carry the dust to great altitudes. Wind from the south
usually reaches its maximum velocity between 2 p. m. and 5 p. m., the period of greatest frequency of soil
erosion. Northerly winds appear to develop greatest ve-
locities shortly after setting in and maintain them steadily until near the end of the period. The lapse rate within
the air is a major factor in the extent to which dust is
carried high aloft and transported long distances. In response to inquiries sent into the affected areas,
came reports of tillage practices which included examples both of effective and of careless forms of farm manage- ment. Fields which continued to lose topsoil are those
where owners failed to learn the lesson taught during the
summers of 1934 and 1935, that bare ground always blows if unprotected. Some farmers put in large acreages of
nothing but beans, while others put in protective strips of
and % olores River valleys, where the heaviest dust in the
i 98 MONTHLY WEATHER REVIEW JUNE 1936
corn or feed crops, or plowed up ridges crosswise to the prevailing winds. These ridges serve a dual purpose. In
addition to the ridges serving to minimize soil blowing, they tend to hold the moisture on the fields where it falls.
In recent years, when the grain brought high prices, the majority of farmers in the pa,nhandles of Texas and Okla-
homa and in the vicinity of Clovis, N. Mes., raised live- stock and row crops of corn, milo, maize, kafir, sorghums, and other fodders.
A few succeeded with wheat; others felt they could do likewise, and cultivated their grassy pasturelnnd, forgot livestock and row crops, and drilled in every available acre
to nothing but wheat. The region soon became known as
the w-heat empire of the Southwest. One farmer northeast of Dalhart, ‘rex., planted 7,000 acres (nearly 11 square
miles) in wheat, while another farm which comprised
10,000 acres was planted with a like crop. Reports as of
April 1, 1936, state that several hundred thousand acres of wheat in the twin panhandles and in the wheat districts of New Mexico and southwestern Kansas were either
blown out or smothered with dust deposits during ht1arc.h.
In extreme southeastern Colorado, 150,000 acres of wheat planted this year has been blown away.
Generally speaking, wind erosion takes place under
extremely dry conditions with a wind veldcity of 20
m.p. h. on all land which has been in cultivation 3 or more yeais. A seeder operates to pulverize and level off the top soil, thereby malring it more susceptible to blow-
ing and to the action of the full force of the wind. Most
of the land that had been in continuous cultivation for several (5 or 6) years lost considerable topsoil. How-
ever, some lands that were in cultivation 5 or 6 years
ago and then abandoned have been healed over by pIant growth and have resisted erosion. Native buffalo sod
does not erode from direct action of the wind, although
in some places, nest to an eroding field, it may yield to The sod may appear to be “blowing” some-
times, when in reality i t is merely captured deposition
that is moving off from the impact of the wind in a new
direction. Effective forms of farm management would necessarily have to be adopted on a community basis,
as an isolated farmer would not benefit if adjoining fields were tilled in a careless manner. The worst offender in
allowing fields to become “blow spots” is the owner who lives in town or city and conies out to his farm only long enough to put in crops and harvest them. Seemingly,
such owners are more careless; farmers who have to endure
the dust if they allow it to rise are more careful and do more planning. Geologlc,al history shows that wind laid the soil in ihese
great grassland areas orer centuries of time, and Nttture
healed the land by providing buffalo sod and other gra,sses
to hold it there. Now that the native grasses have been distcrbed, it is nature.1 thnt the wind claims the soil. In
summarizing the possibilit>ies of preve.ntion of soil erosion
by the wind, it seems that proper use of the land is the answer. From the evidence that has been presented, it
is believed that if we should reach a state of ratiocinative
grace, and impart’ially consider the net resu!ts of dry farming west of the one hundredth meridian in the dis- trict referred to, where the land is subjected to the full
force of t,he wind, it wouldbe w-ell to retire much of the dry land that now is in cultlvation, encourage return of
the nat,ive grasses, and confine the use of farntlng iniple ments to very selec,tive spots. It is also evldent that
duststorms are not only an attendant of unavoidable
drought, but also, in a large degree, a resnlt of destruction of native sods. Careless methods have allowed the top-
soil, which contains humus accumulated for centuries, to
. attrition.
be blown away, evidence of the fact that restoration of
the soil to its predust-era condition will take a consider-
able number of years. Abandonment of the land is not necessary, as good returns can be obtained by selective
tilling of some of the area and by using the remainder for
grazing purposes.
The thanks of the author are clue to the Weather Bureau offices a t Topeka, Oklahoma City, Houston, Ania- rillo, and Albuquerque for their generous cooperation in
furnishing data.
TABLE 1.-Duststorms at Amarillo, Tex.
I-
Year, 1955 Jan. 18 .___________________
Jan. 21-22 _________________
M i l e s z
0
J 3 3
3 1
3
3 2 1.
39 6
5
1 2
0
0
i’ 2
1
1 0 2
6 1 1
1
0 0
5
1
0 1
6 1
1>
3. 5
2
5
4 4 6 1 2
4 1
5
5 5 1
2
1 1 2
5 0 2
3 2
0
1 2
4 5 0
3 0
1 1
19
?,!
54
54
1%
8 14
ti
5 4
3 15
7 11
8 14 20
5
2
2
10
8
6 13
9 19
0
2 13 34
6
10 2 4
11
6 18 11
4
7 18
20
8 2 3 8 11
5
3 7 2
10
14 5
10 2 6 2 6
5 12 4
9 2
4
8 2 3 2
8 8 8 27
10 4
9 6 3 5
16 8 5 12
16
2 12
Duration o lonest vis-
ibility
Hour8 I
t
1 1
: 1 1
t
1
2
5
t
1
2 1 1
4
1
I 2 I
2
I 6 2 1
1 1 1
1 1 3 4 1
1 2
1
3 2 1 3 3 2 2 1 1 4
1
5
1 4 2
2 2
I 1
Highest wind
velocity during
dust- st.orm
M. p . h .
45
60 30 42
28 42
39 42
40
42
40 30 33 30 32 38 3s
44
48
39 42
26
13 48
30 44
40 36 34
40
42
rn 60 30
30 6.3 42 26 24 12
45
40 7.6 30 42
30 44
47 40 31 32 32 28 30 40
38
46
40. 33
38 42 30 51 30 46 20 30
61 44 32 44 50 I6 40
46
40 3s 41
18 46
a7
J U N ~ 1936 MONTHLY WIZATHER REVIEW 199
Prevailing Vfsibil- wind direc- Date ity lion during
duststorm
TABLE 1.-Duststorms at Amarillo, Tex.-Continued
Duratior
oi2:E-
Year, 19S4-Continued
I Affler
Apr. 12 __________________.
Apr. 14..---.-- _____.__..
Apr. 22 ____._________..._.
Apr. 2.3. - _____ ______ _. __ _.
A pr. 2-1. - -. - __ -. - -. - - .4pr.25...---.---..-.-.... Apr.26.. ____________. ___.
Apr.27 ....--.-..-.--..... hlay 3 ____________
~
______. May 4. __________________.
May 13 ___________________ May 14. .____________.____
JunelS.---.--.--....----- June 20. ___ _________
~
____ June 22 ___________.______.
July 5............-.--..-. Aug. 2 ___________________. Scpt. 2 ____________.__.___. Sept. 5 .__________________.
Sept. 6 ._____._____________
Sept. 20 -_.-----.-------_-. Sept. 22 ___________________ Sept. 25 ___________________ Oct. 8 _.__________________. Oct. 15 .__________.____.___
Nov. 2 _.________.__.._____
Nov. 4 _.___.___._._.______
Nor. 27 _.______.___.____._
Dec.1 ..--......--.--..... Dec. 2 .___________________
Dec. 6 .___________________
Dec.31-..----.----.------
Y e a r , 1955 Jan. 3 ..................... Jan. 12 ___________________.
Jan. 12-13 ______________.__
Jan. 16. ___________________ Jan. 17 ____________________ Jan. 19 _____
~
______________ Jan. 20 .................... Jan. 21. ___________________ Feb. 13 _________ ____ _____
~
Feh. 14 ___________________ Feb. 15 ___________________ Feh. IS ___________________ Feb. 17 ___________________ Feb. 18 ___________________ Feb. 21. - _______________ __ Fe b. 2 1-22. - - - - - - - - - - - - - -.
Feb. 22 ___._..____________
Feb. 23-24-25. - ___ ___. ____ Feb.27 .--.-.--.-....----. Feb.28 ......-........-.-. Alar. 3 __._...___.._.._.._.
Apr. 13.--.--..---..-.-.-.
2
3 4 2
3 2
S 1 2
4 2
1 5 5
5 6
2
6
5 6 3
G
5 2
3 1
F 4
3
13
3
5
3
5
2
2
5 6
4
0
3
4
9 6 6
IF
F 0 4
0 6
14:
0
35
Prevailing
tion during oi2:$ duststorm
wind direc- Duration
4 4 2
6 11
19 5
6 5 2 4
5 3
1
5 4 8 5 8 4
2 7 1 2 1
8
11 8
14 4
7 1
4
7
4
8
10 4
1
2 1
0
1
11 2
1 7
12
11 3
56 1
9
4
Duration of lowest vis- ibility
Hourr 2 2 1
2
7 7
3
3
2
1 2 1 1 1
3 4
1 5 1 1 1
1 1
2
1 3
1
2
2
2 2
1
4
5
2
2
1 1 1 1 1
2 1 1 2
1
3 2
1
1
1
1
2
11
Highest wind velocity during dust- storm
h1.p.h. 14
28
30 26
1s 24 30
26
34
35 19 40
28
1s 17 32 20 1B
Bo 40 49
48
43
40 17
44
44
46 36 39 28 26 53
27 41 39 56
29
29
26
19
40
24
30 34
8 15
42
4G
7 44 19
34
48
TABLE 1.-Duststorms at Amarillo, Ta.-Continued
l’ear, 19S6-Conlinued
hfsr. 4-5 _.___.__ _.________
hlar. 5 _____._._.___.___.__
Mar. 6 _._.___.____.__.___.
Mar. 8 ..___..____._______.
Alar. 13 .___. _._.___._._._.
Mar. 15-16-17 __._.________
hlsr. IS .___._.___.___.____
Mar. 19 ____________
~
__ __ __ Alar. 20 ______.____________
hiar. 21 ._____._._________. Mar. 22 _.____________._.-. hlar. 2G ._________________.
Mar. P i ____._._._._.._____
Mar. 29-26-3&31 ... .-. ___.
Y e a r , 1996
Feb. 3 ______.._._________.
Feb.7-S ..-...---....--.-. Feb. 9 _.______________.__.
Feb. 12-13 ________________ Feb. 13-14 _______.__.__._.
Feh. 14 ________________._.
Feb. 16 _________________..
Feb. 17 -..-.-.-.-..--...-. Feb. 23 ______._.____-.-.-_
Feb.24 ...........-.-.-... Feb.95 .--..............-- Feb. 25 ___._.....____...__
Feb. 26 _______...._____.__
hlar. 1 .................... Mar. 3 .................... hlar. 4 _.__________________
Mar. 5 ..__________________
Mar. 10 ... ________________ Mor. 12 ____-___-_--------- Mar. 13-14 ________________ Mar. 14-15 ________________ hlar. 1516 ________________ Mar. 16 ___________________ hlar. 17 ___________________
hlsr. 19 ___________________ Mar. 21-22 ________________
Mar. 23-24 ________________ MRr. 24 __________________- Mar. 25 _____ __ __ __ __ ______ Mar. 26 ___________________ hIor. 27 ___________________ hlar. 28 ___________________ Mar. 29 _________________ __ Mar. 30 ___________________ Mar. 30 __________.________
Mar. 31 ______._.._________
>far. 17 ___________________
h m . 22.-. __ ____ ______ __ __
6 4 S 1
55 1
14
8 5 1 2
19 s3
10 23
15 21 22 5
3
8 12
r! i 8 4
4
10 4 13 16
‘2 i 7
6
12
8 17 13
19
4
15
14
10 3 6
5
11
S
-
Duration of lowest vis- ibility
1
1
8 6
1
5 2
2 1
; 2 2
3 1 1
1
1 1
1
1
1 1
1 a
Highest wind velocity during
storm dust-
AI. p . h.
55 37 44 3.5 20
so 38
28 44
30 2s 25
46
36
38
40
40 35 44 18
30 30 38 44
10 32 20 40 20 38 30 40 33 29
30
46 24
16 40
38
42
52
56
36 42
28
13
33 27
28 40
38
WINTER AIR-MASS CONVERGENCE OVER THE NORTH PACIFIC
By ROBERT W. RICHARDSON
[University of Callfornla, Berkeley, May 19381
This paper is the result of a study of storm tracks in their relations to frontal zones and to the distribution of
air masses over the North Pacific Ocean. Upon the
completion of cyclone frequency maps, the question of the location of the Pacific polar front appeared to be signifi-
cant; a comparison of the maps with Bjerknes’ map of the principal fronts in the Northern Hemisphere reveals
discrepancies that can hardly be accounted for by me- chanical errors or incomplete data. It is admitted that
the frontal zone shifts its latitude with the seasons; the
question dealt with here concerns only the mean position of the polar front during the winter. Figure 1 is based on the map and figure employed by
the authors of Physiknlische Hydrodynamik, and illus- trates the application of their theory in establishing the
position of the Pacific polar front in winter from the mean pressure map for February. The underlying principle is
that a front is produced by a particular combination of
air movements that may graphically be represented by
1 Presented before the Association of Pacific Coast Geonaohers. Los Inaeles, Calif.. _.
June 27 1935.
2 Djerhnes, V.. et al., Physikaliscbe Hydrodynamik, Berlin, 193% 708, fig. 130.
* Op. cit., 703, 5g. 121; 708, fig. 130.
stream lines and is termed a deformation field. A typical
frontogenetic deformation field is illustrated in the inset in
figure 1. Considering the stream lines to conform to gradient winds in the Northern Hemisphere, the distri- bution of pressure will be as indicated. If the deformation
field is to produce a front, however, it must be superim- posed on an appropriate temperature field. The iso-
therms of such a field are indicated by the broken lines-
temperature decreasing toward the top of the diagram. The warm air entering the deformation field from below,
and the cold air brought in from above, escape along the axis of estension to the left and to the right. It is only
along the front to the right of the intersection of axes, where the direction of tho gradient wind produced by the
frontal solenoidal field is opposite to that of the wind in the warmer current, that the equilibrium of the atmos-
phere is disturbed, and wandering cyclones are generated.
If these principles are applied to the map of mean pressure for February, upon which the sea level tempera-
ture field for February‘ has been superimposed, it is to
4 Isotherms from Schott, Q., Geographie des Indischen und Stillen Omans, Earnburg,
1835. plate VI.