I,..._""'.,.,_..,...,... ~ _,._. _.._..., , , ll'dn"-:,.,._..,_.,..._..._... ~..,;

I I

S.AJ:ii FE:RNANDO VALLEY CLIH.ATE ,,

A thesis submitted in partial satisfaction

of

the

requirements

for

the degree

of

l'laster

of Arts in

Geography

by

Joseph Ray Glantz

,?""'""

June,

1977

,__ ... _""' ________ .,.,_..,. __ ...., ... __

^~

... -... _, ___ ._. _ _.._ ... ,.. .. -.. .... -,. ... ---··----.. -·,--··--·-ll'dn"-:,.,._..,_.,. .. ..._ _ _..._...._. ..

^~

.. ,;

. i

I I

The Thes 1 s of Joseph ~Rat G 1 an tz is approved:

Robert B. Lamb

David A. Henderson (Date

Arnold Court, Chairman (Date)

California State University, Northridge

i i

understanding of those individuals associated with the author during the creation of this report be recognized. On occasions, a strain in the atmosphere surfaced when they observed the author continuously working on this paper over a rather long period of time. They would then ask in disbelief, "Haven't you finished that thesis yet?"

Special thanks to my wife, Phyllis, who recently acquired a husband who she thought was on the verge of fulfilling his goal of attaining a Master's degree in Geography. Thanks to Dr. Robert. Lamb for his many sug- gestions which have been followed in this report; Lucille Steinberg, who undertook the mammoth task of editing it; Hope Parshall for her illustration expertise; and Marilyn Thompson for the final typing.

A final nod to Dr. Arnold Court, often understandably referred to as Simon Legree (his own description) , for not only encouraging me to complete this, but also for the loa~

of a good portion of his personal library for research material.

i i i

1.

TABLE OF CQNTENTS

INTRODUCTION 1.1

1.2 1.3 1.4 1.5

Purpose . . . . Background. . . . Physical Situation. . . . Climate . . . • . History . . . .

2. WEATHER OBSERVATIONS . 2.1

2.2 2.3 2.4

Background . . . . San Fernando. . . . . Cooperative Stations. . . . . Aviation Stations . . . .

3. TEMPERATURE . . .

4.

5.

3.1 3.2

Background.

Temperature Trends. PRECIPITATION . .

4.1 4.2 4.3

Background. Topography.

Precipitation Trends . . OTHER ELEMENTS .

5.1 5.2 5.3 5.4

~'Jind. . . . . . Humidity. . . . Clouds . .

Air Quality 6. CONCLUSIONS . . . 7. BIBLIOGRAPHY

8. APPENDIX . .

iv

PAGE 1 1 2 3 7 12 16 16 17 19 20 26 26 33 44 44 46 50

60 60 65 69 70 76 79 84

Figure 1 Figure 2

Figure 3 Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Figure 12

Figure 13

Figure 14

Figure 15

Figure 16

i

San Fernando Valley . . 4 Drainage Basins, San Fernando

Valley. . . • . . . . ⁶

Valley Soils. ⁸

Successive Locations of the San

Fernando Weather Station. . . 18 Names and Elevatiom of Weather

Stations Used . . . . 25 Mean Wintertime Maximum and

Minimum Temperatures (1965-1973). 27 Mean Summertime Maximum and

Minimum Temperatures (1965-1973) . . 28 Average Number of Nights Requiring

Orchard Heating • . . . 32 Mean Annual Temperatures, San

Fernando, December and January. 38 Mean Annual Temperatures, San

Fernando, February and ~1arch.

Mean Annual Temperatures, San Fernando, June and July . Mean Annual Temperatures, San Fernando, August and September. San Fernando and Los Angeles Seasonal Rainfall (July-June) Mean Annual Precipitation, 1951-1960 . . . . . Mean Annual Precipitation, 1951-1960, and Elevation. Percent of Average Seasonal

Rainfall for San Fernando plotted Against the Lynch Percent of

Average Seasonal Rainfall . . . . . 39

40

41

45

47

48

57

\,,_, .. _ __... ______ ... ...,...,._ .... . . , - - - - · -... _ ... _... _ _ , _ _ _ _,_._.._ .. ._ __ .._... _ _ _..-....,_..., ____ , _ _ .._, ______ ..,.., .. _... .. _... _ _ .~-·-..·-··"-y...:,•,7~

v

,._~-~---.·~-~----'-..._r ___________ ~~---·---~-~---_.. ----~L---

Figure 17

li'igure 18

Figure 19

Figure 20

Figure 21

Figure 22

Figure 23

A 200-Year Rainfall Record as Percent of Decadal Average Map of Typical Sea Breeze

Streamlines in General Vicinity of Los Angeles . . . . Surface Streamline Maps for

8 August 1957. . . . . Streamlines of Most Frequently Observed Wind Directions for 0500 PST . . . .

Average Number of Santa Ana Wind- storms at Northridge, July 1957 to September 1966 . . . . Los Angeles Air Quality

Monitoring Stations.

Location of Stations Used in Table 19 . . . .

vi

PAGE

58

62

63

64

66

71

100

'

I

Table 1

Table 2

Table 3

Table 4

Table 5

Table 6

Table 7

Table 8

Table 9

San Fernando Valley Weather Stations, in Chronological Order of Estab-

PAGE

lishment . . . 21 Average Number of Days, 1951-1960

with Temperatures > 90°F and < 32 F . . 30 6 San Fernando Mean Monthly

Temperatures. . 35

Mean Seasonal Temperatures by Periods

and Inter-Period Changes (°F) . . . 42 Average Number of Days with Maximum

Temperatures > 90°F and Minimum

Temperatures < 32°F . . . 42 Number of Days with Annual Mean

Precipitation > .10 and > .50 Inch,

1951-1960 . .

-=- . . .

49 San Fernando Seasonal and Annual

Precipitation Totals, 1879-1973 . . 51 Ten-Year Seasonal and Annual Rainfall

Averages for San Fernando . . . 54 Twenty-Five-Year Average Seasonal

Rainfall for San Fernando . . . 54 Table 10 Lynch Seasonal {Jul-Jun) Rainfall

Indices (Percent of Average) for the

Period 1770-71 to 1929-30 . . . 56 Table 11 Percent Frequency of Wind Speeds

> 17 and > 28 Knots . . . 61 Table 12 Mean Relative Humidity at Burbank

(Percent) 4 to 4:30 p.m. . . . . 67 Table 13 Total Number of Days with a "Trace ••

of Rain or More in July and August,

by Decades. . . . . . . 68 Table 14 Average Percentages of Sky Cover

Days, 1951-1960 . . . 70

... ,.~--. ~---~·-- ·---~---·

---·--

vii

Table 15

Table 16

Table 17

Table 18 Table 19 Table 20

Table 21

Table 22

Table 23

Table 24

Table 25

Table 26

Table 27

Table 28

'

Annual Averages of Daily One-hour Maxima of Various Pollutants for Burbank and Reseda, 1955-1974, in Parts per Million • . . . • . Monthly Instantaneous Maxima of Various Pollutants for Burbank and Reseda, 1974, in Parts per

Million . . • . . . • . .

. .

^..

.

San Fernando Temperature and

PAGE

74

75

Wind Data . . . • . . 88 Prec. for Water Year (Oct to Sep) 89 Monthly Prec. Totals, 1931-1960 . 93 Probability that a Given Day will be

Wet or Dry in San Fernando. . . . 101 Precipitation Means and Probabilities for 1-, 2-, and 3-Week Periods . . . . 102 Uniform Summary of Weather

Observations for Burbank and

Van Nuys Airports . . . 103 Low Ceilings and Low Visibilities

for Burbank Airport . . . 107 Occurrence of Fog, Low Visibilities,

and Low Ceilings, Burbank Airport . . 110 Frequency of Daily Temperatures for

Van Nuys Airport. . . 114

~ecommended Outdoor Design

Temperatures for Locations in the

San Fernando Valley (1931-1962) . . . 117 Local Temperatures Between 1931

and 1952. . . . . . 118 Evaporation in Inches for San

Fernando Valley Stations. . . . . 119

.. _,~.,..._,.. ___ ..,_._ .. "'~·-~·----...-...--·~---···-··---·~·-*·-~- -~----·~~-~---··~~~-~---· _.. ---,.---·--·---···~~~-~~~~-~· ~-~~-....-... "' ... ,.,._.~,_,_..,_,_.~_..____,_..._.. _{... _}

viii

I !

ABSTRACT

SAN FER.l~ANDO VALLEY CLIMATE

by

Joseph Ray Glantz

'Haster of Arts in Geography

The San Fernando Valley has been mentioned only in

!passing in previous studies of Southern California climate.

j

The availability of temperature and rainfall measurements

made on and near the city of San Fernando £roE

1878

to

1974,¹

hmveve.r, permits a closer look at Valley climate. The

temperature record is missing for

25

years:

1904

to

1928;

betHeen

1878-1903

and

1929-1974-

the Valley became progres-

si vely warmer, more so in \•?inter than summer, especially in

i

the urbanization phase after

1950~

Rainfall data have been published for all years except

three:

1904

to

1906.

This record cm1 be extended back to

~---~-,.,.... .. ~-.. ---~--·--· ... --;..-,_...-~.-...-..

---

ix

' I

p

·---,----,

!about

! !

1770 from the reconstruction, by H. B. l.ynch in 1931,

~ ~ [

!of seasonal

! rainfall from numerous chronicles starting with!

!the mission

'

days around 1770 and ending in 1930. Conbina-

I !tion of the reconstructed data with actual measurements

!taken at San Fernando has provided nearly 200 years of

! I

~seasonal

rainfall records.

!dence of any cyclical character in the rainfall picture.

This series gives little evi-

I

_I

\-leather station observing neD:v-orks slmvly evolved in

I !the San Fernando Valley until the 1930s. Published infor-

i

I

i '

lrr.ation from these stations is included with a brief histor-i

I.

^{1 d · · f h · 1 ,...}

tl.ca escrlptl.on o t ese statl.ons to supp ement .:>an

!Fernando data and contributes to·ward a better understanding

I lof Valley climate.

L--·---.--~. ~---~---n---··--~--·~·----·---- X

The purpose of this thesis is to assenilile, record, l 'and evaluate the available weather information about the :san Fernando Valley. These data may then be used to study ,possible fluctuations in climate over the past century. 'Data include temperature, rainfall, humidity, cloudiness,

·air quality, and to a limited extent, certain aviation ,weather parameters associated with the two major airports

in the Valley, Burbank and Van Nuys.

y~fter World War II, housing tracts replaced groves .and farms as the Valley became a densely populated segment

;of Los Angeles, having a million peoplE(~(as outlined in .Section 1.5).

1 The San Fernando Valley is about 15 miles northwest .of downtown Los Angeles, surrounded by mountains ranging in elevation from 1,500 feet along its southern border to about 5, 000 feet in the extreme northeast·· More geographi- cal information is given in Section 1.5. Climatically i t represents a transition between the marine regime on the

! 'shores of the Pacific Ocean, south and west of its borders,;

;and the extreme temperature regimes of the mountain and ,desert regions to the north and east~~ Meteorological con- iditions are discussed in Section 1.4.

At the end of the 19th century, Valley land use ranged .from scattered ranches and farms to large-scale crop

l

,..

_{__}

:production. The local water supply depended on wintertime :rains, as well as the underground water supply. By 1913,

following completion by the city of Los Angeles of its Owens River Aqueduct, most Valley communities elected to :join the city of Los Angeles to be eligible to use its

' ^.

lwater. Heavy irrigation replaced the previous dry farming

i

; practices;<jt Section 1. 4) •

:

_.

.;.,,

;

'1.2 Background

No other region of continental United States has had :its climate more publicized than the Southern California

area, including the San Fernando Valley. XIn the past cen- .tury, periodic mass migrations westward were the result, 'direct or indirect, of the extolling of its climatic ivirtues. In the late 19th century, climatic inducement

!

; and bargain railroad fares attracted hundreds of respira-

;tory sufferers, many of whom sought employment in the

'

growing citrus industry .(:(}The more affluent purchased land,

~·~--../

hoping to become successful citrus growers. A traveler of the time wrote:

"Most of them are a combination of ill-health, intellectuality, and comfortable circumstances. They can sit on the verandas of their pretty cottages - the refined essences of abstract existences - inhaling the pure air of the equable climate, reading novels or abstruse works of philosophy, according to their

mental activity, from day to day, and waiting from year to year for their oranges to grow." Baur (1959, p. 116).

XCornpletion in 1913 of the Owens River Aqueduct and annexation to Los Angeles, gave the Valley an almost

2

r---· --. ...,...-.-....

"~""""""'---""-s.-

_..

~

_..

.flC>'~ .. • ... ...,.~ .... . . , . . . , . , . . - -. . - -... . . _ ... _ , _ _ _ _ _ _ _ _ _ _ _ ... ._... ... _~-~-

'unlimited supply of fresh water when its voters approved :annexation to the city of Los Angeles. The resulting :agricultural boom persisted until the late 1940s

!

.)(The most recent chapter in Valley population growth .:occurred during the 1950s and 1960s, stimulated to a large

;degree by the moderate climate and jobs in the growing

!aerospace industry. Farmlands were replaced by homes,

I

:apartments, schools, and large shopping centers. The num- :ber of people in the Valley rose from 80,000 in 1930 to

400,000 by 1950, reaching one million by the mid-1960s/~) J:';l...._···-.,-/

:(Preston, 1965; Security Pacific Bank, 1967).

11.3 Physical Situation

~The San Fernando Valley covers approximately 192 :square miles in the western sector of Los Angeles County

(Figure 1) . Of several gaps in the surrounding mountains ,(Section 1~1), the largest is in the extreme southeast

corner in the Glendale area (north of downtown Los Angeles) along the eastern edge of the hills comprising Griffith

;Park. A second opening is Cahuenga Pass, seven miles to .the west.

~Stretching along the northern border, the San Gabriel

Mountains, 3,500 feet to 5,000 feet high, consist of com- :plex fault blocks dissected by many steep, V-shaped youth-

I

iful canyons (Pappas, 1952). In the extreme northwest, the Santa Susana Mountains, rising to almost 3,500 feet, are separated from the San Gabriels by the San Fernando Pass

4

)

eBeverly Los Angefes

• .Culver City eHawthorne

Figi.Ire 1. San Fernando Valley

:Railroad. Along the western margin, the Santa Susanas :merge with the Simi Hills, a range of low-lying hills con-

taining a large number of sandstone outcroppings. Highest elevation is about 2,200 feet.

The Santa Monica Mountains, 2,000 feet up, extend 'along the southern border, separating the Valley from the

Los Angeles coastal plain. Their relatively low altitude allows for easy access. Two major freeways pass through these mountains: The San Diego Freeway through Sepulveda Pass from Westwood and Sawtelle, and the Hollywood Freeway through Cahuenga Pass 10 miles to the east. Along the Valley's eastern edge are the Verdugo Mountains, rising to

2,700 feet, an apparent adjunct of the higher San Gabriel :Mountains farther east.

Vegetation in the hilly areas includes scrub oak, chapparal, and minor stands of oak trees. This indigenous vegetation plus the bunchy grasses and potential fruit- bearing trees would describe i t as being Mesothermal Sub- tropical (Russell, 1926). The native Valley floor flora was replaced by a succession of domestic plants, starting 'with dry farming grains in the late 19th century, large :areas of truck farming crops and orchards by the early

20th, and present-day decorative-type shrubbery.

The general boundaries of San Fernando Valley are shewn in Figure 2. The geographical boundary, according . to the areal drainage patterns of the Los Angeles River

···-···-···-···-···-...

""':-

_..

_{-···-··· ....} .,.,..

... _-

~···

\

\

i'

--

... . Los

.>-:

c:l

_..

ul

~

I :

:::;

:;:,1

QJ!

hi flranlll YIIIIJ

LANDFORMS AND DIAINAGE

l-· .. -... 1

Intermittent streams

lfi2£J

Wuh areas

1-·---1

Channelled stream

Contour Intern! 500 flit H

^~.

⁰ ._____L ___ •. ^{1 2 . 3} - , miles ^·.

San Fernando Valley (Eder, 1960) _{~ ..=)3}

0'\

iwatershed emcompasses more hilly land than Valley floor

!

. (Blevins, 197 5) :

Acres Sq. Mi. Sq. Km. Percent lHills and Mountains 206,000 322 834 63

'Valley Floor 123,000 192 497 37

Total 329,000 514 1331 100

The Pacoima and Tujunga Washes originate in the San Gabriel, iMountains and flow across the eastern part of the Valley

l

!into the Los Angeles River, which flows easterly adjacent

!

:to the Santa 1-ionica Mountains and enters the Los Angeles area through the gap near Glendale (Figure 2).

Much of the area is covered by two large alluvial

;fans, one extending southward from the San Gabriel and

!

:santa Susana Mountains, and the other westward from the :verdugo Mountains. The most fertile soil is near the

i

;junction of these fans where the fine texture provides

!optimal conditions for intensive irrigation (Ewing, 1939). :secondary soils, classified as good, occur at the heads of 'each fan where they tend to be coarse and high in alkaline icontent. Poorest soils are in the vicinity of the washes

which are sandy; similar conditions exist in the hilly intrusions where the adobe content is high (Figure 3). 1.4 Climate

)\, Wet winters and warm, dry summers characterize the

!climate of the Valley along with the surrounding portions .of Southern California. This "Mediterranean" climate is

;the consequence of location on a west coast at the southern

CHATSWORTH RESERVOIR

[From Storie's Index Map]

! 3 4

5: I'L ) i n?l

Excellent Good Fair Poor Very Poor

bJUI'ILANU 6 [ / / / d Nonagricultural

';{.'tl'~ ^/ w;;;;;;)x;ny;p 77/77 ):1

Figure 3. Valley Soils

co

jedge of the belt of westerly winds. Southern California's

l

I •

•cl1mate is dominated by one of the major features of the

;

;world's atmosphere, the eastern Pacific anticyclone. In bune, when the noontime sun is directly overhead at the

!Tropic of Cancer (23.5° north), this semi-permanent anti-

!

.

;cyclone is about 1,000 miles west-southwest of Southern

•California. Storm fronts are deflected far to the north, jresulting in a relatively rain-free period from June to

\September.

;<. Thereafter, as the sun moves to its southernmost

! . .

tposJ.tJ.on over the Tropic of Capricorn (23.5° south) by

i iDecember, the semi-permanent High shifts more to the west.

;This allows weather fronts to move south along the west :coast of North America, occasionally reaching Southern

fCalifornia. These are the primary causes of precipitation,

;generally from November to April. Practically an entire _year's rainfall (snowfall in the higher elevations) falls

>during this period. This precipitation, accompanying

1weather frontal passages and, on some occasions, a low

;pressure center at upper levels nearby, is associated with

I

;southerly airflow. South-facing mountain slopes receive

!more precipitation than the adjacent flatlands because of

~th~ orographic lifting of the rain-pro~ucing clouds pro- ipelled by southerly winds. After this moist air crosses [the mountain summits, i t dries when descending into the valleys and deserts to the north of San Fernando Valley and

, - - -

_..

---~~---~,.-~---··~--·---·---·---·-· -·---·~----"'·.

(produces less precipitationZ~As discussed in Section 4.1,

\

:the Antelope Valley receives approximately one-third the 'annual totals in the San Fernando Valley.

After a low pressure center passes too far north to

·produce local precipitation, the following cold, dense air

;settles over the western plateau region, developing into

1

:a high pressure center. The severe pressure gradient be-

'

!tween this High and the lower pressures near the coast causes a strong flow of air through the local mountains. Constricted into the valleys, these winds become very ,strong, moving through the lower coastal canyons to the :Pacific Ocean. These Santa Ana winds, blowing from north

j

:to northeast, rapidly warm by compression and become very dry. On occasions like this, coastal areas experience

!Warmer weather than those farther inland.

\

~ Summer is normally a period of fair, warm weather with

;little or no rainfall, characteristic of subtropical eli- mates. The only rainfall of consequence is the result of .periodic intrusions of warm, moist air from the Pacific

;ocean to the southwest, or the Gulf of California adjacent (to ! Baja, California (Section 8). Air subsiding on the

i i

'eastern edge of the Pacific anticyclone contributes to an

J alinost daily occurrence of a temperature invension. This

i separates the cool, marine air from over the ocean and the

!

: \varm, dry air above i t resulting from the subsidence. On 'occasions the steady northerly flow of wind at Point

. Arguello (about 120 miles northv1est of Los Jl...ngeles) extends

10

; in diminishing force to the Channel Islands (Santa Catalina': Island and those to the immediate northwest). A relatively_

\shallow low pressure center, known as a Catalina Eddy, forms between the Islands and the coast, enabling the sea 'air reaching the coast to flow from a more southerly

direction, especially from the Los Angeles area northward

I

!toward Ventura, 60 miles away. The eddy lifts the tempera- ture inversion significantly, which lowers San Fernando

;maximum temperatures, and carries atmospheric pollutants further inland to the western and northern portions (Sec-

· tion 6) . '

Because the Valley is situated between the cooler

~ocean climate to its south and west and the more extreme

·temperature regime experienced inland to its north and

!

:east, i t is a temperature transitional zone (Section 3.1).

!Both seasonal and even daily maxima and minima have greater extremes than in downtown Los Angeles where the ocean

influence is more pronounced. These extremes, however, are not as great as in the high desert and mountain areas

;to the northeast.

Night and early morning cloudiness is more common to

;the eastern Valley, where ocean breezes first enter, than ,in the northern or western portions. The reverse holds

true during Santa Ana winds in winter, which are more com- mon to the west and north, adjacent to the higher mountains. A more detailed examination of Valley climate follows in ..

:?..~~~~q~~n: ^{!=_. __} s._~_s: :~:-_~_?._~§_-~--G~- .. --·---,---.. ·---.. ·-·· .. ---·---·· .·

; 1. 5 History

~he recorded history of the San Fernando Valley goes back barely two centuries. Earliest references appear in individual diaries of members of Gaspar de Portola's 1769 expedition (Englehardt, 1927). Portola had been instructed

;by the Spanish government to establish a series of missions between San Diego and San Francisco. The only climatic conditions mentioned in the journals are in occasional references to high water marks observed along local streams, especially in the present Los Angeles River.

San Fernando Mission opened in 1797, less than a mile :west of the present city of San Fernando in the north por-

tion of the Valley (Figure 4). Its location, according to ,Englehardt, was influenced by the availability of water, i fertile land, abundant limestone for kilns, and a rela- 'tively large Indian population. Cattle, introduced by

Spanish settlers in the late 18th century, roamed freely over the countryside, feeding upon the various grasses, wild mustards, and tules (Pappas, 1952). Grain planting, especially barley and wheat, and subsequent harvesting, :were faithfully recorded by the mission fathers. Their

records, 150 years later, helped provide a better under-

• standing of rainfall patterns in the early days of the 'Valley (Section 2.2).

Secularization of the mission system by the Mexican government in the 1830s led to the abandoning of crop and , cattle raising on a large scale (only small farms and

'-~-7c .... .>.-~o'•<'-> .... ..._ ... , • • , --~-·--·~----~ ... ~0 ... ~-- ·~--· ,.,.,~-·-7 ·-~~----~···---·~·--~ c-A·--~ ~---~"'-:""""r .. _,.~-~-~--... ~·"••·O••-~~...-·~·"·~ _____ .,_.__ .. ~ , ___ ...,...,..,_...,. .. ,.._..,. ... ,,..~~-~-·· ~·-..-.--~ ---···~· y . ~··

12

California joined the United States in 1848. During most of the first half of the 19th century, travelers rode through the Valley on El Camino Real (a trail followed by the present day Ventura Freeway). By the late 1860s, pro- meters acquired large tracts of land from Mexican grants

1and poured huge amounts of capital into the area, seeking

I

:quick profits by subdividing and selling small land par- i :eels (Robinson, 1967). More than 40,000 head of sheep

!roamed over the area until a severe drought in 1878 vir-_!

!tually wiped out most of the herds, once again ending large-scale cattle and sheep raising (Robinson, 1967).

During the 1870s, the Southern Pacific Railroad was ,built northwest along the eastern edge of the Valley from

downtown Los Angeles to the present city of San Fernando.

;completion in 1876 of a railroad tunnel several miles farther northwest extended freight and passenger service :on a continuous line to northern California. Weather ob- ,serving stations were opened at various depots along the

i

route, including the one in San Fernando (Section 2.2). Their data were used in encouraging people to settle in

l

!the Valley.

In the early 1890s, another railway line across the :southern part of the Valley led to establishing farms along: l

!its route. In 1904, a third trunkline was laid diagonally :across the Valley, meeting the southern line at Chatsworth

:and proceeding by tunnels through the Santa Susana

I

'

•Mountains. This gave the Valley two railroad outlets to

! I

'northern California.

Fruit culture was introduced into the Valley by the :late 19th century, attracting many people. The loam-like

'

isoils could retain large amounts of winter rains, favoring 'Citrus, deciduous fruit, grapes, and walnuts (Ewing, 1939). 'In 1901, a large olive oil establishment was opened north iof the city of San Fernando, adjacent to an 1,800 acre 'tract of olive trees (Zierer, 1934).

Following completion of the Los Angeles Owens River jAgueduct in 1913, irrigated croplands increased to 30,000

!

l !acres by 1917 and over 50,000 by 1920 (Eder, 1960). Giant

;land holdings gave way to smaller cultivated farms and ranches, now that a large supply of water was available at a low cost. Citrus totaled about 25 percent of the entire production. The sandy soils of the washes around Burbank

favored grapes, while the deeper and potentially moist -soils in the southern Valley supported large stands of 'alfalfa (Ewing, 1939).

This profitable use of the land changed after World iWar II. Attractive climate, gainful employment, and in- ,expensive housing lured hundreds and thousands of people to

the area. Land was turned over to the building of a large 'metropolis at the expense of agriculture.

All these changes in the character of the Valley may .have caused corresponding changes in its climate. The two

... ^~.._.... ... ___ ...,._,__ .. _...,_ .. _, ___ ^~-~---.. ---~~~...-...~----~·~·A·-_.. ~T~---·-·P-~-n·----~---~-·~~~----·..-.-,.,.~~-·-... -. ... ^--.~~ .. --... -·6~~~-~--~·-~~~· .. , ... ^~.A-~.--· ... "'"''~ _.. =-~~-_.. ~-~~-·-~-"·~····-·"""'''

14

;significant changes in Valley land use are with the

!availability of aqueduct water (1913) and the suburbaniza- tion, which began around 1952. Since the station at San .Fernando has the longest Valley temperature and rainfall

records, the available data from 1878 to 1974 will be

~divided into three periods (Sections 3 and 4). Ranching: 1870-1913

Irrigation: 1914-1951 Suburban: 1952-1974

As a basis for discussing possible changes to Valley :climate, Section 2 will discuss the evolution of the

;weather observing network which has led to a better under-'

: standing of climate in San Fernando Valley.

2. WEATHER OBSERVATIONS : 2. l Background

K

The first official weather station to represent the iSan Fernando Valley was opened by the Southern Pacific

Railroad at its San Fernando depot in 1877 (Section 1.5).

!Although an old state publication (California, Office of

;the State Engineer, 1886) listed the station as "San Fernando", subsequent

u.s.

Weather Bureau publications,

,r:'"'-·"'1

until 1903, carried i t as "Fernando"(X,'(monthly average

temperatures for those early years appear in Section 3.2).

'/'~ During this period, weather observations countrywide

:were under the aegis of the Army Signal Corps from 1870 i

;until the Weather Bureau was established in the Department :of Agriculture in 1891 (Hughes, 1970). In 1972, the

:weather Bureau became the National Weather Service, and its climatic section the Environmental Data Service. Under the old Signal Corps, primary meteorological functions were limited to storm bulletins and weather forecasts. The num- 'ber of volunteer weather observers throughout the country

1declined until the mid-1880s, when the Corps altered its

i [policy to encourage climatic stations.

"/, When the Weather Bureau took over the entire meteoro-

; logical program, the number of observers had increased

I

,dramatically. The Valley did not benefit from any of these , changes since San Fernando remained the only observing

16

1station until the early 1900s, with one minor exception . (Section 2. 3).

;2.2 San Fernando

f

At San Fernando, daily maximum and minimum tempera- .tures and rainfall were measured. Half a century later, .the importance of the rainfall records was demonstrated by :their use in a study of rainfall probability for the

western United States (Appendix, Table 20).

In 1916, after nearly 40 years of operation by the .railroad, the weather station was moved to the warehouse :of the Lemon Growers Association, a mile or so west-south-

west of the depot and about 100 feet lower in elevation

!

: (USWB Substation History - California 1958). Citrus rais- )ing had become a major factor in Valley growth, and winter-: 'time temperature minima were of prime interest.

Association employees manned the station for the next 45 years, until 1961, and even though the weather instru- ments were moved on several occasions during this period, each location was called San Fernando. Each location was

·between 950 and 1,000 feet above sea level, and each site

I :

:had minimal interference to temperature and rainfall read-

;

ings from large buildings or paved-over areas. Climatolo-

1gically speaking, the data were sufficiently compatible at

i

l

all locations to justify their representing San Fernando

_{/2) ·}

I

L/ .

(Figure 4).

In the 1950s, the large-scale removal of orchards to

1929-1947

0

Rinaldi St~

San Fernando IMtsslon Blvd.

Chatsworth St

-

(If

:-gl'

.:::3: 01'

el

< ~

'

_....

-

^~

:I: ~·­

:Z:,

<(I

(J')I

?

^{I I I I} 5

^I

^t•ULES

I______

Figur:e 4.

Success~~~-~~cations _Q_:L.!_~

_ _l)_9.n__ferng!).g.Q __ Weather

:N

.0.

-CAL I FORN lA ·

:Map loeat loA

1-' 00

:closure of the Association headquarters. This resulted in :a final move of the weather instruments to the grounds of :a large insurance company a few blocks north of its pre-

;vious location. The insurance company employees continued

i

idaily observations from 1961 until 1974, when increasing

j

:vandalism led to closing the station in April. Thus ended

:

!the long-term record of the oldest weather station in the iSan Fernando Valley and one of the oldest in the state.

San Fernando rainfall measurements have appeared in 'Various publications during its 97-year tenure. However, il904 and 1905 records were lost in the 1906 San Francisco

earthquake. Temperature records are more difficult to

!come by. From 1877 to 1903, mean monthly readings were tpublished, except for a few individual years. Data for 23 [years, from 1906 to 1938, are not available, possibly lost

!in the archives of the Environmental Data Service. 2.3 Cooperative Stations

The second official station in the Valley opened in : 1897 in its southwest quarter on the Girard Ranch (present

:

day Woodland Hills), adjacent to a siding on the new

Southern Pacific Railroad branch line (Section 2.1). Two

;years later, in 1899, a fire destroyed the ranchhouse and

I

;burned the weather records and equipment.

In 1910, daily rainfall measurements began just

;northwest of the center of Glendale, in the southeast ,corner of the Valley. The station is still maintained by

•the Los Angeles County Flood Control District, the original

,..,.._n_..,..._._._,.,..,.,,.,., ... ,...,...,_.,...,_-.,.r.;,a••""-... -.·-->4•"",.._..,_. •• ._.,_~,....,M""-•-*r>O-""""'_"" _{_ _ _ _} ,._,.,...,.~....-~-~..,u..._,.,..,.,..._...,_ _{_ _ _ _ , _ _} """"'_....-~...,.,_.,_...,. _{__} ,_,_.,_..~,,..,_,._

r-

~~---~---·---~·--·---,

(records stored in their archives. A chronological listing i

i

l

1 (

iof stations opened in the Valley in ensuing years is shown in Table 1. Sources for these data were: (1) "Substation History- California", released in 1958 by the Weather Bureaui (2) "Decadal Census of Weather Stations, Califor- :nia", a Weather Bureau publication in 1964; and (3) "Climaj

i

;tological Stations in California, 1971", issued by the

~

;california Department of Water Resources.

The number of weather stations operating within San Fernando Valley, at 10-year intervals, was:

1890 1900 1910 1920 1930 1940 1950 1960 1970

1 2 2 6 13 22 25 26 25

r 2.4 Aviation Stations

As air commerce developed after World War I, the Weather Bureau issued special bulletins and weather fore- casts for both domestic and military flights and training flights (Whitnah, 1961). Primary user of this service was

i

' the Post Office following the establishment of two air mail:

'

routes, one between Chicago and New York and the other fro~

New York to Washington, D.C., around the early 1920s.

Southern California became involved in the commercial aspect of this air age in 1928 at the old Grand Central

i

i Airport in Glendale, a mile or so east of Griffith Park in Los Angeles. Airways observations consisted of sky condi- tion, visibility and temperature.

20

I

SAN FERNANDO VALLEY WEATHER STATIONS, IN CHRONOLOGICAL ORDER OF ESTABLISHMENT

l I

I i

I I

I I

I

'

I

_i

I I

;

Station and No. of Locations

San Fernando 6

Girard 1

Glendale (Stapenhorst) 7 Girard Brant Ranch 1 Sunland

Pacoima Dam 5

Northridge DWP 2

Reseda. 5

Bell Canyon 5

Sherman Oaks 1

Granada Hills 2

Chatsworth 1

Van Norman Lake 1 Burbank Fire Dept. 1 Burbank Airport 1 Van Nuys Warehouse 1 Chatsworth Reservoir 1 Tujunga Spreading Gnd. 1 Encino Reservoir 3 Olive View Sanatorium 1

Record Elev. ^@ Lat. ^@ Long. @ Obsns.* Began Ended (ft) (north) (west) T ^p R

vi

E 1877 1974 965 34°16' 118°28' X X

1897 1899 UNK UNK UNK X

1910 530 34°09' 118016' X

1912 1952 876 34°10' 118°36' X

1915 1460 34°16' 118°18' X X

1917 1500 34°20' 118°24' X X

1920 810 34°14' 118032' X

1923 1960 785 34°10' 118°32' X

1925 1960 930 34°13' 118°39' X

1927 1941 900 34°09' 118°28' X

1927 1280 34°17' 118°31' ^X

1928 957 34°15' 118°36' X

1930 1150 34°17' 118°29' X

1930 1972 680 34°11' 118°18' X

1931 1966 699 34°12' 118°21' X _~ X X

1931 695 34°11' 118°27¹ X X

1931 865 34°13' 118°37' ^{X X}

1932 1945 815 34°13' 118°25' X

1932 1000 34°09' 118°31'

r

^{X X}

1936 1971 1425 34°20' 118°27'

(Continued on next page)

A H

_#

a a b b a b b b b b b b b b

X X a

b b b b b

_I

I

_'

.!

rv

I-'

r~

_..

_0<_, • ., --~ -.· -··-·· ·~ ~ ~ ~--- ..,.... _ _ _ ...., _ _ _ _ _ .. ~ ··-·~--·--· • .,·-~. -••• ____ , , _ , . . < , . -·~-~-""" · - · - --~--·-··-~---._..., _____ ···-···---·-·..,---··...,·~··

·~<'-·-··1

I

Table 1 (Continued)

l,stat.ion and No. of Locations

_L

Record lElev. ·@ I Lat. Long. ^@

1

Began !Ended 1 (ft) 1 (nort

!North Hollywood 3

_I

~>I

^(west)

!Hansen Dam

•Studio City

!van Nuys Airport

!

jBirmingham Hospital

~~Chatsworth Heyn. Rch. San Fernando P.H. #3

!canoga Park (Pierce)

! !Simi Hills

~Northridge (CSUN)

~Burbank APCD , Reseda 1\.PCD

!Burbank Pump Plant

!

3 1 1 1 1 1 1 1 1 1 1 1

1936 619 34°10'

1938 1100 34°16'

1939 680 34°08'

1942 720 34°13'

1944 722 34°11'

1945 1959 1000 34016'

1945 1248 34°19'

1949 794 34°11'

1959 1910 34°14'

1960 860 34°14'

1962 660 34°11'

1965 810 34°12'

1966 655 34°11'

118°24' 118°24'

XIX I _I a

X b

118°24' ^X b

118°29' xl

_I

X X _c

118°30' X b

118°36' X b

118030'

118°34'

_~~x

X _a

X X a

118°42' 118°31' 118°18' 118°31' 118°21'

~I~

^lx

I~ _I

^{IX \X}

^I

^{d d}

X ^le

X _le

X IX I I

j @At.latest location #Original records stored at:

Ia

Temperature (maximum and

minimum, occasionally hourly) P Standard Rain Gage

I

I*T

I

I I

_I

I l

R Recording Rain Gage

W Wind Direction and Speed E Evaporation

A Airways Observation H Relative Humidity

a National Climatic Center

b L.A. County Flood Control District c Federal Aviation Administration d At Station

e L.A. Air Pollution Control District (now So. Cal. Air Quality Management

District)

I _I

I

J I

N N

---·---·---··---~--~---·-··~-·~·

In 1931, the aviational observational program was transferred to Burbank Airport (Hollywood-Burbank Airport at present) , 12 miles northwest of downtown Los Angeles and approximately 3 miles west-northwest of the main Bur- bank Post Office. It remained here until a disastrous fire in the middle of 1966 caused the Weather Bureau to close its offices and let control tower personnel observe aviation weather.

Airway observations included ceiling and visibility readings, pressure, temperature, moisture content, and wind. Temperatures were read from atop the roof, 38 feet up, until 1961, when ground level readings were made. At first the rain gage was at ground level, but moved to the roof in 1941. Wind measurements were recorded from the control tower roof at about 81 feet above the ground; raised another 8 feet in 1956 to minimize wind-eddy effects. Some of the machine-run summaries of weather parameters for the station, available at the National Climatic Center, appear in the Appendix.

A third major airport weather reporting station was opened by the Army Air Forces at Van Nuys in 1942. This installation is still operating, 8 miles west-northwest of ; Burbank Airport at an elevation of 780 feet. Following World War II, jurisdiction was transferred to the Federal Aviation Administration. Up to 1961, airway observations consisted primarily of ceiling and visibility readings, . and wind measurements. However, except for a few brief

....

^~.,

...

-l'<>r.---~..._,.,

... --.... --.,-,.,_·_· _____ ._.. ___ ..__ ....

~-"---·_...---·---,..

_..

.--~

... - ... ....,_,,.._,.., ___

.,~....,....--~

_...

~.

~---··-·---,---

!periods when Van Nuys' observations were transmitted

I

!independently on an airways teletypewriter circuit, avia-

l ~

1tion weather conditions at Burbank Airport were regarded

I

ias being applicable to Van Nuys also. After 1961, follow-

;

_I

/ing the installation of a ceilometer and runway temperature)

!instrument, Van Nuys

l

l

weather observations were transmitted

!independently of the Burbank report.

I

The anemometer was atop the control tower, 53 feet

!above the ground, until the late 1950s when i t was placed

I

l

!20 feet above ground, adjacent to the runway.

I

The various weather stations used in this report in

i '

! iand around San Fernando Valley are shown in Figure 5.

i

!

!area of interest is contained in a northeast-southwest

!

I

!corridor, 30 to 45 miles wide and 60 miles long in Los The

;Angeles County. i It is bordered by Lancaster to the north,

f

I Los Angeles International Airport to the south, Pasadena to the east, and the Ventura County line (including a portion of the coast) to the west.

f

24

LOS ANGELES COUNTY

& 20Miles

~~~--~~--~--~ ¹⁰

15

_.!~~- ^I

< ...

.A: •. ..-··----

Tem<Jcraturl! :md

Pracipitation ~

... -~

r

Figure 5. Namesand Elevationsof Weather Stations Used: JYalley stations are inside dashed line.)

3 . TEMPERATURE :3.1 Background

~The transitional nature of the Valley climate (Sec- tion 1.4) can best be seen in its temperature pattern.(0) Figures 6 and 7 represent average winter and summer tern-

;peratures over a selected portion of Los Angeles County. 'December, January and February represent winter; July, :August and September constitute summer.

The period used for these figures is 1965-1973. In addition to San Fernando, Burbank and Canoga Park, three more temperature stations not found in official publica-

I tions were selected for use: Northridge, Rye Canyon, and :simi Hills. Northridge is on the California State Univer-

sity campus, eight miles southwest of the city of San Fernando. Rye Canyon is the Lockheed-California Company

i i

:research center, 16 airline miles north\vest of San Fernando at 1,260 feet. Simi Hills is the rocket engine testing facility of Rockwell International, three miles due west .of Chatsworth Reservoir and 21 miles southwest of San

Fernando, at 1,910 feet.

X

Wintertime minima are in the upper 40s near the coast,; about 40 in the Valley, and near to slightly below freezing

!in the interior and mountain regions. Maxima are in the 50s inland and approach the mid-60s elsewhere. Summertime . low temperature readings favor the mid-60s near the coast, 'about 60 in the Valley, and range from 55 to 65 inland.

26

0

•

San Fernando Valley,

LOS ANGELES

COUNTY

5 10

CSUN GEOGRAPHY

Figure 6.

15 20Miles

Mean Wintertime Maximum· (left) and Minimum (right) Temperatures

(1965;_1973)

Lancaster

59 30

e

^Palmdale

58 30

0

San Fernando Valley,

LOS ANGELES

COUNTY

5 10

CSUN GEOGRAPHY

Figure 7.

15 20 Miles

Mean Summertime Maximum (left) and Minimum (right) Temperatures

(1965-1973)

28

8 LANCASTER

·gs 60

PALMDALE

• 96: 61

\

Maximum readings are warmest in the high desert (Antelope Valley), in the mid-90s, drop to the low 90s in San

Fernando Valley, mid-80s in the Los Angeles area, and down to the low 70s in the vicinity of the coast. The San

Fernando Valley mean maxima are 15° higher than the Los

;Angeles area, but 10° lower than further inland.

X

;<

Another indication of the transitional character of

lvalley temperatures is the average number of days with

!temperatures 90° or warmer in summer, and of days 32° or i :cooler in winter. Antelope Valley experiences over 100

i

idays over 90°, San Fernando Valley close to 70 days, and

!

! jLos Angeles less than 25 days. Freezing weather, or colder,;

i i

!occurs on 80 days per year in Antelope Valley, just under

l

i30 days in San Fernando Valley, and rarely occurs in Los

; !?-

;Angeles( -i-~ Table 2) .

~ \_/

Within the Valley confines, lowest wintertime readings'

•show some variation. Minima at Burbank (eastern edge) and icanoga Park (western edge} average slightly lower than in

the central Valley. The higher incidence of drainage winds,

!along the northern border reduces the radiational -effects )of nighttime cooling. An early study of wintertime low

!temperature by the Fruit Frost Service of the Weather

i

I

!Bureau (Figure 8), shows that the greatest number of nights: requiring orchard heating to be just west of Canoga Park

(Ewing, 1939). This and other low-lying areas appear to be ,protected from the slight air drainage more characteristic

·of other portions of the Valley. Lowest minima during the

',._~...,_,., ... -,.-... ,,.. __ ,..,.__._.,._,L~•---·~• .. ~~---~----·-•~--.-.~··~~-·-~.o.-_. ..,,,.,.L> . ..,-.. v~•r"'<-<'"<''<''"""-~·,_.-~•.,u-·.m~•~- _.,...,,,,,,~··•...,•>T><••.L--~;· . ..-.~.·.-.-.-~»-·•· ~>-•-<•·•~~••~""''-·-,.,.

---·--·--···--·--· ..

---···---·---·-·--·---·---··--·---,

TABLE 2

AVERAGE NUMBER OF DAYS, 1951-1960, WITH TEMPERATURES > gooF AND < 32oF

- -

,Jan Feb Mar Apr May

_{- - - -}

Jun Jul Aug Sep Oct Nov Dec Total Valley Stations, Days ~ 90°F

Sunland 0

_*

0 2 4 11 25 24 21 9 2 1 99

Canoga Park 0 0

_*

3 4 11 24 23 19 8 1

_*

93

San Fernando 0 0 0 2 3 8 22 20 18 7 1

_*

81

Pacoima Dam 0 0 0 1 2 7 20 15 16 5

_{* *}

66

Van Nuys 0 0 1 2 1 7 15 17 12 3 1 0 59

No. Hollywood 0 l

_*

2 1 2 12 12 14 5 1

_*

50

Burbank 0

_*

0 1 1 3 13 11 13 4 1

_*

47

Outside the Valley {nor~heast to southwest) , Days > 90°F

Lancaster 0 0 0 1 7 17 29 28 22 7

_*

0 111

Pasadena 0

_*

0 1 1 4 14 12 14 5 1

_*

52

Los Angeles 0 * 0 1 * 1 6 4 7 3 1 0 23

Santa Monica 0 0 0 0 0 *

_*

0

_*

* 0 0 *

(Continued on next page)

._....,.. .• ~--·--.,...,,_ .. -·-~---·---.... -~-... ~ ... ...,.._...,.,.,.., __ .. __ ...,.._~. , ... __ ., ·--~-·- ~-,-·R ··-•·-~-....-.-~-"--''_,.. __ ,,_, .... ,4 .... . , . . . ,.. . . . _ . . . _ , , • ...,,..,,..,_,,,__, _ _ _ _ _ _ _ ... --.••"'~ ... ~ . . . , , , , ... '""''-~

w

0

.. ---... ---·---.----· ·-.---.--.-______ ,.. _______________ .. _______ ... , _____ , ___________ .. ________________ --l

TABLE 2 (Continued)

Jan Feb Mar Apr May

_{- - - -}

Jun Jul Aug Sep Oct Nov Dec Total Valley Stations, Days ~ 32° F

Canoga Park 8 7 3

_*

0 0 0 0 0

_*

2 5 25

Van Nuys 7 2 1

_*

0 0 0 0 0 0 1 5 16

No. Hollywood 2 2 1 0 0 0 0 0 0 0

_*

2 7

San Fernando 3 2 1

_*

0 0 0 0 0 0

_*

1 7

Sunland 2 2 1

_*

0 0 0 0 0 0

_*

1 6

Burbank 1

_*

0 0 0 0 0 0 0 0

_{* *}

1

Outside the Valley (northeast to southwest) ^! Days < 32°F

Lancaster 19" 14 8 1 0 0 0

·o

0 1 15 22 80

Pasadena 1

_*

0 0 0 0 0 0 0 0 0

_*

1

Los Angeles 0 0 0 0 0 0 0 0 0 0 0

_{* *}

Santa Monica 0 0 0 0 0 0 0 0 0 0 0 0 0

*less than one day

··~··~~--- ~~,-. ,c,_,_, ••• •"'-·1-·~-~ ~·· ~-··---··~-... ''""'~''••- -·~-_o -·~·-~-·-· .. -,' ·.--F.-'' _,.,. ... - -... ~~·--'<----~-.---.---·-•oo'""' ....,., ,...__,,, ,...,, ^{~-··••-}·O "' .00 ·-··~----·-·•0 __ ,_,_..,.,_._,~-·-... -~ ~'"'• '"'"'"'.'~'- ~--.. ,.._,.~---... ,_,_, '0"~ • ._~,--,_ ,.,,._.,,_,_...,._,

w

I-'

·q

^·\I,

,.

II ' ' ( ' ^{f "··,} ~ '

_l

-f" --- --'

^{'~'2 \: . . ~ l}

',

,,'\~

Figure 8. Average Number of Nights Requiring Orchard Heating

l

w

(\.)

ldecade of 1951-60 include: Burbank 28°, San Fernando 26°,

;

:North Hollywood 24°, Sunland 22°, and Canoga Park 20°. Ain summer, highest daytime temperatures follow a

!predictable pattern. The less hot weather occurs in the

:)...

;southeastern portion where the sea breeze enters the area

• (Section 1.4), and gradually warms towards the west and

/""'.

north. C~)Iighest decadal readings (1951-60) include: North Hollywood 110°, Burbank 111°, Canoga Park 115°, and Sun- ,land 116°.

:3.2 Temperature Trends

At San Fernando, the only Valley weather station with

1a relatively long temperature record (Section 2.2), daily :averages were taken as one-half the sum of each daily maxi-:

! mum and minimum temperature. This format is the one used

since the establishment of the Weather Bureau in 1891. :However, the early San Fernando readings were extracted

from original Southern Pacific Railroad records which ap- peared in an old California publication (State Engineering

;Department of California, 1886) covering the period from '1878 to 1885. In those days, the mean daily temperature

was·calculated by taking one-third the sum of the readings for 0700, 1400, and 2100 hours Pacific Standard Time.

A recent study at California State University at :Northridge compared 16 years of temperatures recorded on :campus using both methods to arrive at monthly mean read- : ings. It was found that the Weather Bureau system (one-

half the sum of the maximum and minimum) tended to be 0.5°

!higher in summer and practically the same for winter :months than the Southern Pacific Railroad method. Since .the listing of mean monthly temperatures under Table 3 :embraces both systems (1878-1885 the old one and the re- 1mainder using the Weather Bureau one), conclusions based 10n these data later in this section should be valid.

To determine possible temperature trends, the data

!under Table 3 have been divided according to the periods

' :discussed in Section 1.5: The "Ranching" includes data .from 1877-1903; "Irrigation" 1929-1951; "Suburban" 1952

to 1974. Time periods selected were winter (December, :January, February and March), summer (June, July, August

!and September), and a yearly average. In Figures 9 through' il2, monthly means for the winter and summer for each of the'

i

!periods are shown with reference to the means for each

;period. Mean temperatures for each season, and the annual, :are shown in Table 4, with changes between periods.

Winters become progressively warmer, but summertime in

~"Irrigationfl tended to be the coolest. Perhaps at the

~eight of fruit and vegetable raising in the 1920s to the :1940s, greater evaporation resulted from heavy irrigation

i

:and led to a slight lowering of average summer maxima. ' Another aspect of temperature trends is the number of 1days reaching 90° or warmer over a given year, and also 32° br colder (Table 5). San Fernando represents the eastern Valley and Canoga Park the western; two decades are used. :summertime at both locations in the 1960s had about five

34