January

February

March

April

May
June

July

AVERAGE MONTHLY RELATIVE HUMIDITY (PER CENT) FOR TWELVE YEARS.

70

37 17

81 60 48

864 796 760 516 334 3, 195.7

CLIMATE OF SANTA CLARA
CLARA VALLEY.

The Santa Clara Valley lies between the Santa Cruz Mountains on the west and the foothills of the Coast Range on the east. In some respects it is an ideal valley in which to study the general movements of the air and the modifications of the same due to topography. We are fortunately able to discuss the principal climatic factors for the following stations: Menlo Park, San Mateo, San Jose, and the Lick Observatory at Mount Hamilton. Mount Tamalpais and San Francisco should be added to the list for a proper understanding of the air movement through the valley. As has been stated before, the prevailing westerly winds are strongly intensified at the Golden Gate, and the whole effect of the topography is to force the air down the valley. Some interesting relations showing the effect of topography upon rainfall become apparent. The mountain stations show a much larger rainfall. On Tamalpais the rainfall exceeds that of San Francisco in the ratio of 3 to 2, and at Mount Hamilton the rainfall exceeds that of San Jose in the ratio of 2 to 1. The gradual increase in rainfall even in so short a distance as 50 miles is also apparent. The mean annual rainfall at San Jose is 14.88 inches, at Menlo Park 16.43 inches, at San Mateo 20.71 inches, and at San Francisco 23 inches. In other words, within a distance of 50 miles, from San Jose to San Francisco, we find an increase of nearly 8 inches in rainfall, and this increase is nearly in proportion to the distance from San Francisco.

The following comparative data for the six stations for a period of three years serve excellently to show the general features of the climate of the Santa Clara Valley:

MEAN MONTHLY AND ANNUAL TEMPERATURE (DEGREES FAHRENHEIT).

1898.

Mount Tamalpais

San Francisco

Lick Observatory.

San Jose..

Menlo Park

San Mateo..

1899.

Mount Tamalpais
San Francisco....
Lick Observatory.
San Jose....
Menlo Park
San Mateo

1900.

Mount Tamalpais

San Francisco

Lick Observatory.

San Jose.

Menlo Park

San Mateo..

Jan. Feb. Mar. Apr.

May. June. July. Aug. Sept. Oct. Nov. Dec. Mean.

40.8

46.7

52.6

51.2

35.7

42.8

39.1

46.4 46.6 56.0 50.5 61.9 70.0 67.5 62.6 59.4 51.2 45.7
54.5
52.6 59.0 56.0 58.0 59.0 61.2
55.4 49.7
50.8 47.9 62.0 71.4 70.5 61.8 56.0 46.5 43.3
57.7 57.0 65.3 65.1 65.2 63.1 61.1 55.0 46.3
59.1 58.7 66.7 69.0 66.6 65.1 ! 60.5 52.4 46.0
61.3 61.5 70.2
67.8 71.8 64.9 62.8 54.3 47.6

45.4 53.3 51.8
44.9 52.6 49.9
45.9 52.6 54.7

47.8 47.6 44.6 51.6
53.0 51.6 52.2 54.6
42.5 41.0 39.6 47.6
56.4 52.2 54.7 59.1
47.8 50.0 53.5
57.4
49.0 52.8 56.4

51.6

51.2. 66.8 71.0 61.4 73.2 55.3 49.4 47.7
52.6 56.9 55.9 58.3 58.2 59.3 56.8 49.6
46.3 63.7 71.6 61.3 69.8 50.2 46.9 45.6
58.4 67.8 67.4 65.5 66.0 60.4
50.4
58.4 66.0 65.9 65.8 63.3 59.2 56.0 48.6
58.0 68.3 67.7 67.4 64.9 61.0 58.8 49.3

57.6

47.4

48.4

50.7

47.5

52.2 48.6 55.4 62.2 69.8 64.0 60.4 55.2 53.6 48.8
53.6 55.2
54.0 57.0 57.6 58.2 59.7 63.3 58.9 56.3 50.2
43.0 48.1 43.0 52.9 63.8
71.6 62.1 56.3 51.6 51.1
47.2
51.3 53.5 56.4
58.8
64.8 67.8
69.8
70.0 68.4 63.4 58.2 50.6
51.4 52.6
62.4
66.5 67.3 67.3 65.6 59.4 57.4 50.2
52.2 52.0
70.0 67.7 66.1 60.7 59.4 48.6

57.6

55.9

58.3

59.2 ļ 65.8 66.9

111

54.9 54.7 52.3 57.2 57.6 59.6

55.6 54.9 52.2

59.7 57.7 58.8

55.5 56.2 53.2

61.1

59.5 60.6

1898.

Mount Tamalpais.

San Francisco.

Lick Observatory..

San Jose.

San Mateo..

Menlo Park

1899.

Mount Tamalpais

San Francisco..

Lick Observatory.

San Jose

San Mateo..

Menlo Park

1900.

Mount Tamalpais

San Francisco

Lick Observatory.

San Jose...

San Mateo.

Menlo Park

MONTHLY AND ANNUAL PRECIPITATION (INCHES AND HUNDREDTHS).

Jan. Feb. Mar. Apr. May. June. July. Aug. Sept. Oct. Nov. Dec. Annual.

The mean annual temperature for three years at Mount Tamalpais is 55.3, and for San Francisco for the same period is 55.3°, which is practically the temperature of the Pacific Ocean near the Golden Gate. The mean annual temperature for San Jose for a period of twenty-six years is 58°, and for San Francisco 56. The mean annual temperature on Mount Hamilton is 52. At both of the mountain stations there is a well-marked seasonal curve of temperature. Comparing the mean annual temperatures it would appear that from sea level to 1,000 meters elevation the fall in temperature is 1.5° C. In January the mean temperatures are, from sea level up, 10.1, 7.4°, and 5.5° C., or there is a vertical gradient of 1° fall for 260 meters. But in July this gradient is inverted, and the temperatures run from sea level upward as follows: 13.7°, 21.3°, and 22° C., or at the rate of 1° rise for 44 meters. These temperature inversions are due to the water vapor carried by the winds through the Gate from the Pacific and down the valleys. Under normal conditions ascending air cools at the rate of 1°C. for 100 meters of ascent. But our experiments at Mount Tamalpais show that often the temperature contrasts are more marked than the general averages quoted above would indicate. Sea-level temperatures (12 or 13° C.) and fog often prevail to the 50-meter level, while at 700 meters clear weather with temperatures of 25 or 26° C. prevail. For example, on July 15, 1900, at 5 p. m., the temperature at San Francisco was 11° C. (52° F.), at Point Reyes 13° C. (55° F.), and at Mount Tamalpais 27 C. (80° F.). It is also interesting to note that the temperatures at Red Bluff, Sacramento, and Fresno were respectively 39° C. (102° F.), 33° C. (92° F.), and 38° C. (100° F.). The mean of three temperature values at different points in fog gave a temperature of 10° C. or 50° F. as the temperature of condensation or the dew-point. The maximum weight of the water vapor per cubic foot at this temperature is 4.076 grains.

MENLO PARK.

Menlo Park is situated about 25 miles in an air line south of San Francisco, in latitude 37° 27' north, longitude 122° 11' west. To the east lies the southern portion of San Francisco Bay, about 4 miles wide. The average elevation is between 50 and 60 feet. About 3 miles to the west the land rises, varying in elevation from 100 to 500 feet. To the southwest, at a distance of about 8 miles, the Montara Mountains attain heights of from 1,000 to 2,000 feet.

The mean annual temperature, based upon records covering a period of twenty-three years, is 57.7°, which is practically the same temperature as that of San Jose and about 2 warmer than that of San Francisco. The coldest month is January, with a mean temperature of 47.3°, and the warmest is July, with a mean temperature of 67.7°. The highest temperature recorded is 1060, in June, 1891, and the lowest 20°, in December, 1879, and January, 1888. Reliable frost data are not available.

1878. 1879.

1880.

1881.

1882.

1883.

1884.

1885.

1886.

1887.

1888.

1889.

1890.

1891.

1892.

1893.

1894.

1895.

1896.

1897.

1898.

1899.

1900.

The mean annual rainfall, based upon records covering twenty-three years, is 16.43 inches, or nearly 2 inches more than at San Jose and 7 inches less than at San Francisco. With the single exception of a heavy rainfall in 1886, no rain has fallen in July. December is the month of heaviest rainfall, and more than half of the annual rainfall occurs in the months of December, January, February, and March. During the past twenty-three years there have been but two years when the rainfall did not exceed 10 inches. In 1878 and 1898 the annual rainfall was but slightly above 9 inches. There have been five years when the annual rainfall exceeded 20 inches. In 1889 the total rainfall for the year was 26.90 inches.

MONTHLY AND ANNUAL MEAN TEMPERATURE (DEGREES FAHRENHEIT).

1878..

1879.

1880.

1881.

1882.

1883..

1884.

1885. 1886.

1887.

1888.

1889.

1890.

1891.

1892.

1893.

1894.

1895.

1896.

1897.

1898.

1899.

1900..

Year.

Mean (23 years).

Average (23 years)

Jan. Feb. Mar.

Apr. May.

46.3 49.3 55.4
44.8 52.5 55.0
44.1 44.7 48.9
49.3 53.1 53.9
52.1

46.6

15.4

43.4

45.8

46.4

48.1

47.8 51.6

47.8

52.3

46.4

47.1

45.8

52.5

47.5

51.2

59.2 64.9 67.6 67.0 64.2 61.9 58.2 51.6 45.8
59.9 62.5
71.9 67.6 69.2 65.0 59.1 50.5 45.8
55.3 64.2 66.1 67.0 66.3. 63.8 57.8 49.3 50.3
61.0 64.2 67.3 69.7 66.8 63.2 56.0 50.3 49.0
55.7 65.5 66.6 69.0 67.7 63.3 58.8 50.7 49.4
54.1 55.1 62.6
67.5 65.4 64.4 65.9 55.8 49.8 46.5
T
53.2 57.2 65.6 65.3 69.3 66.0 59.6 56.5 53.3 48.3
55.3
58.5 62.4 63.4 68.0 66.3
64.4 58.6 54.2 49.2
50.1 54.9 61.7
65.5
66.5 65.4 61.1
55.5 48.7 50.5
55.8 55.4 61.0
66.5 64.2 63.6 64.6 60.8 53.6 48.3
51.4 59.0 60.4 67.0 70.2
69.5 66.7 61.7 55.2 53.1
56.3 59.1 61.9 66.4 66.0 67.9 66.3 61.3 55.8 50.3
i
53.7 57.0 62.8 63.7 66.9 66.3 64.9 58.9 54.6 48.6
55.4 56.1
61.1 67.3 67.6 68.2 64.8 60.91 56.1 48.3
52.9 56.5 55.3
64.8 66.5 67.9 68.0 65.4 59.6
54.6
47.5 49.1
51.5 52.8 57.2 63.1 66.2 66.9 61.1 58.1 54.4
47.7 49.9 58.6 59.7
63.1 67.8
67.3 66.4 61.2 55.5
52.3 53.0 56.6 60.4 66.6 64.8 66.8 63.8 61.9 55.0
54.7 55.5 54.8 60.5 67.5 71.6 67.5: 64.4 60.5
50.4 50.4
60.0 65.0 68.0 68.6 65.5 64.3 60.2
44.9 52.6 49.9 59.1 58.7 66.7 69.0 66.6 65.1
60.5
47.8 50.0 53.5 57.4 58.41 66.0 65.9 65.8 63.3
51.4 52.6 57.6 55.9 62.4
66.5 67.3 67.3 65.6
47.3 50.2 53.4 57.1 62.1 66.3 67.7 66.7

45.4

47.8

48.7

51.4

50.3

51.4

50.9

47.0

49.0

47.5

48.2

52.7

52.1

51.5

47.5

51.9

48.0

52.4 46.0 59.2 56.0 48.6 59.4 57.4 50.2 59.2 53.2 49.0

64.1

MONTHLY AND ANNUAL PRECIPITATION (INCHES AND HUNDREDTHS).

0.65

2.38

3.35

1.89

4.97

0.72 4.92

3.17

1.36

0.65

2.98 2.21 1.72 1.36 0.10 0.00 0.00 T.
0.00 0.00 0.42 0.25
3.09 2.73 4.27 1.19 0.98 0.03 0.00 0.00 0.00 0.48 1.66 3.96
1.92
1.79 | 1.65 6.44 0.69 0.00 0.00 0.00 0.00 0.00 0.59 8.93
3.70 1.56 0.68 2.66 0.00 0.24 0.00 0.00 0.00 0.36 0.71 1.97
1.17 3.71 0.67 0.18 0.00 0.00 0.00 0.23
1.25 1.69 0.52
0.52 2.70 0.76 2.49 0.00 0.00 0.00 0.20 0.73 0.28 0.85
4.07 4.80 3.40 0.00 3.16 0.00 0.05 0.04 1.86 0.27 4.92
C. 12 0.50 1.98 0.04 0.00 0.00 0.00 0.02 0.09 6.22 2.17
0.37
1.65
3.34 0.08 0.00 0.24 0.00 0.00 0.86 0.40 1.26
0.46 1.18 0.01 0.00
0.00 0.00 0.22 0.00 0.85 2.16
2.31 0.02 0.37 0.09 0.00 0.00 0.98 0.00 3.72 2.59
0.54 5.75 0.69 1.08 0.00 0.00 0.00 0.00 4.96 2.38 10.85
7.45 3.27 2.76 0.51
1.48 0.00 0.00 0.00 0.18 0.00 0.00 2.61
0.69 7.02 2.17 1.83 0.38 0.03 0.00 0.00 0.28 0.00 0.46 5.26
1.07 1.39 2.91 0.47 1.43 0.00 0.00 0.00 0.00 1.14 4.69 6.55
2.75 4.33 1.26 0.23 0.00 0.00 0.00 0.09 0.09 1.51 1.90
2.80 0.57 0.00 0.99 0.01 0.00 0.00 1.65 1.51 0.46
1.59 2.30 1.44 0.36 0.00 0.00 0.00 0.00 1.16 1.45 1.13
6.76 0.00 2.15 3.65 0.45 0.00 0.00 0.85 0.53 1.13 4.67 3.14
1.69 3.92 4.20 0.13 0.00 0.00 0.00 0.00 0.00 1.91
1.38 2.24 0.58 0.15 0.63 0.00. 0.00 0.00 2.00 0.73
3.62 0.42
6.67 0.34 0.07 0.05 0.00 0.05 0.00 3.08
0.52 1.46 1.00 0.66

2.44

4.60

9.65

7.12

0.76 1.63
0.31 0.99
3.22
2.00 4.61
1.01 1.80

1.87

0.05

1.99

3.69
3.04

0.00 0.00 0.00
0.01 0.04 0.28

2.06

2.62

1.50

0.55

0.16

3.35

June. July. Aug. Sept. Oct. Nov. Dec. Annual.

57.6 58.6

56.5 58.6

57.6

56.4

57.4

58.3

56.7

57.3

59.4

59.2

57.6 58.8

59.4

56.6

57.8

58.1

59.7

50.3

57.6

57.7 59.5 57.7

9.04 18.39 22.01 11.88 10.07 10.91 25.92 13.03 13.17 10.52 14.61 26.90 18.26 18.12 19.65

14.60

22.24

16.55 23.33

14.24

9.01

19.39

15.98

16.43