492 MONTHLY WEATHER REVIEW. NOVEMBER, 1900
a temperature of 62O, although the air temperature a t the
time was 94O, a difference of 3 2 O between the air temperature
and the sensible temperature. This is worthy of notice, as i t
is somewhat explanatory of the freedom from prostration from heat sunstroke, for which this section is noted. The
sensible temperature is influenced by the relative humidity,
and the low relative humidity during the warm portions of
theyear isone of the most important factors in freedom from
sunstroke a t Spokane.
The preceding meteorological data are taken from the eighth
annual report of the Board of Health of Spokane, Wash., for
the year ending December 31, 1899.
The above tabular statement gives the average relative
humidity for this place as 66.1 per cent, but during the warmer
months of the year the relative humidity often falls in the
afternoon, about the warmest part of the day, to as low as 10
per cent, sometimes lower. For example, a t the afternoon
Observation (taken in Spokane a t 5 p. m., Pacific time), Au-
gust 16, 17, and 18, 1895, the relative humidity was respec-
tively 8, 7, and 5 per cent, but a t the morning observation
(6 a. m., Pacific time), August 17, 18, and 19, the relative
humidity had risen to 51, 52, and 53 per cent, showing that
the air does not remain long enough dry to he hurtful in some
respects.
Each year, excepting the years 1595 and 1897, the temper-
ature has fallen below zero a t Spokane, but i t is also shown
that during the winters of 1888-89,1894-95,1895-96,1897-98,
and 1899-1900, the temperature did not fall as low as zero a t
this place. The lowest temperature recorded a t Spokane since
the opening of the Weather Bureau office here was 30.5O be-
low zero, January 16, 1888, but it should be borne in mind in
this connection that the winter of 1887-85 was one of great
severity throughout the whole country. The lower tempera-
tures do not prevail for many days a t a time, but have days
with much higher temperature between them.
The prevailing winds are from the southwest, and have a
marked influence in tempering the cold of winter or heat of
summer. The greatest velocity of wind ever recorded a t Spo-
kane was 48miles per hour, once in 1890 and once in 1891,
lasting for about five minutes each time ; this place has re-
markable freedom from violent winds, due in a great measure
to the topography of the surrounding country.
Thunderstorms are rare and seldom if ever of the violent
kind experienced in the Eastern States, many of the thunder-
storms recorded in the following table have been reported for
only a peal or two of distant thunder.
It has been estimated by agricultural experts that from 15
to 20 inches of precipitation per year suffice for the produd-
tion of good crops in the agricultural sections near Spo-
kane. Weather Bureau reports referring to Washington and
Oregon indicate that ‘. Agricultural operations are more fruit-
ful with a small rainfall than in some sections of other States
with considerably larger precipitation.” An examination of
the preceding table which shows the amount of precipitation
for the greater part of the agricultural year indicates that a
sufficiency of precipitation for agricultural needs has always
fallen in this section ; and the same table shows that in gen-
eral the rainfall has been well distributed during the period
critical for agriculture.
The actual atmospheric pressure given in the first part of
the tabular statement should be of interest to physicians and
others from a physiological point of view.
FOG STUDIES ON MOUNT TAMALPAIS.
By ALEXANDER 0. YCADIE, Forecast Official.
I n a previous paper attention was called to the prevalence
of fog on the central coast of California, especially in the
vicinity of the Bay of San Francisco. A few illustrations of
fog drifts as photographed a t the Weather Bureau observatory
on Mount Tamalpais were given in the July issue of the
MONTHLY WEATHER REVIEW. The differences in tempera-
ture, humidity, and air motion are so marked within com-
paratively small distances, both horizontally and vertically,
in tbe bay district, that i t seemed advisable to tabulate in
comparative form the nieteorological elements for a year at
the higher station (elevation approximately half a mile) and
the station a t sea level. The present paper Rims to present,
with some photographic evidence of fog forms and drifts, a
rough study of the air drainage of the locality, in which
fog streams and counter streanis are of such frequent occur-
rence that they serve.excellently as exponents of air motion.
The topography of the section is remarkable, because of the
close juxtaposition of ocean, bay, mountain, and foothill.
A valley, level as a table, 450 miles long and 50 miles wide,
having afternoon temperatures of looo or over, is connected
by a narrow water passage with the Pacific Ocean, the mean
temperature of the water in this locality being 55’3. Thus
within a distance of 50 miles in a horizontal direction there
is frequently a difference of 50° in temperature, while in a
vertical direction there is often a difference of 30° in an ele-
vation of half a mile. High bluffs, ridges, and headlands are
a t such an angle to the prevailing strong westerly surface air
currents that an air stream is forced with increased velocity
through the Golden Gate, and there must of necessity be con-
siderable piling up of both air and water vapor a t this point.
The locality niay indeed be considerecl as a natural laboratory,
in which experiments connected with cloudy condensation of
water vapor are daily wrought, and i t is therefore of more
than passing interest to the meteorologist.
Much faithful work has beeu done in physical laboratories
on the behavior of water vapor a t varying volumes, pressures,
and temperatures. Regnault, Thomson, Broch, Aitken, Kiess-
ling, R. von Helmholtz, Hertz, Rayleigh, von Bezold, Barus,
Marvin, and others have worked upon the change of state from
vapor to liquid and from liquid to solid, and while many
irregularities are noted in the behavior of water vapor, the
general problems of decreasing volumes and increasing press-
ures until condensation points are reached, have been solved ;
and i t is well understood that the vapor-liquid and liquid-
solid condensations are in themselves but two phases in a
chain of condensation phenomena. The problem of fog is
therefore a limited one. It may be considered as a special
case of cloud development, occurring in the first and second
stages of Hertz, viz, the unsaturated and saturated stages.
Condensation in the free air, as in these fog formations,
takes place under conditions different from those obtain-
ing in the laboratory. There are no fixed restraining walls,
though the strongly stratified outlines suggest sharply limited
air streams. Again saturation as i t occurs in free, constantly
changing air and true adiabatic saturation are not identical.
Saturation in the free air must be studied under disadvan-
tageous circumstances, for the work must be done a t a distance,
with instruments neither sufficiently delicate nor accurate,
and there is no control of conditions possible. In passing, i t
may be noted that, except for traces of salt, the air of the sec-
tion under consideration is partially filtered, as it presum-
ably comes from of-f the broad ocean and is as free from land
dust and smoke as normal air can be. Off-shore winds are
infrequent and light.
An attempt has been made a t the Mount Tamalpais station
to correlate the surface pressure conditions with fog. There
are, however, many different types of fog. The conditions prevailing in winter, when tule fog, formed in the great val-
leys, drifts slowly seaward, are very different from those pre-
vailiug in summer, when the sea fog is carried inland. A
typical pressure distribution accompanying sea fogs has been
NOKEMBER, 1900. MONTHLY WEATHER REVIEW. 493
recognized. I n general, a movement southward along the
coast of an area of high pressure in summer means fresh
northerly winds and high temperatures in the interior of the
State, with brisk, westerly winds, laden with fog, on the coast.
A kite meteorograph a t the station has been used frequently
i n the following way: A descent from the station to sea level
can be made by the train, a distance of 8 miles, in fifty
minutes. The meteorograph was attached near the top of an
open canopied car, insuring a good circulation, and carried in
this way a number of times through the fog. We make in
this way a rough cross section of the fog.
I n fig. 1, Plate I, is shown perhaps the most common type
of fog. It may be of interest to compute roughly the weight
of water vapor existing under such conditions. From
a number of records, a fair average dew-point temperature is
51° F. (10.6O C.). It is estimated that an area 10 miles east
and west and an equal distance north and south is covered
with fog. The upper level of the fog may be taken as half a
mile. If the fog were solidly packed, we could not lie much
in error if we estimated its bulk a t 50 cubic miles.
There are, therefore, 5280’ x 50 cubic feet of water vapor a t
a mean temperature of 51° F. A cubic foot of vapor a t this
temperature weighs 4.222 grains, and we therefore have as a
gross weight 2,219,535 tons of 2,000 pounds each. But most
generally the fog disappears between sea level and 1,200 to 1,500
feet altitude, and there are also wide swaths or channels fog
free. The amount given above, therefore, would need to be cut
in two, and a liberal estimate of the weight of the water vapor
in a fog outside the Head3 is 1,000,000 tons. This is carried
through the Golden Gate by westerly winds, blowing 22 miles
per hour, from 1 to 5 p. m.
For each square mile of surface there would be about 10,000
tons of water vapor and for each acre about 15.63 tons. This
is equivalent to a rainfall of 0.14 inch.
I n Waldo’s Modern Meteorology’ an example in the use of
Hertz’s graphical tahles for following the chauges in a given
quantity of water vapor under varying conditions is given.
With little change, the problem will apply in this case.
At San Francisco the mean actual pressure is 29.87 inches
(758.7 mm.) and a t Tamalpais 27.55 inches (699.8 mm.);
the elevation of the latter station is 721 meters, and the
former is practically a t sea level.
With a pressure of 750 nim. and a temperature of 2 7 O C.
(80. P.), a given mass of air, half saturated, lifted upward
under adiabatic conditions, will not change its initial 11
grams of water contents per kilogram, until a t an elevation
of 640 meters, when condensation would begin. At an eleva-
tion of 700 meters, the pressure being 687 mm., the tempera-
ture would be 19.3‘ C. (67O F.).
At 640 meters the dew-point would be 13.3c (56O F.) or 2.5O
lower t h m the initial dew-point 15.S0 (GOo F.), the difference
being due to the increased volume. At 1,000 meters the tern-
perature would be 8.2O C. (49O F.), or a t a rate of 0.51O C.
decrease per 100 meters elevation.
It is pointed out, however, that in all theoretical values the
assumption is made that the kilogram of mixed air and water
vapor retains its mass unchanged, but this can not be the case
with a mixture in free air performing a journey of any ex-
tent. It is a100 to be remembered that in the actual case
before us the horizontal movements of the given mass would
be of far more significance than the vertical movements.
I n von Bezold’s third paper on the Thermodynamics of the
Atmosphere (see Mechanics of the Atmosphere, pp. 257-288),
the effect of mixing different air masses is considered. If two
masses of saturated air a t Oo C. and 20° C., respectively, and
’Page 236. The paper i n full is translated in Professor Abbe’s
[Im- roved me’thods are given by Professor Bigelow in his Report on the Mechanics of t h e Earth’s Atmosphere, No. XIV, p p 198-211.
fnternational Cloud Observations, Washington, 1900.-E~.]
6 7 4
a t 700 mm. pressure are thoroughly mixed, the greatest amount
of rainfall that can occur is 0.75 gram per kilogram of air
and water vapor. The temperature of the mixture will be
llo C. (52O F.). The warmer mixture would have yielded the
sanie amount of rainfall by raising i t 310 meters or cooling it
1.6O C. by elevation and 0.8O C. by contact.
Direct cooling by contact or radiation is shown by von
Bezold to be more efficient as a cause of rainfall than cooling
by mixture, but in the production of fog i t is probable that
cooling by mixture (except in the case of ground fogs) is the
most important factor to be considered. It is to be noted
that reverse pressures should also be studied, for perhaps a
close watch upon the conditions prevailing when fog is rap-
idly dissipating might conversely t,hrow light upon the order
and relative importance of the three ways of cooling, viz,
mixture, expansion, and radiation.
Von Bezold’s deductions may be thus summarized : More
vapor condenses when a stream of air and vapor a t low tem-
perature impinges on a mass of warmer air than with reversed
conditions. Ocean fogs as a rule form when cool air flows
over warm, moist surfaces, but in the case under discussion,
where the ocean surface temperature is 13O C. (55O F.) and
the air temperature may reach 2 7 O C . (SOo F.), i t is evident
that the above does not hold. It is more probable that con-
densation is the result of the sharp teniperature contrasts a t
t,he boundaries of certain air currents having different tem-
peratures, humidities, and velocities, and that the contours
of the land play an important part in originating and direct-
ing these air currents. The suinnier afternoon fogs of the
San Francisco Bay region then are probably due to mixture,
more than radiat,ion or expansion. The winter tule fogs of
the Sacramento and San Joaquin valleys are probably pure
t,ypes of radiation fog, where the process of cloud building is
from the cooled ground upward. Occasionally in summer,
when the warm air has been pumped out of the valleys and
there is rapid radiation, ground fog forms. An illustration
of this is given in fig. 2, Plate I, where fog covers a number of
valleys. Gummer sea fog is shown in fig. 3, Plate XI, and, as
said above, is probably due to mixture. The wave motions or
Luft Wogen of von Helmholtz are shown in fig. 4, Plate 11,
a11d also the surgings or splashings. where a certain condenea-
tion results from the mechanical uplifting.
--t -_
THE WATER SUPPLY FOR THE SEASON OF 1000 AS
DEPENDING ON SNOWFALL.
On November 9 the Chief of Bureau called for reports from the sec-
tion directors for Colorado, Idaho, Montana, New Mexico, Utah, and Wyoming on the water supply of the last season, the value of snow data published in special snow bulletins, and the verification of any predic- tions based upon the experience of past seasons. The replies summarize the work done in the respective sections, and especially the data pub- lished in the special bulletins and the ice and snow charts of the Climate and Crop Division during the early spring of 1900. No better method of presenting this important subject to our readers could be desired than the publication of these excellent reports, which follow herewith. The importance of forecaRting t h e supply available for irrigation was discussed by the Chief of Bureau and others a t the Irrigation Congress held in November in Chicago.-ED.
COLORADO.
By F. €I. BRANDENBURG, Section Director.
The Weather Bureau began the collection of snowfall sta-
tistics in Colorado four years ago, believing that i t would be
possible to forecast accurately the prospective volume of water
in the streams, and that such information would be of mate-
rial advantage to agricultural interests.
In the early days, when agricultural operations were neces-
sarily limited, the flow during summer was of comparatively
little moment, but with the increase of population agriculture
t
I
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FIG. 1.-Morning fog over valleys. View from U. S. eath her Bureau Observatory, Mount Tamalpais.
L
I
FIQ. 2.-Lifted fog. Height above ground about 500 meters. View from U. S. Weather Bureau Observatory, Mount Tamalpais.
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FIQ. 3.-Sea fog pouring over Sausalito Hills ana tnrougn Golden Gate.
E
FIG. 4.-Fog waves. View from U. S. Weather Bureau Observatory, Mount Tamalpais.