Measurements of Fog Water Deposition on the California Central Coast

doi:10.4236/acs.2012.24047

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Atmospheric and Climate Sciences, 2012, 2, 525-531

http://dx.doi.org/10.4236/acs.2012.24047 Published Online October 2012 (http://www.SciRP.org/journal/acs)

Measurements of Fog Water Deposition on the California

Central Coast

Cyrus Hiatt1,2, Daniel Fernandez1, Christopher Potter2

1California State University Monterey Bay, Seaside, USA

2NASA Ames Research Center, Mountain View, USA

Email: [email protected]

Received May 3, 2012; revised June 6, 2012; accepted June 16, 2012

ABSTRACT

Fog deposition is a notable component of the water budget of herbaceous-shrub ecosystems on the central and southern

coastal regions of California. This paper presents an analysis of fog water deposition rates and meteorological controls

in Big Sur, California. Mesh-screen fog collectors were installed the Brazil Ranch weather station sites to measure fog

water during the summer seasons of 2010 and 2011. Fog deposition occurred during 73% of days recorded in 2010 and

87% of days recorded in 2011. The daily average deposition rate was 2.29 L/m2 in 2010 and 3.86 L/m2 in 2011. The

meteorological variables which had the greatest influence on prediction of fog deposition were wind speed, wind direc-

tion, and the dew-point depression (difference between air temperature and dew point). Based on these results, we hy-

pothesize that high rates of summer fog deposition help sustain the productivity of California coastal vegetation through

periods of low rainfall.

Keywords: Fog; Deposition; California; Water Budget

1. Introduction

The presence of fog on the California Central Coast has

been linked to numerous important bio-physical pro-

cesses, including moderation of surface temperatures and

increase of relative humidity [1-3]. With respect to vege-

tation interactions, coast redwood [Sequoia sempervirens

(D. Don) Endl.] has been studied for effects of cloud

moisture within the canopy, which may reduce the at-

mospheric water vapor pressure deficit and cause tran-

spiration and movement of vascular fluid to slow mark-

edly [4,5]. Under certain wind conditions, coastal shrub

foliage may trap advected fog water and drip moisture

into the surrounding soil below to mediate losses due to

evapotranspiration [4,6,7].

The physical processes leading to coastal fog forma-

tion on the Pacific coast have been studied for decades.

Warm surface air blowing over cold upwelling ocean

water near the California coast is cooled to create a sur-

face-based inversion. Petterssen [8] reported that radia-

tive cooling of the fog layer, together with heating from

the sea surface, initiates mixing and lifting of the marine

inversion. The well-mixed marine boundary layer is

topped by such an inversion at a height of 100 - 600 m

[9]. Subsidence acts to strengthen the inversion above the

stratus cloud top and forces lowering of clouds [10], which

can move ashore with sustained winds to generate fog

over land.

Despite a strong understanding of fog formation pro-

cesses along the Pacific coast, little is known about the

amounts of water that can be deposited on ocean-facing

ridges of the coastal California at different times of the

year. This paper presents an analysis of fog water deposi-

tion rates and meteorological controls on shrub- and

grass-covered slopes of Big Sur, California. Coastal shrub

cover on ocean-facing ridges in this region have a high

degree of biological diversity and endemism, and provide

critical habitat for a large number of rare, endangered,

and threatened animal and plant species [11]. The water

budgets of mixed herbaceous-shrub communities are of

interest because they dominate the central and southern

coastal regions of California.

A number of other studies have examined the rela-

tionship between some of these meteorological controls

and the prediction of fog. For example, one study in

Monterey worked to predict west coast fog by applying

large-scale synoptic weather events with inversion-based

statistics [12]. Others have attempted to apply large-scale

numerical models to the prediction of fog on a meso- to

large-scale basis [13] or at a single site for the purpose of

prediction of fog at an airport [14]. Other, more regional,

studies have examined the direct relationship between

relative humidity, temperature and wind patterns in rela-

tion to fog water collection [15]. While a significant

number of studies have sought to predict or forecast the

C. HIATT ET AL.

526

occurence of fog based on apriori conditions, relatively

few have examined the relationships between fog and

existing meteorological conditions with the intention to

determine valid proxies for fog deposition water to the

land surface. The objective of this study was to under-

stand and quantify the meteorological controls, namely

temperature, dew point, wind speed, direction, and rela-

tive humidity, on fog water deposition rates for the

coastal grassland and shrublands of central California

using measurements collected over two field seasons.

2. Study Site

The primary research site is located at the Brazil Ranch

(center coordinates: latitude 36.35˚N, longitude 121.88˚

W) near Big Sur, California (Figure 1). The Brazil

Ranch is named after Tony and Margaret Brazil and the

pioneer family that worked to establish the land as a farm,

ranch, as well as a dairy operation in the early 20th cen-

tury. Today, the property serves as a primary research

site for the US Forest Service to monitor and manage

vegetation, wildlife, water quality, and sensitive coastal

habitats.

Drier, southeast-facing slopes share a relatively equal

distribution of coyote bush (Baccharis pilularis) and

California coffeeberry (Rhamnus californica) along with

some California sagebrush (Artemisia californica) [16].

The coastal scrub community is usually a successional

plant community that, in the absence of fire, gradually

moves into herbaceous cover where the soil depth transi-

tions from the shallowest to intermediate depth. The

herbaceous plant community includes California annual

grassland series and California oatgrass series. Coastal

sage scrub and chaparral are known as secondary pioneer

plant in California grasslands, which invade grassland

and increase in the absence of fire or grazing. We noted a

propagation of the introduced Cape ivy (Delairea odo-

rata) during our field work. Cape ivy, a vine native to

South Africa, has become a significant threat to coastal scrub.

3. Data and Methods

Two Campbell weather stations, each equipped with a

Figure 1. Study site.

CR800 data logger, a 03001 R.M. Young Wind Sentry

Set, and an HMP45C Temperature and Relative Humi-

dity Probe were installed at the Brazil Ranch in 2007

(Figure 2). The on-going hourly meteorological data

collection included air temperature, humidity, wind

speed and direction, solar irradiance, and precipitation

(as rainfall).

Mesh-screen fog collectors were installed at the Brazil

Ranch weather station sites in June 2010 (Figure 2). The

fog collector design (also called a Standard Fog Collector,

or SFC) is a polypropylene mesh (triangular weave of

flat fiber, 1 mm wide and 0.1 mm thick) fastened to a

metal 1 × 1 meter vertical frame [17] (Figure 3). A metal

trough directly under the frame collected water that

dripped from the mesh and funneled the water flow to a

Figure 2. Fog collector locations.

Figure 3. Standard fog collector.

C. HIATT ET AL. 527

tipping bucket rain gauge. The tipping bucket rain gauge

on the fog collector recorded the amount of water col-

lected over 15.0 minute intervals. Measurements based

on water collected from the mesh screens were removed

from the fog record when they occurred within an hour

of precipitation events recorded on the adjacent Camp-

bell weather station, this helps to minimize the risk of

erroneously interpreting water from rain as collected fog.

Meteorological and fog deposition data were collected at

the fog collection site during 128 days in 2010 and 74

days in 2011. Two of the installed fog collectors were not

included in this study because insufficient fog deposition

was recorded for deposition analysis. Figure 2 shows the

location of the fog collectors and weather stations.

4. Results

The maximum wind speed during the 2010 fog collection

period was 9.42 m/s with an average daily maximum of

5.38 m/s and an average wind speed of 3.13 m/s. During

the 2011 fog collection period the maximum wind speed

was 12.54 m/s with an average daily maximum of 4.32

m/s and an average wind speed of 2.44 m/s (Figure 4).

Wind rose plots in Figure 5 show the diurnal variation of

wind direction and speed over three 8-hour periods. The

prevailing wind direction was generally from the north-

east with winds from the northwest and southwest in-

creasing in frequency and magnitude in the between the

hours of 7:00 and 15:00. A histogram of wind direction

for all hourly records also shows most winds approach-

ing from the north—between 350˚ and 40˚ with winds

directed from the south and north east with less fre-

quency (Figure 6).

The dew-point depression ranged from 0.33˚C to

38.80˚C with an average of 4.44˚C. The distribution of

dew-point depression values is skewed sharply to the

right. Thirty-nine percent of hourly records had a dew-

point depression below 1˚C (Figure 7).

Fog deposition occurred during 73% of days recorded

in 2010 and 87% of days recorded in 2011. Average

hourly fog deposition in 2010 was higher than 2011 dur-

ing the early half of the summer, but lower during the

latter half. In 2010 the month with the highest average

hourly fog deposition was June at 0.15 L/m2, but in 2011

the peak occurred a month later at 0.24 L/m2. Figure 8

shows a comparison of the trends in monthly fog deposi-

tion. However, only 3 days in June and 10 days in Sep-

tember are compared in this chart because the 2011 data

was collected from June 28 through September 10.

In 2010, the total amount of fog deposition was 292.77

L/m2 with a daily average of 2.29 L/m2 and a daily

maximum of 13.62 L/m2. 2010 had an hourly average of

0.1 L/m2 and an hourly maximum of 2.31 L/m2. In 2011

the total fog deposition was 285.66 L/m2, the daily ave-

rage was 3.86 L/m2, and the daily maximum was 17.75

Figure 4. Wind speed during the 2010 and 2011 fog collec-

tion periods.

Figure 5. Diurnal wind direction during the 2010 and 2011

fog collection periods.

Figure 6. Wind direction during the 2010 and 2011 fog col-

lection periods.

Figure 7. Dew-point depression during the 2010 and 2011

fog collection periods.

C. HIATT ET AL.

528

L/m2. In 2011 the hourly average was 0.17 L/m2 with an

hourly maximum of 2.42 L/m2. Figures 9 and 10 show

the hourly values over the 2010 and 2011 fog collection

seasons, respectively.

Most fog deposition occurred during the night and

early morning hours. Average fog deposition was ap-

proximately 0.16 L/m2 between the hours of 20:00 and

2:00 increasing to a peak of about 0.21 L/m2 between

3:00 and 9:00. After 9:00 average fog deposition steeply

declined to nearly 0.0 between the hours of 14:00 and

18:00. The median dew-point depression followed an

opposite diurnal trend indicating fog deposition occurred

when the dew-point depression was low (Figure 11).

Figure 8. Average hourly fog deposition by month.

Figure 9. Hourly fog deposition 2010.

Figure 10. Hourly fog deposition 2011.

The dew-point depression was a strong indicator for

the presence and absence of fog deposition. Table 1

shows a confusion matrix for the prediction of fog depo-

sition when the hourly dew-point depression was at or

below 0.5˚C. Using this threshold, the rate of correctly

predicting fog deposition when fog deposition occurred

(true positive rate) was 76.9%. The rate of predicting no

fog deposition when no fog deposition occurred (true

negative rate) was 94%. The precision of predicting fog

deposition, which represents the ratio of correct positive

predictions to the total number of positive predictions,

was 88.1%. Adjusting for the imbalance between the

number of fog events and no fog events, we used the

geometric mean as an additional measure of performance

[18]. This value was 82.3%.

High wind speeds typically accompanied increased

rates of fog deposition during periods of low dew-point

depression. A time series plot of the 2010 fog collection

period shows a roughly similar trend between changes in

fog deposition and wind speed (Figure 12).

A linear relationship between wind speed and the

quantity of fog deposition was observed in hourly re-

cords with a dew-point depression below 0.38˚C (Figure

13). This relationship was slightly stronger during peri-

ods that not only had a dew-point depression under 38˚C,

but were also subjected to prevailing winds (between

350˚ and 40˚) (Figure 14). The number of fog records

that met these meteorological criteria was, however, lim-

ited. Only 209 records had a dew-point depression under

0.38˚C and of these only 123 occurred during prevailing

winds.

Table 1. Fog deposition confusion matrix based on an

hourly dew-point depr e ssion at or below 0.5˚C.

No Fog

Predicted Fog Predicted True Prediction

Rate

No Fog 2985 144 95.4%

Fog 319 1062 76.9%

Precision 88.1%

Figure 11. Diurnal fog deposition and dew - point de pression.

C. HIATT ET AL. 529

Figure 12. Fog deposition and wind speed time series.

Figure 13. Fog deposition vs. wind speed.

Figure 14. Fog deposition vs. wind speed during prevailing

winds.

5. Discussion

Due to the absence of other fog collection studies on the

California Central Coast, there is no local benchmark to

compare the relative quantity of fog water collected at

the Big Sur site. However, according to Schemenauer

[19], a standard fog collector generally collects between

1 and 10 L/m2/day. Although the daily average 3.01 L/m2

during peak fog months places the Big Sur fog collector

at the lower end of that range, the Big Sur quantities are

likely conservative due to the fact that some fog deposi-

tion was presumably excluded by omitting records con-

taminated by rainfall.

Fog deposition at the Big Sur site was typically found

to occur when the dew-point depression drops below 0.5˚C.

Dew-point depression has long been used as an indicator

of fog formation; however, its reliability can vary by

region and synoptic weather conditions [20]. Although

our study only examined meteorological variables that

influence fog deposition, other studies have drawn simi-

lar conclusions regarding the formation of fog. Grace and

Ferriere [14], for example, demonstrated at a site in

southeast Australia that a dew point depression below 1.0˚C,

predicted the occurrence of fog with a probability be-

tween 60% and 90%.

During periods of slightly lower dew-point depression,

the quantity of fog deposition can be accurately predicted

due to a positive, linear relationship between wind speed

and the amount of fog intercepted by the collector. Wind

speeds below 2.0 m/s show very little correlation with

fog deposition, but the relationship is evident at higher

wind speeds. Estimates of fog deposition are more reli-

able when winds are moving in the prevailing direction.

This may be due to physical characteristics of the pre-

vailing winds, or simply due to the fact that the collector

screen was oriented perpendicular to the prevailing wind

direction allowing the screen to intercept fog droplets

more effectively.

Despite the positive relationship between deposition

and wind speed observed at the Big Sur site, the diurnal

timing of wind speed does not favor fog deposition.

Wind speeds were highest during the late morning—the

same period in which the dew-point depression tends to

increase and fog deposition decrease. Conversely, wind

speeds generally drop in the late evening and remain low

through the morning. Figure 15 shows difference in the

wind speed and fog deposition trends. This decrease in

wind speed coincides with a drop in the dew-point de-

pression and higher rates of fog deposition. Lundquist

and Bourcy made similar observations in analyzing data

from 13 meteorological stations along the California

Coast. They found that observed fog was typically pre-

sent when both air temperature and wind speeds were

low [21].

Figure 15. Diurnal trends in wind speed and fog deposition.

C. HIATT ET AL.

530

Wind speed and direction are frequently cited as drivers

of fog water deposition in fog collection studies [22,23].

This is due simply to the fact that faster and more direct

winds transport more fog droplets horizontally into the

collector mesh. How much fog deposition occurs verti-

cally in the absence of wind is unaddressed by this study.

However, Frumau et al. note that, in many cases, only

small amounts of fog fall vertically to the ground relative

to horizontally deposited fog or rainfall.

Another reason why wind speed is an important com-

ponent of fog deposition is the efficiency of the SFC in-

creases with increasing wind speed. Schemenaur [23]

found that for a screen with a mesh density of 70%, fog

collection efficiency was only 30% at 0.5 m/s, but at 8.0

m/s, collection efficiency increased to just over 60%.

Gains in efficiency, however, decreased with increasing

wind speed. Fog collector efficiency is also dependent on

droplet size. Schemenaur also found that efficiency in

collecting large droplets (15 µ) increased faster with in-

creases in wind speed than did the efficiency of collect-

ing smaller droplets (11 µ). Changes in efficiency at dif-

ferent wind speeds and the dependence of fog collector

efficiency on droplet size may explain some of the re-

siduals observed in our regression model results.

A better understanding of the timing between fog col-

lection and changes in wind speed may also improve our

ability to predict deposition. Because the fog collectors

are in a remote location, we were unable to directly ob-

serve the timing between increases in wind speed and the

collection of fog deposition by the logger. In matching

our fog deposition records to anemometer readings we

observed many instances of elevated wind speed prior to

increases in fog deposition. We suspect that acceleration

of wind speed would more closely match increases in fog

deposition were it not for these apparent lags.

We surmise high rates of summer fog deposition, by

increasing soil water content, may help sustain the pro-

ductivity of Big Sur vegetation through periods of low

rainfall. Joslin et al. [24,25] demonstrated that trees col-

lect fog deposition at approximately the same rate as a

passive string fog collector. These results suggest Cali-

Figure 16. Average monthly precipitation for years 1981-

2010 in Big Sur, CA and average hourly fog deposition for

the 2010 and 2011 fog collection periods.

fornia Coastal shrubs may also function as highly effi-

cient fog collectors, intercepting fog deposition and

channeling the water to their root systems via fog-drip.

On the Point Reyes peninsula, a California coastal area

approximately 200 km north of Big Sur, Ingraham and

Matthews [6] compared water isotopes of soil water be-

fore and after the summer fog season. They not only

found the isotope matching fog drip permeated to the

root zone of conifer trees, but also that fog drip was pre-

sent in the tree cores of conifers. If fog drip is a source of

soil water in Big Sur as it is in Point Reyes, the timing

would likely be advantageous for Big Sur vegetation.

During both the 2010 and 2011 summers, fog deposition

at the Big Sur site was highest in July and August. This

increase in fog deposition coincides with a period of very

low rainfall characteristic of California’s Mediterranean

climate. Figure 16 shows a 30-year average of monthly

rainfall at a NOAA [24] weather station approximately

15.8 km southeast of our study site. This figure also

shows that as rainfall decreases in the summer months,

fog deposition increases, and as the region receives more

rainfall in early autumn, fog deposition declines.

The amount of fog deposition may compose a large

portion of the overall water budget for the region. To

estimate the amount of fog deposition that the ocean-

facing slopes of Brazil Ranch may potentially receive,

we calculated the average slope of the ocean-facing

coastline within Brazil Ranch. At an average slope of

27.01 degrees, a 1 m × 1 m fog collector would represent

2.2 m × 1 m of hillside surface. The daily average of fog

deposition for both seasons was 3.01 L. Assuming that

the 2.2 m2 receives the same amount of deposition as a 1 m ×

1 m SFC, the daily average fog deposition on the hillside

surface is approximately 1.3 cm/day.

This value well exceeds the average rainfall for the

same period which was only 0.2 cm/day.

This study is an initial step towards a better under-

standing of the quantities of fog deposition that occur on

the California Coast, the local weather conditions that

drive fog deposition, and how fog deposition may affect

coastal vegetation productivity. Future analysis will in-

clude data from more fog collector locations to better

characterize fog deposition in the region, as well as

comparisons between fog collector efficiency and the

collection efficiency of California coastal scrub plant

species.

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