# G. Moxon's Wind Data for Fallbrook, CA

## T. Chester and G. Moxon

G. Moxon uses an automated weather station to accumulate Fallbrook weather statistics at his location in north Fallbrook every 30 minutes since 1993. The figures below shows the temperature and two measures of wind speed for two typical days:

Note that in order to show the temperature and wind speed on the same scale, the temperature has been divided by 10. Thus the peak temperature on 8 August was 100° F., and on 9 September it was 88° F. (However, note that there is probably about a 6° F. high bias in the peak temperatures, so the true temperatures on those days were probably around 94 and 82° F., respectively.)

The humidity is not shown in order to prevent cluttering the plot, but it is essentially 100% from the time the temperature levels off at night at around 9 p.m. on both nights until the time the temperature begins to significantly increase in the morning.

The two measures of wind speed are the instantaneous wind speed measured in the last 15 seconds, labeled "inst. wind", and the peak wind speed measured in the last half hour previous to the plotted point, labeled "peak wind".

The peak wind speed is the appropriate wind speed to use, for the following reasons:

• Wind is a highly variable quantity, making the average wind speed a less than useful quantity. It doesn't help to know that the average wind speed is 2 mph over the day, if 75% of the day has an average wind speed of 0 mph and 25% of the day has an average wind speed of 8 mph, close to the actual situation in Fallbrook.

• The peak wind speed in a given interval is what will blow your hat off if it is strong enough, or blow your trees down if it is stronger yet. This is just like the sea surface height, where surfers want to know what the typical wave heights are, the peak ocean height, rather than the average sea surface height of zero.

• While we have no measurements of the wind speed measured over intervals of minutes, from personal observation it is probably correct that one would derive nearly the same peak wind speed using intervals of anywhere from ~5 minutes to ~4 hours, during the ~4 hours of peak wind during the day. For example, the wind chimes at T. Chester's house, located on a ridgetop, ring nearly continuously in the afternoon, almost to the point of annoyance. Also, in the afternoon on nearly every day of the year, it is usually impossible to do activities like move compost from one pile to another without it being blown around, or to use a leaf-blower to clear a driveway of leaves. Hence the peak wind speed is probably the best measure of average wind conditions, recognizing that the wind ebbs and flows, and that a significant fraction of every 5 minutes has wind speed of less than the peak wind speed.

• Due to the regularity of the daily wind pattern in Fallbrook, in which there are many hours that are characterized by the same peak wind speed, independent of the measurement interval over which the peak is calculated, over the range of ~5 minutes (possibly less) to 4 hour intervals. Hence you will experience that peak wind speed if you are outside for more than ~5 minutes during the 4-6 hour interval where the wind has "come up". (See below for more on this.)

Fallbrook has an extremely regular daily wind, temperature and humidity pattern, and these plots are quite typical of most days except for Santa Ana or storm conditions. The wind is nonexistent from about midnight until the sun "burns away" the marine layer, which can vary from 6 a.m. on the hot day of 8 August 1998 to as late as the 11:30 a.m. on the cooler day of 9 September 1998. (Thus in fact these plots show the extremes of a typical day in Fallbrook.)

When the temperature in Fallbrook begins to increase, a temperature differential is created between the land and the ocean, which begins to create a sea breeze. The humidity drops from 100% when the temperature increases. The sea breeze becomes stronger as the land temperature increases, and very quickly becomes strong enough that it brings in cooler ocean air and prevents the temperature from rising further. Except on the hottest days, the sea breeze brings in cooler air and the temperature drops in the afternoon much more quickly than typical in locations farther from the ocean. As the temperature drops, the wind speed decreases. Eventually the wind speed drops to nearly zero as the marine layer re-establishes itself over Fallbrook and the humidity becomes 100%.

The actual wind speed during the peak sea breeze varies strongly with location, with ridgetop locations receiving a strong breeze and with valley locations sheltered from the wind. This was most dramatically shown to me one early afternoon when I walked up Camino Corto. At the middle of the street, about 100' below the ridgetop to the north, there was essentially no wind. As I walked to the top of the street, gaining only 50' of elevation, I could no longer keep my hat on my head, with a stiff breeze constantly blowing. As I walked back down the street, the wind vanished again. The transition seemed fairly sharp on that day, which was probably not an unusual day. Any hiker knows the saddles and ridges are nearly always associated with a high wind, and just below the ridgeline is a much less windy place to eat lunch.

Thus please remember that whereas the general behavior of the wind speed as measured by Gaylord Moxon is undoubtedly the pattern in most of the Fallbrook area, the specific values are correct only at his location. Your mileage may vary.

From the above, it is clear that in order to study the variation of wind speed over the year, the appropriate variable is the maximum wind speed for each day.

Those individual values for the maximum wind speed for each day are plotted below, followed by a boring plot of the median wind speed computed from a 15-day running median average, 7 days before a given date and 7 days after, using all recorded years. (It is boring due to our weather patterns - the plot shows the median wind speed is nearly constant throughout the year!)

1.5 years are plotted to easily see any seasonal variation.

 Year Color 1993 Orange 1994 Yellow 1995 Red 1996 Green 1997 Blue

(Click on graph for bigger and better image.)

Again note that the above graph plots the median of the daily peak wind speeds, which is different from the median wind speed of a given day, which would be much closer to zero mph.

We don't know of any reference source that could be used to validate these wind measurements, but they seem reasonable. The Beaufort wind scale #3 is a "gentle breeze", 8-12 mph, in which "Leaves and twigs [are] in constant motion". That seems to describe the general afternoon situation at T. Chester's house, in which the leaves on the driveway are nearly always in constant motion then, which is one validation of Moxon's wind speeds. If any reader can provide further validation of these measurements, please email one of us.

## Summary of Fallbrook's Wind Variation

The median peak wind speed is 8-11 mph on every day of the year, as measured at G. Moxon's house on Via Alegre. Wind speeds measured at other locations may differ.

Individual days with high winds occur most often in the winter. Days with peak wind speeds of 15 mph or greater essentially only occur from November through April, although they do occur more rarely outside that interval.

We thank Brian McPartland for a good question that stimulated us to add a much more complete explanation of the wind speed in Fallbrook, and Jane Strong for providing the sea breeze weblink.

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