T. Chester's Fallbrook, CA Weather Records

Weather is one of those subjects where everyone forgets the extremes sometimes, and where everyone forgets what is "normal" sometimes! For example, Southern California averages one good rainstorm in the summer every year, but everytime it happens, everyone comments how "unusual" it is to have rain in the summertime. And of course, one cannot trust the news media to report accurately how unusual a given event is. The only way to know how unusual some event happens to be is
  1. to have good records of what has happened in the past,
  2. to have some idea of how to analyze those records, and
  3. to understand statistics.

I have been collecting weather data for Fallbrook since 25 September 1994, so I don't have a lot of data for Fallbrook and thus cannot report on events that happen only once per 10 years or longer. Nonetheless, the data that I have already collected give a pretty accurate picture of the mean temperature and sunshine percentage variation with the day of the year. The data also give some indication of the "normal" type of variability that happens within each year. The data were collected at my house in the Winterwarm area of Fallbrook at 690' elevation at 33° 20.33' latitude, 117° 13.24 West longitude.

I use two Taylor max-min thermometers, and manually read the recorded max and min temperatures once a day near sunset when I am home. If I am away, I record the max and min temperatures when I get back, and use the official Fallbrook temperatures to assign those measurements to the appropriate day.

I also record my estimate of the percentage of possible sunshine each day that I am in Fallbrook all day. I define the percentage of possible sunshine as the total number of hours of actual sunshine divided by the number of daylight hours.

My high temperature comes from my thermometer under my porte-cochere, mounted on the north side of a supporting post, separated from the wall by half-inch blocks of wood at the mounting screws. My low temperature comes from my thermometer mounted on a wood post in the ground several feet away from the west wall of my house. The thermometer is again mounted a half-inch from the post with wooden blocks. In addition to being fairly close to the house, two large trees block the view of most of the sky from that thermometer, which probably prevents recording temperatures as low as might be experienced in the middle of an open field.

For rainfall, I use a standard cheap plastic rain gauge available at hardware stores, placed in an open area in my yard. I read the gauge near sunset.

I have over a decade of weather for Altadena, Ca that I'll put on the web when I have time that will be useful for extending the rainfall comparison between years, and for determining events that happen only once per 10 years or so.

There are several caveats to be noted about the data, which are explored further in Analysis of Biases in Fallbrook Temperatures:

  1. My high temperatures are about 4o higher than the official Fallbrook temperature for official temperatures less than 90o and about 2o higher than the official Fallbrook temperature for official temperatures over 90o. My "high temperature thermometer" is located underneath my porte cochere, and the concrete underneath it traps solar heat and may keep the temperature there a bit warmer. However, there is no noticeable effect versus the amount of sunshine, which leads me to question whether there is any significant bias due to this cause. Thus this bias may simply be due to a bias in my thermometer.

  2. My low temperatures are about 2o higher than the official Fallbrook temperature for official temperatures over 40o. Below that temperature, my low temperatures can range to over 10o warmer than the official temperature, which is probably caused by my location in the Winterwarm area compared to the location of the official Fallbrook temperature near downtown. My "low temperature thermometer" is located in a natural area fairly close to my house and close to a large Canary Island Pine, which may produce a slight high bias for the low temperatures. I don't believe the magnitude of this effect is large, but I'll place another high-low thermometer in a more open area sometime to check it.

  3. I am absent from Fallbrook about 9 days per month, usually for 3 days per week for 3 weeks, and hence the high and low recorded temperature for that period of 3 days will be recorded accurately, but two days of "lower highs" and "higher lows" were not recorded. This biases the mean high and low temperature record slightly high and low, respectively. My analysis shows that this causes a bias of much under 1o, probably due to the high correlation of temperatures from day to day.

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Copyright © 1997, 1998 by Tom Chester.
Permission is freely granted to reproduce any or all of this page as long as credit is given to me at this source:
Comments and feedback: Tom Chester
Last update: 1 January 1998.
The plots may have been updated later - see the date at the top of each plot.