High Temperatures in the San Diego County Mountains

Observed High Temperatures and Analysis


The mountains are always thought of as being cooler than the valleys. After all, the mountains get snow in the winter, and the valleys do not. Nearly everyone on vacation has experienced the delightful coolness of higher altitudes compared to the summer heat of lower elevations. Many of us have learned in school that temperatures fall with elevation at a rate of 3-5° F per 1,000', and so we can easily calculate that temperatures at 5,000' should be 15-25° F cooler than at sea level.

Like much of common knowledge, there is a lot of truth to all the comments above. However, in the summer, the San Diego County Mountains are nearly always hotter than most coastal San Diego County communities, shattering all the beliefs above.

There are two fundamental reasons why summer temperatures in the San Diego County Mountains are hotter than expected by most people:

The Temperature Inversion Layer

Southern California's inversion layer is famous for creating our cloud-free sunny summer days and for trapping pollutants to form smog. The inversion layer is a hallmark of our relatively-rare Mediterranean climate, found in only a handful of other places in the world.

The Mediterranean climate, and the inversion layer, are formed under a persistent high-pressure area, where the air is descending to the Earth's surface, heating up by compression as it descends. Under normal conditions, the air descends to a certain altitude that is then the top of the inversion layer. Small horizontal movements of air complete the flow of the descending air.

The top of the inversion layer is where the air reaches its highest temperature. The air below that level cannot ascend. (Because each parcel of air below has a cooler temperature, it is therefore denser, and gravity keeps it trapped in place.)

The result of all of this is that temperatures decline with altitude only for a few thousand feet to the bottom of the inversion layer, followed by a rapid increase in temperature with altitude to the top of the inversion layer. Only then does the temperature fall again with elevation.

Most of the time, hikers ascending the San Diego County Mountains above 3000-5000' can tell precisely where the top of the inversion layer is at any moment. The top is the boundary between the hazy lower air and the clear air above it. During the summer, the top is often found at an elevation of 2,000-4,000' and hikers above that elevation can clearly tell that they are above the top.

The top can be as low as several hundred feet, creating smog days in coastal San Diego County, or at much higher elevations, allowing smog to penetrate the mountains. Since pollutants tend to concentrate at the top of the inversion layer, some places in the mountains are often the smoggiest places in San Diego County. For example, Crestline, at 4710' in the San Bernardino Mountains is now the smoggiest monitoring location in the L.A. Smog Basin, and Alpine, at 2000' in San Diego County, consistently is the smoggiest monitoring location in San Diego County. Most likely, locations at even higher altitude are smoggier than Alpine, but no smog monitoring stations are located at higher altitudes. (See Crestline's Air Quality May Take Breath Away (LAT 7/26/98) and San Diego: An Introduction to the Region, Pryde, Third Edition, 1992, p. 263.)

Due to the variable altitude of the inversion layer, it is difficult to make general statements about the net cooling or warming with altitude. The data below show the extreme variability of this effect.

Distance From The Ocean

Most areas of San Diego County above 4000' are 25-40 straight-line miles from the ocean, which eliminates much of the moderation in high temperatures due to the maritime influence. In contrast, most of the populated area of San Diego County is within 15 miles of the coast and therefore experiences summer high temperatures cooled significantly by the ocean.

The maritime cooling is ~10-20° F at a distance of 12 miles from the coast and ranges up to 30° F at the coast. Thus even in the absence of an inversion layer, the normal temperature lapse rate of 3° F / 1,000' predicts that elevations of 3,000-7,000' in the San Diego County Mountains would have the same high temperature as Fallbrook or La Mesa, both about 12 miles from the coast. Since the existence of the inversion layer increases the expected temperature of a given elevation above it, one would expect those elevations in the Mountains to be hotter than Fallbrook or La Mesa on a normal summer's day, and significantly hotter than coastal communities.

Observed High Temperatures and Analysis

Beginning on 12/13/98, I have kept track of the high temperatures of Fallbrook (698'), Palomar Mountain (5550'), Julian (4240') and Mt. Laguna (5760') as reported in the North County Times. My major goal was simply to try to understand what temperatures to expect when hiking during each time of year. I have used Fallbrook as my base comparison simply because I live in Fallbrook, but the results would be similar using other cities about 12 miles from the San Diego Coast.

These stations give interesting comparisons. Fallbrook and Palomar Mountain are only ~17 miles apart, and thus should experience nearly the same air masses, showing directly the variation of temperature with altitude and distance from the coast. Julian is about 20 miles from Palomar Mountain, farther from the ocean, and possibly in a different air mass at times than Fallbrook and Palomar Mountain. Mt. Laguna is yet another 17 miles from Palomar, and will be in a different air mass more frequently.

Beginning on 6/3/00, I added the Temecula temperatures, since they are representative of the lower parts of the Dripping Springs Trail.


High TemperatureDifference In High Temperatures
FallbrookPalomar Mountain minus Fallbrook
Palomar MountainJulian minus Fallbrook
JulianMt. Laguna minus Fallbrook
Mt. LagunaMt. Laguna minus Julian
TemeculaPalomar Mountain minus Temecula

The plots of the differences in high temperatures also show the expected difference in high temperature using the dry and moist lapse rates, 5.5 (yellow line) and 3.3° F (pink line) per 1,000', respectively.

The date range of the data set used for the conclusions below is 12/13/98 to 3/20/00. The plots will be updated later to keep track of current temperatures.

The plots show the following:

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Copyright © 2000-2003 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: 9 September 2000 (two urls updated 12 April 2003).