Observed High Temperatures and Analysis
The mountains are always thought of as being cooler than the valleys. After all, the mountains have regular 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 Gabriel Mountains can sometimes be hotter than many places in the L.A. Basin, shattering all the beliefs above. And frequently the temperatures in the San Gabriel Mountains can be essentially the same as the temperatures at the L.A. Civic Center, as many a hiker has found out the hard way.
There are two fundamental reasons why summer temperatures in the San Gabriel Mountains are hotter than expected by most people:
- Southern California frequently experiences a temperature inversion layer, with the temperature increasing with altitude instead of decreasing.
- The San Gabriel Mountains are farther from the ocean than many parts of the L.A. basin, and hence receive little or no ocean cooling.
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 almost the definition 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 south slopes of the San Gabriel Mountains can tell precisely where the top of the inversion layer is at any moment. The top is the boundary between the hazy basin 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 heavy smog days in the basin, 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 the L.A. Basin. See (4710') Crestline's Air Quality May Take Breath Away (LAT 7/26/98).
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 of the San Gabriel Mountains are 25-50 straight-line miles from the ocean, which eliminates much of the moderation in high temperatures due to the maritime influence. In contrast, the L.A. Civic Center is 12 straight-line miles from the ocean, and is 10-20° F cooler than other places in the L.A. Basin that are 25-50 miles from the ocean.
Hence low elevations in the San Gabriel Mountains should have summer high temperatures 10-20° F hotter than the L.A. Civic Center simply due to the lack of maritime influence. 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 Gabriel Mountains would have the same high temperature as the Civic Center. Since the existence of the inversion layer increases the expected temperature of a given elevation above it, one would expect these elevations to be hotter than the Civic Center on a normal summer's day.
Observed High Temperatures and Analysis
Beginning on 12/13/98, I have kept track of the high temperatures of Pasadena (864'), Mt. Wilson (5,709') and Wrightwood (~6,000') as reported in the L.A. Times. My major goal was simply to try to understand what temperatures to expect when hiking during each time of year. It would have been nice to have a station at higher altitudes, but I don't know of any.
A majority of the hikes in the San Gabriel Mountains are at elevations between that of Pasadena and Wrightwood, and thus these plots directly show the range of temperatures expected during the year. However, a significant number of hikes are at higher elevations, and hence one must extrapolate (see below) to deduce the expected range of temperatures.
These stations give an interesting comparison, since Pasadena and Mt. Wilson are only ~7 miles apart, and thus nearly directly show the variation of temperature with altitude. Mt. Wilson and Wrightwood are at nearly the same elevation but 26 miles apart, most of that distance also being increased distance from the ocean. Hence that comparison shows the influence of maritime cooling.
High Temperature Difference In High Temperatures Pasadena Mt. Wilson minus Pasadena Mt. Wilson Wrightwood minus Mt. Wilson Wrightwood Wrightwood minus Pasadena
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 and 3.3° F per 1,000', respectively.
The date range of the data set used for the conclusions below is 12/13/98 to 10/18/99. The plots have been updated later to keep track of current temperatures. For the date range analyzed here, there are 257 plotted data points for Pasadena, 256 for Mt. Wilson and 191 for Wrightwood, 83%, 83% and 62% of the days. There are fewer points in the difference plots: 234, 172 and 176, for Mt. Wilson - Pasadena, Wrightwood - Pasadena, and Wrightwood - Mt. Wilson, respectively, 75%, 55% and 57% of the days, since the absence of data from either station prevents the difference calculation.
The plots show the following:
- Mt. Wilson and Wrightwood usually have high temperatures in the 80s in summer and fall, with occasional cooler days. Temperatures in the 80s are tolerable for hiking, but are not considered cool temperatures by nearly anyone.
- There are essentially no days in summer and fall where the Mt. Wilson temperature is as cool as predicted by either the dry or moist lapse rate from the Pasadena temperature. This clearly shows the persistent presence of the inversion. Instead of the 16-28° temperature difference predicted by the lapse rate, the observed temperature difference is usually 0-13° cooler, with temperatures hotter at Mt. Wilson than Pasadena on 5-10% of summer days!
Although the plot shows that the inversion was present during most of early 1999 as well, that may have been due to the light rainfall, warm winter of that year.
- The Wrightwood - Mt. Wilson comparison clearly shows the decreased maritime influence at Wrightwood. Wrightwood is 0-5° warmer than Mt. Wilson in summer, instead of the 1-2° of cooling expected due to the altitude difference. The comparison also shows a wide variability in the temperature difference in winter and spring, presumably due to a boundary between air masses located between the two places. Such boundaries exist in Southern California only during that time of year.
Since the inversion layer is usually found below 6,000', it is probably a good assumption that high temperatures are roughly 3° F/1,000' lower than Wrightwood above 6,000'. Thus normal high temperatures at 9,000' would be expected to be in the 70s in the summer, a pleasant hiking temperature.
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Copyright © 1999-2003 by Tom Chester.
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Last update: 22 March 2000 (two links updated on 8 April 2003).