Eastern Mojave Vegetation Climate of the Eastern Mojave Desert  

Tom Schweich  

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Literature Cited
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Other articles: Glossary climate Vegetation of the Mid Hills 35400 Frasera albomarginata 135000

Locations: Santa Barbara. Ulan Bator.  


We use "climate" to describe broad patterns of heat and cold, and precipitation and other factors. We say, "the Mojave has a warm desert climate," "Santa Barbara has a coastal Mediterannean climate," and "Ulan Bator has a continental climate." Each of these statements implies something about the patterns of seasonal temperature and precipitation, based upon years of experience. Ulan Bator is very cold in the winter, with scant snow as precipitation, accompanied by cutting winds. Santa Barbara has dry summers with nighttime temperatures moderated by coastal fog. The desert aspect of the Mojave implies a small amount of rainfall. Traditionally a region that receives less than 10 inches (250 mm) of precipitation per year is classified as a desert. But the descriptive statements, as true as they may be, are a subjective evaluation and difficult to compare one location to another. To characterize a climate objectively, we turn to weather data.

Other articles: Glossary weather  

"Weather" generally refers to daily or even more frequent bservations of sun, clouds, temperature, precipitation, wind and other sensible aspects of the outdoors environment. We tend to talk about today's weather, as "sunny, with highs in the 80s," or "cold and windy," or "cloudy with a chance of meatballs."

Locations: San Francisco.  

Some weather data is easy to collect and report on a daily basis. Daily high and low temperatures can be measured on a simple thermometer and collected once a day. Rain can be collected in a cylinder that is read and emptied out once a day. Wind data is a little more complicated. The wind may blow from the south for a while, then from the west, and finally from the north, as it often does when a storm passes through the San Francisco Bay area. How would the wind be characterized for that day?
  Nevertheless, we tend to use the easily measured and processed weather data, summarized over many years, to characterize the climate of a given location. This is usually limited to daily high and low temperature, and daily sum of precipitation.

Locations: Asheville.  

For my study of the climate of the Eastern Mojave Desert, I obtained weather data from the National Climatic Data Center, Asheville, North Carolina, telephone: (704) 271-4800.

Locations: Mitchell Caverns. Providence Mountains State Park.
Full Size ImageMitchell Caverns headquarters at the end of Essex Road.  

The data selected was the TD3200 - Cooperative Summary of the Day. These data are from agencies that cooperate with the National Climatic Data Center. For example, one of the locations that I use for weather data in this study is Mitchell Caverns, California. The caverns were formerly privately owned by Jack and Ida Mitchell who operated the caverns as a tourist attraction. Jack and Ida maintained a small weather station at the Caverns. They were paid a small sum by the National Weather Service to collect the temperature and rainfall data every day and mail it in once a month. Mitchell Caverns was purchased by the California State Parks System and renamed Providence Mountains State Park. California State Parks employees at Mitchell Caverns have continued to collect the weather data and send it to the National Climatic Data Center, formerly the National Weather Service.
  The data available in the Summary of the Day includes the daily maximum temperature, minimum temperature, precipitation, snowfall and snow depth. The processing time for Cooperative station data is six months, so the data for any year is not available until July of the following year.

Locations: Lobo Point. Mitchell Caverns. Wild Horse Mesa.  

The reason I use weather data from Mitchell Caverns is that it is the nearest weather station to Lobo Point and Wild Horse Mesa. Mitchell Caverns is 15 km southwest of, and 50 m lower than, Lobo Point. My data set includes nearly 40 years of consistent weather data for Mitchell Caverns. It begins January 1, 1959 and ends April 30, 1997.

Locations: Mountain Pass.  

I have also obtained weather data for Mountain Pass, CA. This location is on Interstate 15, the Los Angeles to Las Vegas highway, and is the location of Molycorp's rare earth mine. Mountain Pass is 46 km north of and about the same elevation as Lobo Point. I have 14 years of data from Mountain Pass, beginning in January 1, 1980 and ending April 30, 1997.

Other articles: Field Notes Homer, September 22, 2000

Locations: Baker. Colorado River. Las Vegas. Lobo Point. Los Angeles. Needles.  

Baker, CA and Needles, CA are the two next closest weather stations to Lobo Point. However, both cities are at much lower elevations. Needles, CA is on the Colorado River, at an elevation of about 200 m, and about 70 km southeast of Lobo Point. The weather observations are made at the airport, on a mesa a few kilometers south of town at an elevation of 250 m. Baker, CA is on Interstate 15, the main Los Angeles to Las Vegas highway at 200 m elevation, and about 70 km west of Lobo Point.

Other articles: Example of TD32000 10000  

The TD3200 - Cooperative Summary of the Day data, is delivered on a diskette in a really obscure format. I takes some work to organize the data so that it can be analyzed. When I first obtained some weather data, I was using dBase III, and wrote the dBase code to decode the input data and store it in a data base. When I updated my data set in 1997, dBase no longer existed as a product and I have migrated to Microsoft Access as a data base software. I had to re-write the code to import the weather data in Visual Basic.

Locations: Mitchell Caverns.
Full Size ImageSummary of the Day, Mitchell Caverns, July, 1980.  

There are many kinds of reports and other ways to analyze weather data. The simplest way is to chart daily temperature and precipitation. Figure 4-2 shows data for the month of July, 1980. I chose this month to use as an example because it contains the highest temperature that was recorded at Mitchell Caverns between January 1, 1959 and April 30, 1997. The maximum temperature that day was 43.3° C, which is about 110°F. There are a couple of other things to note on the chart. The morning of the hottest day that August was also the coolest morning that month. The range of temperature that day was 28.9°C (52°F). Then a couple of days later, it began to rain. Over the next four days Mitchell Caverns received 7.9mm (0.31 in) of rain.

Locations: Mitchell Caverns.
Full Size ImageSummary of the Day, Mitchell Caverns, December 1990.  

At the opposite end of the spectrum was December, 1990, for which daily weather data is shown in Figure 4-3. In this month is the lowest temperature recorded at Mitchell Caverns in my data set. Over a period of several weeks, the temperatures slowly declined until reaching a minimum of -11.7°C (11°F) on December 22, 1990. After a period of at least 48 hours in which the temperature stayed below freezing, the temperature slowly began to climb to more normal winter temperatures. The other thing I notice is that the only precipitation in December 1990 was at the beginning of the month, when temperatures were higher. While we don't know for sure, the weather was most likely clear and dry when temperatures were so low. This would be because drier air holds less heat, and a clear sky permits more radiation of heat at night.

Locations: Mitchell Caverns.
Full Size ImageSummary of the Day, Mitchell Caverns, 1995-1996  

At some time though, we have to begin summarizing the weather data over a period of years in order to characterize the climate. Figure 4-4 at left shows the Summary of the Day for 1995 and 1996 at Mitchell Caverns, California. Easily visible is an annual cycle of rising and falling maximum and minimum temperatures. Imagine, though, a chart showing all 40 years of available daily data. It would just contain lots of red and green dots that would be very hard to analyze.
  Precipitation is also difficult to analyze visually. It occurs as isolated events. Summarization by month, to aggregate the total rainfal for a month into a single number, may provide a little more useful data.

Full Size ImageMonthly Averages, Mitchell Caverns, 1959-1997  
When you chart a lot of weather data points, you get a lot of points but not much intelligence, as shown in Figure 4-5. The variance of the weather data as it follows the annual cycle of the seasons is so great that you need some way to smooth the data and look for trends.
  The chart at left, Figure 4-6, shows the same data as above, except that it has been normalized and smoothed. The normalization simply expresses each data point as a percentage of the average for that month. This procedure should dampen the seasonal rise and fall of temperature and seasonal precipitation.

Full Size Image12-Month Moving Averages, Monthly Averages, Mitchell Caverns, 1959-1997  
The smoothing I've done on this chart is a 12-month moving average. This may not be too bad of an approach, under the (unproven) theory that the effects of temperature and precipitation may accumulate over time, but the most direct effect will be in the current month. Often a different form of statistical analysis will be used. The most common is a fast Fourier transform. All of this comes under the heading of "Time-Series Analysis."
  When I first prepared this chart, I thought I had a problem in the source data. There is a very obvious difference in precipitation between the period 1959-1975 and the period 1976-1997. However, I have recently found some other analyses of Mojave weather patterns that indicate a similar occurrence. See, for example, Long-term climate variation in the Mojave Desert Ecosystem by Richard Hereford of the United States Geological Survey.
  However, we still have the problem of looking at 40 years of data and trying to characterize the climate in a few meaningful statements.
  If we assume that the annual cycle of the seasons dominates the climate of a location, and that temperature and precipitation are two key indicators of climate, perhaps a simple chart relating those two variables can be a reliable indicator of climate.

Literature Cited:
- Walter, H, 1963.
Full Size ImageDescription of climate diagrams after Walter (1963).  

Walter (1963) did just that when he suggested a climatic diagram to relate precipitation to temperature as a surrogate for water balance, as described at left.

Locations: Mitchell Caverns. Mountain Pass.  

With the weather data noted above, I prepared a climatic diagram each for both Mitchell Caverns and Mountain Pass.

Locations: Mitchell Caverns.
Full Size ImageClimate diagram for Mitchell Caverns, California  

The climate diagram for Mitchell Caverns is shown at left. The temperature curve follows a nice sinusoidal curve, with a minimum in January and maximum in July, although the temperature seems to build a little more slowly in the Spring, and fall off more rapidly in the Fall than a simple sinusoidal curve would indicate.

Literature Cited:
- Zar, Jerrold H, 1996.

Locations: Mitchell Caverns.  

The distribution of rainfall can be characterized as bi-modal. That is, there is a Winter peak of precipitation, which occurs in February, and a Summer peak, which occurs on August. Zar (1966) notes that it is interesting to report the number of modes detected in a population, if there are more than one, because a bi-modal distribution may indicate a combination of two distributions with different modes. The Total Quality Management press takes the concept a little farther, stating that a bi-modal distribution often indicates that two processes are being measured by one variable. And that is probably what is happening here; the weather process that beings Winter precipitation is different than the process that brings Summer precipitation, and the two processes are being measured by one variable, precipitation at Mitchell Caverns.

Locations: Mountain Pass.
Full Size ImageClimate diagram for Mountain Pass  

The climate diagram for Mountain Pass, shown at left, shows that the climate there is very similar to Mitchell Caverns, except slightly colder winters.
  Overall, the climate diagrams for the eastern Mojave show a humid period from November through March, and an arid period from April through October. The favorable growing season, defined by absence of frost, is limited to June through September. May, the best month for flowering displays, may have frost and is arid. The most striking feature of the diagram is the summer precipitation peak. August is the wettest month in the eastern Mojave. However, the climate in August is still classified as arid.
  The presence of a summer monsoon in the California desert is well known. There are two papers I have found that address it directly.

Literature Cited:
- Douglas, Machael W., Robert A. Maddox, Kenneth Howard, and Sergio Reyes, 1993.  

"The pronounced maximum in rainfall during the warm season over southwestern North America has been noted by various investigators. In the United States this is most pronounced over New Mexico and southern Arizona; however, it is but an extension of a much larger-scale phenomenon that appears to be centered over northwestern Mexico. This phenomenon, herein termed the “Mexican monsoon,” is described from analyses of monthly mean rainfall, geostationary satellite imagery, and rawinsonde data. In particular, the authors note the geographical extent and magnitude of the summer rains, the rapidity of their onset, and the timing of the month of maximum rainfall. Finally, the difficulty in explaining the observed precipitation distribution and its timing from monthly mean upper-air wind and moisture patterns is discussed." Abstract, Douglas, et al (1993).

Literature Cited:
- Adams, David K., and Andrew C. Comrie, 1997.  

"The North American monsoon is an important feature of the atmospheric circulation over the continent, with a research literature that dates back almost 100 years. The areal extent of the North American monsoon is large, extending over much of the western United States from its region of greatest influence in northwestern Mexico. Regarding the debate over moisture source regions and water vapor advection into southwestern North America, there is general agreement that the bulk of monsoon moisture is advected at low levels from the eastern tropical Pacific Ocean and the Gulf of California, while the Gulf of Mexico may contribute some upper-level moisture (although mixing occurs over the Sierra Madre Occidental). Surges of low-level moisture from the Gulf of California are a significant part of intraseasonal monsoon variability. Variability of the North American monsoon system is high, but it is not strongly linked to El Niño or other common sources of interannual circulation variability, " from the Abstract, Adams and Comrie (1977)..

Literature Cited:
- Sharon, David, 1972.  

Prevailing wisdom is that summer monsoon precipitation can be quite spotty. The standard reference on this issue is Sharon (1972) who concluded that most convective cells are well separated from each other in time and space. This means that on any given day 20% percent of a basin could be receiving rainfall. Spottiness mostly occurs in late spring and fall, and spatially uniform in the mid-winter at least in southern Israel. How this translates to the eastern Mojave is, I believe, an open question.

Literature Cited:
- West, N. E., K. H. Rea, and R. J. Tausch, 1975.

Locations: Mitchell Caverns. Mountain Pass. Sedona.  

West, Rea and Tausch (1975) published climatic diagrams for 16 stations where pinyon-juniper woodlands are located, ranging from California to Texas, and north to Idaho. Visually, the diagram for Sedona, Arizona is the most similar to the eastern Mojave stations. However, the climatic diagrams for Mitchell Caverns and Mountain Pass are more similar to each other than they are to any of West, et al's (1975) diagrams.

Literature Cited

  A list of all literature cited by this web site can be found in the Bibliography.
  Adams, David K., and Andrew C. Comrie. 1997. The North American Monsoon. Bulletin of the American Meteorological Society. 78(10):2197-2213.
  Douglas, Machael W., Robert A. Maddox, Kenneth Howard, and Sergio Reyes. 1993. The Mexican Monsoon. Journal of Climate. 6(8):1665-1678.
  Sharon, David. 1972. The spottiness of rainfall in a desert area. Journal of Hydrology. 17: 161-175.
  Walter, H. 1963. Climatic diagrams as a means to comprehend the various climatic types for ecological and agricultural purposes. pp. pp. 3-9 in Rutter, A. J., and F. H. Whitehead (Eds.). 1963.. A symposium of the British Ecological Society, London, 5-8 April 1961. The water relations of plants. London: Blackwell Scientific Publications, 1963.
  West, N. E., K. H. Rea, and R. J. Tausch. 1975. Basic synecological relationships in pinyon-juniper woodland. Proc. pinyon-juniper ecosystem: a symposium. Logan, Utah: Utah State Univ.
  Zar, Jerrold H. 1996. Biostatistical Analysis, 3rd edition. Upper Saddle River, New Jersey: Prentice Hall, 1996.
If you have a question or a comment you may write to me at: tas4@schweich.com I sometimes post interesting questions in my FAQ, but I never disclose your full name or address.  

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Date and time this article was prepared: 3/21/2017 9:13:44 AM