The Climatology of Lake Merced, San Francisco, California

Graeme Somerville,Graduate Student,Geography
San Francisco State University

Introduction

Lake Merced is a significant natural area in San Francisco, California. It is located in the Southwest corner of the city and lies 0.25 miles east of the Pacific Ocean. Its exact coordinates are 37° 43' 22'' N,122° 29' 42'' W. This latitude and its close proximity to the Pacific provide an interesting and very changeable climate that is influenced by distinct maritime weather patterns (Gilliam 1962). The climatology of Lake Merced involves an array of meteorological phenomenon, especially wind, fog and precipitation which are derived from the complex interactions between the atmosphere, ocean and surrounding topography. The climate of this area is considered to be an important and significant factor when discussing the biogeography of the lake and its surroundings.

This section will discuss the meteorological and climatologic conditions that prevail over the region and will explain their significance in terms of local and micro climatologic influences that occur in and around the lake.

Regional Climate

San Francisco has a mesothermal climate regime classified as Csb according to the Köppen-Geiger classification system (Ahrens 1994 and Null 1992). This classification depicts San Francisco as having a coastal Mediterranean climate with dry mild summers and wet mild winters (Ahrens 1994). The Mediterranean climate that San Francisco enjoys is further defined by a dry season lasting typically from May until October and a wet season from November until April. However, anyone who has visited the region will know that the meteorology of this area cannot be generally summed up in a few sentences. San Francisco has an complex matrix of micro climates that are especially distinctive and prevalent during the summer months (Gilliam 1962). It is a climate dominated by the fluctuations of the Aleutian Low and Pacific High atmospheric pressure systems. The variety of climatic conditions that San Francisco experiences can be linked directly in some way or another to the dynamics associated with these two pressure systems (Gilliam 1962).

From late October through March, San Francisco receives an average of 95% of its annual rainfall (Null 1978). Cyclogenesis in the Gulf of Alaska produces winter storms which move into northern and central California sometimes bringing impressive rainfall and steady winds. Most of these winter weather systems move through the region fairly rapidly and mild cool conditions follow. Occasionally, a Gulf of Alaska storm will gain strength from sub tropical moisture (often referred to as the pineapple connection) and its severity in terms of precipitation can be extreme.

In summer and fall the climate of San Francisco can be classified differently depending on where you live. Locations adjacent to the ocean are often enclosed in fog with temperatures a cool 60° F while locations further inland or those in the 'fog shadow' are experiencing 75° F and clear (Gilliam 1962). San Francisco often enjoys its best weather in late September and early October. Inland cooling reduces the formation of coastal stratus and modifies its influence on the coastal communities of the city and atmospheric conditions set up an off shore wind flow. As a result of less fog and the strong off shore breeze, temperatures are often the warmest of the year and give San Francisco a well defined Indian summer (Gilliam 1962). Any precipitation that occurs in the summer is usually a result of a tropical moisture flow moving up from the Gulf of California but this is rare and a dry pattern generally prevails (Null 1992).

Although there are as many as 14 locations within San Francisco that have kept climatological records of varying lengths (Null 1978), there are no recording stations at Lake Merced. For an accurate appraisal of the climatology of Lake Merced it was necessary to use climate records from a location as close as possible to Lake Merced, climatologically as well as geographically. The data presented are obtained from the Richmond/Oceanside station and collated by the Western Regional Climate Center (WRCC) a sub-division the National Climate Data Center.

Discussion

The temperature profile in Figure 1 provides a good visual of a mid latitudinal maritime climatograph depicting a typical "flat' graph with only a small diurnal range of temperatures. The annual averages vary only by approximately 10° F and the average maximums and average minimums only about 15° F. Despite the normalcy of this graph there are a few interesting points to note.

• The highest average temperatures occur in late summer indicated by a slight rise in the graph around September/October. This is indicative of the Indian summer referred to earlier.
  

• Despite the relatively mild and stable temperature profile, the Lake Merced area does experience some significant extremes. Occasionally lows dip to below freezing with 26° F (December 1984) and 29° F (February 1986). Maximum recorded highs have reached well into the 90's in August, September and October. This is almost certainly due to a strong off shore "Santa Ana" type wind pattern that occasionally develops eliminating the usual sea breezes. These extremes are usually short lived with an onshore flow returning with fog. It is not uncommon for the area to experience 90° F one day and only 60° F the next.

Figure 1                                                                                                                                  WRCC (2000)

As typical with the Mediterranean climate the majority of the precipitation that occurs in the Lake Merced area comes during the winter (Gilliam 1962). Figure 2, the average and extreme precipitation values from 1948 - 1999, shows precipitation fluctuating from around four inches in winter to around zero in the summer. Figure 2 also shows some indicate interesting irregularities.

• February has had a monthly precipitation total of 14 inches.

• In all the months except August and July the total can be up to two inches or more.

• The Lake Merced region has experienced significant daily rainfall events indicated by the black line. This could lead to hydrologic problems associated with the lake in terms of storm water run-off and bank erosion.

Figure 2                                                                                             WRCC (2000)

Effects of Climate on the Lake

Drought has played an important part in the history of Lake Merced. Periodically the region has experienced extended droughts such as those between 1976-77, 1986-90

Attempts to correlate drought occurrences (low precipitation) with lake level is a difficult task. During periods of drought water is often added to the lake, for instance in May 1978, 400 million gallons was added usually from external sources such as the Hetch Hetchy system (SFWB 1999). Water is also removed from the Lake periodically from well pumping by Daly City Municipal Water District and the local golf courses (SFWB 1999). It is therefore very difficult to make a quantitative statement about the contribution of precipitation to the surface level of the lake. However in Figure 3, which shows graphically lake level and rainfall data from 1996 until 2000, there are some interesting characteristics to note.

• During the El Niño years of 1997 and 1998 the lake level (taken at the end of each month) was seen to rise about 2 ft and 4 ft respectively.
  
• The La Niña winters of 1999-2000 also show an increase in lake level but not as significant.
  
• During the dry summer months the lake level is noted to go down steadily.

Although the graph would indicate a correlation between rainfall and lake level, caution should be exercised and all discharges and recharges fully researched before the actual effect of rainfall can be correctly calculated.

In order to fully understand the climatology of a region on the scale of Lake Merced, it would be valuable to analyze data from a micro climatological level. To fully assess the micro climatology of an area such as this, the energy transfer processes need to be calculated in terms of evaporation, evapotranspiration, soil moisture, soil flux densities leaf area indexes. No such information has been collected for Lake Merced.

However, more general micro-climatological aspects can still be discussed. Some of the more germane points noted are:

• Prevailing winds in the area usually come from the west. These winds have been generally noted to be unhindered by large buildings or tall vegetation.
  
• During storms the pre-frontal rains are usually delivered from the southwest and are blown by southwesterly winds. Therefore the slopes facing the southwest would be expected to receive more direct precipitation than northeast facing slopes.
  
• On clear days, the aspect of slopes plays a significant role. In the northern hemisphere the south facing slopes receive more sunshine than the north facing. Some of the north facing slopes around Lake Merced are densely foliated and may not receive any direct sunlight.
  
• The climatology of fog drip is an interesting subject and it is relevant to mention that the fog, prevalent throughout this region, can provide significant moisture addition to the rainfall totals in the form of fog drip. Snyder (1992) explains that studies in the mountains east of Half Moon Bay, 25 miles to the south have recorded 58.8 inches of fog drip under an exposed, 20-foot high tan oak (Lithocarpus densiflora) in 39 days. Although the Lake Merced area does not have the vegetation and topography like that of the Half Moon Bay region, the fog drip in and around Lake Merced could be deemed very significant to the water balance of the area. As shown by Snyder, precipitation in the form of fog drip can add substantially to the water balance of an area.

Conclusions

The climatology of Lake Merced is an extremely complicated and interesting subject. It is difficult to make precise judgments about the effects of climate due to the lack of data from the actual location. In order to fully understand the climatic and meteorological processes at work, numerous stations recording the usual parameters such as temperature, precipitation and relative humidity need to be installed alongside some of the micro-climatological apparatus to provide an overall picture of the atmospheric conditions and the factors which influence them.

Ahrens, C.D. 1994. Meteorology Today. West Publishing

Gilliam, H.  1962. Weather of the San Francisco Bay Region. Univ. of California Press

Plummer, J. 2000. Friends of Lake Merced. http://www.lakemerced.org/rain2.html (May 00)

Null, J. 1992. The Climatology of San Francisco Rainfall 1849- 1991 M.A Thesis. San Jose State University

SFWB. 1999. The Hydrology of Lake Merced. San Francisco Water Board Lake Merced Watershed Sanitary Survey, November 1999. Camp Dresser & McKee Inc.

Snyder, J.A. 1992.  The Ecology of Sequoia sempervirens (Thesis) 
     http://www.batnet.com/askmar/Redwoods/Redwood_Thesis.html (May 00)

WRCC. 2000. Western Regional Climate Center   http://www.wrcc.dri.edu/cgi-bin/cliMAIN.pl?casfor+sfo (May 00)