San Francisco is famed for its weather. Fog and wind are celebrated, cursed – or both – in song, story and conversation. And while the City is not as wet as Portland nor as sunny as Miami, it does have a sometimes long rainy season and a pleasant Indian summer. And just as well known as the City’s climate is its unpredictability. The mercury may often rise to over 60 on a mild January afternoon or it may drop into the 40s and feel like 30 on a windy day in June. Much of this unique weather is shaped by the microclimates that nestle in the City’s valleys and along the slopes of over 40 hills. The variety and unpredictability of San Francisco’s topography and climate create a matrix of possible weather situations.

A microclimate is defined as the climate of a small area especially when that climate differs from the surrounding areas.

A variety of factors and forces form a microclimate. Not only do meteorological, biological, topographical, chronological and other natural influences help form a microclimate, but also technological and cultural factors play their parts.

Among meteorologists, San Francisco is noted for microclimates. The microclimate entry on Wikipedia mentions San Francisco and notes that temperatures in some of its neighborhoods may vary by as much as 9 degrees Fahrenheit from block to block

In San Francisco microclimates are also influenced by each of the four dimensions – length, height, width and time.

From the Golden Gate to San Bruno Mountain, the length of San Francisco runs along an irregular spine of central highlands. These hills form a steep barricade between the cool, moist maritime climate on the coast and the drier, warmer Mediterranean zone by the Bay.

The width of San Francisco – from the Pacific coast to the Bay – dissipates the fog and wind as they move east. The process is usually a slow, gradual one but sometimes a fogbank at the beach will “burn off” a few blocks inland.

The heights of San Francisco’s hills also act upon the weather. Hills can block fog or shelter it. Rain may fall heavily on one side of the central heights while making just a drizzle on the other.

And time changes the weather in San Francisco more often than in most places. Many spring and summer days begin with a fog that “may chill the air” as the City’s official song reminds us. But by early afternoon, wind or heat may drive or dissipate it away.

One afternoon, the Old Veteran was assigned to work the neighborhoods flanking the central heights. As he rode from Buena Vista Park along the spine of the City to Forest Hill, he was exposed to different weathers in less than an hour.

Most of the day, the weather had been dry though gloomy but in the early afternoon clouds dark with rain moved from the southeast toward the City. But the light gray clouds above the City’s coastal neighborhoods began to dissipate and admit shafts of sunlight here and there. As he steadily ascended a pass between Buena Vista Park and the Upper Market Street area, a beam of sunlight beautifully brightened a freshly painted yellow Victorian house. In the background the sky was a severe deep blue. The contrasts in light and color were vivid. He remembered the punchline from a Steve Martin joke: “But the most amazing thing of all is I get paid for doing this.” Even as he saw the house gleaming in sunlight, a few drops of rain began to gently fall on him. As he rode among the canyons and hills of the Upper Market area, the light rain continued. But on Twin Peaks Boulevard, high above the trees, a strong southerly wind blew more rain into his face. Turning west, he rode downhill and the rain subsided. He then crossed Laguna Honda Boulevard into the Forest Hill neighborhood. But as he ascended fog shrouded Forest Hill, drops of water fell from the trees. On one street near the hilltop, the trees dripped enough to create a persistent drizzle.

Just as seemingly minor natural forces can impact a microclimate, so can seemingly minor human activities cause a microclimate change. These effects may be beneficial or harmful.

A grove of trees or a building can cause microclimate change. San Francisco’s natural environment had very few forested areas. Most of the trees we see in San Francisco were introduced by humans in the last 150 years. Trees can moderate wind and cold by serving as windbreaks or they can decrease heat by providing shade.

In the 1800s, when Market Street, the City’s major transit and commercial thoroughfare was laid, it was blocked by Mint Hill, which separates the Castro and Hayes Valley neighborhoods. Workers leveled the hill’s southern slope to allow Market to pass over what is now Duboce Street. As a result, westerly winds now sweep unimpeded along Market Street.

In its natural state, San Francisco was covered by sand dunes. The western half of the City was covered by sand dunes, some of which extended as far east as the Mission District. Even today, San Francisco’s gardeners must contend with sandy soils in many of the City’s back yards. To tame the moving sands, San Franciscans planted trees and other plants. The planting of trees to make Golden Gate and McLaren Parks were the largest of those efforts.

The filling of San Francisco Bay has directly and indirectly affected the City’s microclimates. Filling the Bay shrunk its size and as a result, the moderating influence exerted by a large body of water also decreased. In addition, the filling of the Bay in San Francisco allowed for the construction of major skyscrapers on the landfill. Today, most of the City’s highest buildings stand on landfill. As in most cities, San Francisco’s high rise building area creates an urban heat island (UHI). Discovered almost 200 years ago in London, the heat island effect is in urban areas with high densities of buildings and low amounts of vegetation. As a result, temperatures are higher in UHIs than in less developed surrounding areas.

These examples show that human actions can affect microclimates in a variety of ways. But if well intentioned, thoroughly studied and properly managed, these actions can actually be beneficial to the environment and even mitigate previous damage done by humans.

By Howard Williams – Veteran MAC Team Member