Artificial and Natural Diversity
San Francisco is not a large city. Located on the tip of a peninsula with the Pacific Ocean on the west and the San Francisco Bay on the east, the city is only about 7 miles (11.3km) square. The famously steep hills contribute to an illusion of greater size—as if what the city lacks in width, it makes up for in height. Although it is nicely compact, San Francisco is composed of numerous distinct neighborhoods that are often profoundly different from each other in terms of architecture, culture, ethnicity, and overall demographics. Chinatown, for example, is just steps away from North Beach (our version of Little Italy), yet the local vibes of the two areas could not be more different. But as you move around San Francisco, you may notice something even more striking than the varying neighborhoods: major shifts in weather. San Francisco is often held out as a textbook example of microclimates.
A microclimate is a weather pattern that’s localized in a small area and different in some significant way from the weather of nearby areas. The variation can be one of temperature, humidity, rainfall, wind, or any combination of these. For example, there are parts of San Francisco that barely see the sun all summer long; along the coast, especially, it’s frequently cold, foggy, and windy. In other parts of the city, though, fog is virtually unknown; it may be sunny and 10 or even 20 degrees hotter than spots just a mile or two away. In all, there are about 30 well-defined microclimate regions within the city, and even more when you travel slightly farther away. When I was commuting to work just south of the city, I often drove out of rain and into sunshine or vice-versa, but nearly always found the southern parts of the peninsula much warmer than San Francisco.
http://itotd.com/articles/223/microclimates/