As noted in the two previous GeoCurrents posts, it is challenging to make a map of climate favorability partly because preferences vary significantly from person to person. This problem is largely avoided on the ingenious website myPerfectWeather, which allows visitors to construct personal maps based on their own preferences. Unfortunately, the variables that can be selected are limited to average daily high temperature, daily average dewpoint temperature, and daily average cloud cover. Despite this limitation, it is interesting to see how maps generated on the site differ significantly due to relatively small changes in the desired conditions. The first of these maps posted below surprised me by giving San Diego County a middling ranking, contrasting sharply with nearby top-ranking Los Angeles County. In both the Camelot Climate Index and the Yahoo! “perfect weather” map considered in the two previous posts, San Diego is ranked as having the most ideal climate in the United States.
As intriguing as it is, myPerfectWeather is useless for my own climate preference, as I care a lot about seasonality and precipitation, neither of which are considered. But I did enjoy changing the parameters and generating different maps. I wanted to make one that puts mild and cloudy San Francisco County in the top position, which was easy to accomplish (see the map posted below).
I focused on San Francisco in part because it is one of the smallest counties in the United States, coterminous with the city of the same name and covering only 47 square miles. Along the Pacific Coast of the United States, mapping climate at the level of the county is highly misleading,* as conditions vary considerably with proximity to the ocean. Even in miniscule San Francisco County, the west side is distinctly cooler and cloudier in the summer than the east side. In large Mendocino County (3,878 square miles) on California’s northern coast, such differences are pronounced, with a mean July high temperate of 65°F (18.3°C) in coastal Fort Bragg and 91.1°F (32.8°C) in interior Ukiah, the county seat.
To illustrate such county-level climatic variability, I extracted Los Angeles County from two different Köppen climate maps of California, both found in Wikipedia. The first of these maps shows five distinct climate zones in L.A. country and the second shows seven. Two main factors account for these significant differences: distance from the ocean and elevation.
Although I admire the Köppen climate-classification scheme and use it extensively, it is not without problems. On the final map posted below, note how most of the city of San Diego has a BSh climate (arid steppe hot), but its northwestern neighborhood of La Jolla is placed in the BSk category (arid steppe cold). Although La Jolla is slightly cooler than downtown San Diego, especially in the summer, the difference is too small to merit classifying the former as “cold” and the latter as “hot.” In the Köppen scheme, an annual mean temperature of 18°C (64.4°F) is the cutoff between “hot” and “cold” arid climates, which is essentially that of La Jolla. I think that the system could be improved by adding a “mild” category between these two extremes. Doing so would slightly simplify the climate map of southwestern California.
* myPerfectWeather acknowledges this problem by noting that “Data is shown at the county level, which may not reflect city-level variation.”