For more than a hundred years, researchers have measured daytime and nighttime temperatures that are consistently higher in cities than in the surrounding countryside. This is the "urban heat island" effect caused, it is said, by the concentration of structures, hard surfaces, people, and the heat-generating mechanical systems that allow cities to survive.
Depending on geography and climate, the nighttime temperature of an urban core can be as much as 22 degrees warmer than its rural fringe.
A new study by Yale researchers has clues to why daytime temperatures are higher downtown, with implications for how Los Angeles may fare as average temperatures rise and the city's landscape is reshaped by development, drought, and public policy.
The Yale research team pins the disparity between urban and non-urban daytime temperatures on the efficiency of heat transfer to the lower atmosphere through a process called convection. Green and leafy landscapes are better at convection, lowering temperatures in rural areas. Slab-sided highrises aren't efficient, trapping heat in the lower atmosphere and baking urban dwellers.
The research also found a paradox in the data. The temperature difference between green landscapes and hardscapes of concrete and asphalt was greatest in the relatively rainy parts of North America. In the dry Southwest, where sparse vegetation surrounds urban cores, daytime temperatures in some cities were slightly lower than the desert fringe. (The paradox disappears at night, when the stored heat in the urban hardscape keeps temperatures higher in both the wet and dry parts of the country.)
The Yale study put Los Angeles toward the middle of the range of temperature differences caused by the heat island effect. New Orleans is about 10 degrees F hotter in the daytime than its surroundings, but in Los Angeles, the difference is about 6 degrees F.
Integrating these findings into the forces that are evolving the landscape of Los Angeles isn't going to be easy.
The several urban cores of Los Angeles aren't surrounded by a rural fringe but by suburbs. As they inevitably grow more dense and built up, the size and shape of the city's heat islands will change. We don't know to what extent. Water conservation and the trend toward lawn-free yards and less greenery will change the balance of microclimates at the urban-suburban edge even more. An urban forest -- the EPA's recommended mitigation for heat island impacts -- makes demands on limited water supplies and the city's resources to establish and maintain.
Former Mayor Antonio Villaraigosa promised Los Angeles one million trees. When he left office, only half that number had been planted. Who knows how many survive today.
On the other hand, more pocket parks in dense neighborhoods, more "living walls" of vertical plantings on highrises, and the general greening of the Los Angeles River should have cooling effects on the city's microclimates.
Better awareness of the heart island mechanism might lead to standards for heat shedding features in new construction. Some cities, for example, are beginning to require light colored roofing materials that reflect sunlight upward and away from the structure.
We know more about the nature out of which -- and into which -- we've built Los Angeles, but we still know far too little as the city's urban centers rise and spread.