The Effect of Cities upon Horizontal and Vertical Temperature Gradients

For three California cities of various sizes, two-meter-level temperature patterns were determined by intensive traverses with automobile-mounted thermistors, and vertical temperature gradients in the lowest 1000 feet were measured by wiresonde simultaneously at urban centers and peripheral open areas. In 35 evening surveys under varying weather conditions, a characteristic horizontal temperature pattern existed for each city. Temperatures increased from peripheral open lands to built-up center in direct proportion to structure density. Characteristics of the urban gradients have been analyzed in relation to city size and to meteorological parameters. Vertical temperature data showed that built-up areas frequently caused instability up to about 3 times roof height in otherwise stable air and that a “crossover” point sometimes existed above which the air over the urban center was cooler than that over surrounding country.