Evaluation of the urban effect of long‐term relative humidity and the separation of temperature and water vapor effects

Abstract Monthly relative humidity values for the period between 1905 and 2003 for Busan and Seoul were used to quantify the impact of urbanization on changes in the humidity in an urban area. In this study, the urban effect on observed relative humidity was separated into water vapor and temperature effects using observations in large urban areas and rural sites. The double‐station Evaluation (DSE) is a method that directly compares an urban (Busan) with rural (Miryang) sites. To estimate the urban effect in Seoul, the intermediate (Incheon) and rural (Suwon) stations were used for multistation evaluation (MSE) method. To quantify the urban effect, we assumed that observations in urban areas were influenced by macro‐scale, local and urban scales. Observations from the rural sites did not include the influence of urban areas. Natural variability in the urban areas was derived by assuming that climatic conditions changed simultaneously in the early period of data collection. The sum of the temperature and water vapor effects might not be responsible for the total urban effect owing to errors. The relative humidity of Seoul has continuously decreased over the long term except for a few short periods, and the urban effect in Seoul increased with the temperature effect in the 1970s. In Busan, the urban effect increased rapidly with the water vapor effect after the early 1980s. However, urban effects in both areas have decreased in the past few years. This might be due to changes in land‐use as urbanization developed. The influence of water vapor has decreased after dramatic increases in the 1970s and 1980s, which may be due to the production of excess water vapor due to lifestyle changes. Copyright © 2007 Royal Meteorological Society