With the use of monthly relative humidity for the period of 1908-2003 in Seoul, the temperature and the water vapor effects on relative humidity changes were analyzed as the causes for urban effect. The relative humidity of Suwon and Inchun since 1960s near Seoul was used for the evaluation of urbanization.

  To analyze the causes of humidity change in urban area, we attempted to examine the urban effect quantitatively and divide it into the water vapor and temperature effect. In this study, we used the evaluation method of composite calculation for urban effect with the data observed in a big urban city and a rural city. We also investigated the   characteristics of climatic change in urban area based on the variability of annual mean NDVI. To quantify the urban effect, we assumed that observations in urban area contain the influence of macro-scale, local and urban scale, and observations from rural site doesn't include urban scale. Natural variability in urban area was obtained by assumption for simultaneous climate condition for early 15 years. The main idea in this study is to separate the water vapor and temperature effects for urban effect in relative humidity. Inversely the sum of temperature and water vapor effect could not be equal the total urban effect.

The climatic change of relative humidity in Seoul has a continuous decreasing trend except a few short periods. The urban effect increased with the temperature effect to 1980s. This is due to the change of land-use type as the urban has developed. After 1980s, temperature effect on urban effect decreased due to the apparent urbanization in Suwon, which is observed in NDVI from NOAA AVHRR data. The influence of water vapor tends to be decreased after the dramatic increase in 1970s with excess of water vapor owing to change of lifestyle.