Structural Characteristics of Atmospheric Relative Humidity During Lightning Activity in Yunnan Province

Using monitored cloud ground lightning data of lightning detection system in Yunnan Province and relative humidity parameter in NCEP/NCAR data from 1 June to 31 August in 2007, structural characteristics of atmospheric relative humidity during lightning activity are analyzed. The results show that lightning activity in Yunnan is most frequent in summer because of high temperature and high humidity, and there's a daily apex which appears from 16:00 to 17:00 and a valley which appears from 08:00 to 11:00 because of daily variation of solar radiation. The lightening activity is uneven and discontinuous on day to day variation because of environmental conditions of atmospheric relative humidity. Some specific environmental humidity conditions are necessary for lightening activity. Too high humidity may restrain ascending motion of convective development, while low humidity is insufficient for convective cloud to develop. Moreover, convective development and intensification are aroused, and convective cloud developing upwards is urged by some dynamic triggering condition when unstable energy runs up to a certain extent. When electric field intensity reaches to a certain extent, the thunder phenomenon occurs. Lightning activity in Yunnan usually happens in the typical environmental atmosphere with certain vertical configuration: The relative humidity is not high at 850 hPa, but at 700 hPa it's very humid and then becomes dry at upper layers. The relative humidity increases with height below the 700 hPa, forming high moist region at the middle layer and deceases above it. This environmental condition of relative humidity is in favor of thunderstorm weather occurrence and thunder formation. First, relative humidity increase below 700 hPa is favorable for ascending motion of water vapor and cloud formation. Second, dry atmosphere at upper layers and moist atmosphere at lower layers cause convective instability so that convective cloud may be urged to develop and promote lightning activity. When lightening occurs, relative humidity at low layer is about 40%—75%, at middle layers of 600—700 hPa it is above 80% and usually reaches up to 90%—95%, and the humidity decreases to about 35%—60% at upper layers of 250—400 hPa. It is also found that deeper middle moist layer will probably result in stronger thunder process.