On the Relationship of Cirrus Clouds and Upper Tropospheric Relative Humidity Fields with Symmetric Instability

The relationship of cirrus clouds and areas of increased relative humidity in the upper troposphere with symmetric instability developing on the subtropical (anticyclonic) side of jet streams are explored. Theoretical arguments are supplemented by case studies related to the Northern and Southern hemispheres. Using satellite images of cirrus clouds and reanalysis data, the values of the vertical eddy viscosity, which compensates for the inviscid growth of disturbances in case of symmetric instability, are estimated. At the levels of 250 and 200 hPa, a significant correlation between high relative humidity and potential vorticity characteristic of symmetric instability is confirmed.