Unrealistic data values may occur for a number of reasons. These are detected and hard flagged by simply comparing the minimum and maximum value of all points in the record to some fixed limits considered unphysical. For this data, the absolute limits are 30 m s−1 for the horizontal wind components, 5 m s−1 for the vertical wind, −20° to +60°C for air temperature, and 2–30 g kg−1 for specific humidity.
Several records in Microfronts95 are hard flagged by the absolute limits test (appendix, Table A1). The majority of these (18 out of 22) are obvious instrument problems. However, four records hard flagged for vertical velocity values exceeding 5 m s−1 are classified as physical after further inspection. In these cases, the large vertical velocities are associated with exceptionally high turbulence levels and strong surface heating, and the hard flags are changed to soft flags.