QFF is atmospheric pressure at sea level.   QNH is the setting which will give airfield elevation when parked on the airfield (n.b. altimeter is calibrated to ISA).   In cold conditions a column of air will shrink so the ‘cold’ QNH will be less than QFF and the ‘warm’ QNH will be more than QFF.

 

 

 

                                   

                                                                               FIELD

                                                                              ELEVATION

 

COLD’ QNH

                                                LESS THAN QFF

                                               

QFF SEA LEVEL

                        PRESSURE

 

                                                                                        ‘WARM’ QNH

                                                                                     GREATER THAN QFF