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