for the met exams you dont needs to go that deep as the exams only ask you which one is greater.
i.e
Q QFF at an airfield located 400 meters AMSL is 1016Hpa the air temp is 10 degree c higher than standard atmosphere what is QNH?
A 1016 hpa
B it is not possible to give a definitive answer
C more than 1016 Hpa
D less than 1016 Hpa
for this problem think of a XNOR logic gate (sorry for my ASCII pictures it meant to be a XNOR Gate!)i.e
Above MSL =1 ___________
Temp above ISA=1________))----1= QNH>QFF (QNH more than QFF)
or if it was the other way i.e
Below MSL =0___________
Temp below ISA =0______))------1= QNH>QFF (QNH morw than QFF)
so in the above question it can be seen that QNH is more than QFF so C is correct.
or i.e.
the QNH california is 69m below MSL and is 1018Hpa the air temp is ISA+10 what is QFF?
A 1018
B not possible to answer
C more than 1018
D less than 1018
draw XNOR logic gate
Below msl=0___________
above ISA=1__________))----------QNH<QFF=QNH more than QFF
so C is correct!
try it!
and if you need a refresh on logic gates look here
Logic gate - Wikipedia, the free encyclopedia