I agree with Keithl's reply... but I also agree it's difficult to explain. I'll have a go at explaining it differently, hopefully one or the other reply will make some sense.
- Pressure increases as altitude decreases
- The rate at which pressure decreases is dependant on the temperature
- At an airfield, it's not possible to measure the sea-level pressure, because sea-level is (usually) underground somewhere. But you can measure QFE very easilly.
- Having measured QFE, you then need to convert that to sea-level pressure.
Following me so far? Right, now it gets fun.
Meteoroligists needs to know what the actual sea-level pressure is. In other words, if you drilled a hole down to sea level, what would the pressure be at the bottom of the hole? To do that, they look at the air temperature, and they work out how much the pressure would change by as you go down to sea level, based on the current temperature. (They make some assumptions about how the temperature will change as you go down towards sea level, too.) The number they come up with is QFF - and this is the number you will see on surface pressure charts.
Now, assume you set QFF on your altimeter, and you're on the ground at an airfield with an elevation of 1000'. The temperature is ISA-10 degrees. Rule of thumb says that for every 10 degrees from ISA, your height above datum will be out by 4%. The datum, in this case, is sea level, so your altimeter will indicate 1040', not 1000' as expected.
That's why pilots prefer QNH. This time, instead of looking at the actual outside temperature, you pretend that the temperature is exactly as per ISA. Because the altimeter makes the same assumption about temperature being ISA, when you set QNH on your altimeter it will correctly indicate the airfield elevation. (But if you drilled a hole down to sea level and lowered the altimeter down it, it would read incorrectly.) The datum, in this case, is the airfield elevalation.
Is that any clearer than Keithl's answer? Or have I confused you even more???
FFF
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