Hi there,
The correction to be applied to station barometric pressure to reduce it to sea level is given by the formula
M = P multiplied by ((10 to the power m) -1)
Where M is the correction in hectoPascals
P is the measured barometric pressure
And m = h divided by (18429.1 + 67.53T + 0.003h)
Where h = the height of the station above sea level in metres
And T = temperature at the station in degrees Centigrade.
If you need to calculate QFF from station pressure use the actual temperature measured at the station. If you need QNH use the standard temperature corresponding to the measured pressure.
Looking at a set of tables derived from the above formulae for a barometric pressure of 1000 hPa at 5 metres above sea level gives a range of corrections from only 0.7 hPa to 0.5 hPa for the temperature range of –15C to +40C. So looks like your assumption was correct.
For a station 250 metres above sea level the values of M range from 33.6 hPa to 27.6 hPa for the same temperature range. Using the standard temperature of 15.1 C for 1000hPa at 250 metres amsl gives a QNH of 1030 (M = 30), whereas the QFF could vary from 1033.6 to 1027.6 for the temperature range –15 C to +40 C.
Hope this helps.
Xacto