Why are 'Regional QNH' and 'aerodrome QNH' often quite different? Leaving the zone I transfer to regional, and often its a ~200'+ difference....I assume its something to do with the update time..........
Regional QNH is the lowest pressure recorded over the whole region. A high pressure gradient across the region (probably indicated by high winds) could therefore cause a big difference.
Neither of those. Regional QNHs are FORECASTS made in advance. It is lowest forecast pressure which might be experienced in the region over an hourly period. Normally the Regional for the current hour and the next hour are available.
This is another example of how the UK antiquated system of Regional Pressure is no use to man nor beast.
It is useful when flying in the Highlands of Scotland and preventing flight through Cumulo Granite. There are very very few aerodromes up there with facilities to get the QNH. And those that do are usually a great distance away. For more populated parts of the UK, then using a local aerodrome QNH does make more sense.