Altimetry
Nice to see that these quetsions are still around! Historically, quite a few pilots cam a cropper when the did not reset their altimeters and flew from an area of high atmospheric pressure to one of low pressure. Regional QNH values - (the LOWEST QNH forecast) - were introduced to meet this problem. "High to low watch out below" sums it up well. Hope the new "Centre of Excellence" is able to write better questions!