Jonny, you’ve got to get past your obsession with air pressure. At or near sea level, the barometric pressure is not substantially different between the poles and the equator. Yes, the troposphere is thinner at the poles, but it’s colder and denser so air pressure is about the same. On average, the barometric pressure is the same in Scotland as it is in Florida. The vapor map you posted shows an amazing correlation with temperature, that’s no coincidence.

Obviously we’re overdue to reign this thread in. This whole atmospheric science discussion was born out of someone’s comment about engine maintenance method X not being good enough for the Scottish climate. So, who’s climate is worse? If you’re concerned about moisture during storage then it’s all about relative humidity. Who’s got higher relative humidity indoors? In the winter months it’s probably Florida. Why? Because their dewpoint is usually higher, the air’s holding more water. If you take air from Scotland and air from Florida and heat it to the same (indoor) temperature, you’ll find that the Scottish air has a lower RH.

If you disagree, grab your favorite psychometric chart and PM me.