The variations of five-day mean sea-level pressure, averaged about selected latitude circles in the northern hemisphere, and the variations of differences between five-day mean pressures at selected pairs of latitudes are examined statistically. The northern hemisphere is found to contain two homogeneous zones, one in the polar regions and one in the subtropics, such that pressures in one zone tend to be correlated positively with other pressures in the same zone and negatively with pressures in the other zone. Considerable difference is found between the seasonal and the irregular pressure-variations which result from mass transport across the equator, but the seasonal and the irregular variations of pressure differences resemble each other closely, as do the seasonal and the irregular pressure-variations which result from rearrangements of mass within the northern hemisphere. The most important rearrangements appear to consist of shifts of mass from one homogeneous zone to the other. These shif... Abstract The variations of five-day mean sea-level pressure, averaged about selected latitude circles in the northern hemisphere, and the variations of differences between five-day mean pressures at selected pairs of latitudes are examined statistically. The northern hemisphere is found to contain two homogeneous zones, one in the polar regions and one in the subtropics, such that pressures in one zone tend to be correlated positively with other pressures in the same zone and negatively with pressures in the other zone. Considerable difference is found between the seasonal and the irregular pressure-variations which result from mass transport across the equator, but the seasonal and the irregular variations of pressure differences resemble each other closely, as do the seasonal and the irregular pressure-variations which result from rearrangements of mass within the northern hemisphere. The most important rearrangements appear to consist of shifts of mass from one homogeneous zone to the other. These shif...