This assumes that future movetnents and changes in intensity of pressure systems may be estimated from past development. Obviously, the shorter the interval between weather maps the better the estimate that can be made of future developtnents, but in any case forecasts for a greater period than 6 hours ahead will not be reliable. Even if only one map is available useful deductions can still be made. Frontal depressions tend to move in families, each depression following its predecessor but in a slightly lower latitude. A depression with a warm sector tends to move with the wind parallel to the isobars in the warm sector at about three-quarters of the wind speed.
Depressions tend to move with the wind around large, well-established anticyclones.
An occluded depression tends to move slowly and irregularly. If the depression has a large warm sector it has a tendency to deepen. As a depression occludes the deepening processes decrease. A non-frontal depression tends to move with the strongest wind circulating around it. A front which is crossed by isobars which are close together will probably be fast moving.