Some Forecasting Rules
	See also: 'Hurricane Safety Rules'
	A shipmaster should not, in general, need to make his own forecast if he has received a direct weather forecast, or a forecast map for his area. However, it may be that an adequate forecast is not available for a particular area, in which case the following rules will enable a reasonable reference to be drawn about the weather conditions to be expected.
	A - If a weather map is available: A series of weather maps will enable simple deductions to be made about the future movement of pressure systems using the rule of persistence. 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. For example: 1. Frontal depressions tend to move in families, each depression following its predecessor but in a slightly lower latitude. 2. 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. 3. Depressions tend to move with the wind around large, well-established anticyclones. 4. An occluded depression tends to move slowly and irregularly. 5. If the depression has a large warm sector it has a tendency to deepen. 6. As a depression occludes the deepening processes decrease. 7. A non-frontal depression tends to move with the strongest wind circulating around it. 8. A front which is crossed by isobars which are close together will probably be fast moving. 9. A front which is parallel to the isobars will be slow-moving. 10. Warm fronts move at about half the speed of the wind at the front. 11. Small anticyclones usually move faster than large ones. B - Even without a weather map the observer who notes carefully the weather changes taking place can form for a period of about 6 hours ahead, a useful estimate of the weather to come. The following points should be of assistance: 1. A falling barometer is an indication of bad weather to come. 2. A rising barometer does not necessarily indicate good weather to come. In unsettled conditions a rapid rise can quickly be followed by a rapid fall. In general, if the barometer rises and stays high for at least 12 hours, 24 hours of settled weather may be expected. If it stays high for 24 hours, several days of settled weather may be expected. 3. If cirrus cloud approaches from the west, and at the same time the barometer is falling, and the wind is backing (N.H.) then bad weather may be expected. 4. Fast-moving, high clouds often indicate that bad weather is to follow. 5. If after the passage of a cold front the barometer falls and the wind backs (N.H.) further bad weather may be expected.