郑在型 郑亨敦:Navis.gr - Some Forecasting Rules

Some Forecasting RulesSee also: 'HurricaneSafety Rules'

A shipmaster shouldnot, in general, need to make his own forecast if he hasreceived a direct weather forecast, or a forecast map forhis area. However, it may be that an adequate forecast isnot available for a particular area, in which case thefollowing rules will enable a reasonable reference to bedrawn about the weather conditions to be expected.
A - If a weather map is available:

A series of weather maps will enable simple deductions tobe made about the future movement of pressure systemsusing the rule of persistence. This assumes that futuremovetnents and changes in intensity of pressure systemsmay be estimated from past development. Obviously, theshorter the interval between weather maps the better theestimate that can be made of future developtnents, but inany case forecasts for a greater period than 6 hoursahead will not be reliable.

Even if only one map is available useful deductions canstill be made. For example:

1. Frontal depressions tend to move in families, eachdepression following its predecessor but in a slightlylower latitude.

2. A depression with a warm sector tends to move with thewind parallel to the isobars in the warm sector at aboutthree-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 andirregularly.

5. If the depression has a large warm sector it has atendency to deepen.

6. As a depression occludes the deepening processesdecrease.

7. A non-frontal depression tends to move with thestrongest wind circulating around it.

8. A front which is crossed by isobars which are closetogether will probably be fast moving.

9. A front which is parallel to the isobars will beslow-moving.

10. Warm fronts move at about half the speed of the windat the front.

11. Small anticyclones usually move faster than largeones.

B - Even without a weather mapthe observer who notes carefully the weather changestaking place can form for a period of about 6 hoursahead, a useful estimate of the weather to come. Thefollowing points should be of assistance:

1. A falling barometer is anindication of bad weather to come.

2. A rising barometer does notnecessarily indicate good weather to come. In unsettledconditions a rapid rise can quickly be followed by arapid fall. In general, if the barometer rises and stayshigh for at least 12 hours, 24 hours of settled weathermay be expected. If it stays high for 24 hours, severaldays of settled weather may be expected.

3. If cirrus cloud approaches from the west, and at thesame time the barometer is falling, and the wind isbacking (N.H.) then bad weather may be expected.

4. Fast-moving, high clouds often indicate that badweather is to follow.

5. If after the passage of a cold front the barometerfalls and the wind backs (N.H.) further bad weather maybe expected.