42 SINGLE OBSERVER WEATHER WATCHKEEPING

162. LOCAL MODIFICATION OF WEATHER NEAR COASTS

The weather near a coast as predicted either in an official forecast or from your own weather map from the general air mass and pressure system is likely to be modified locally in several ways, and all mariners should be aware of where and when these modifications are likely to occur.

If the coastline is steep-to, winds approaching such a shore at an angle are usually deflected along the shoreline and increased a little in speed. Strong winds blowing directly on to a steep coast at right-angles to it, create contrary, gusty winds close inshore;

 The wind deduced from the prevailing pressure system may be modified by land and sea breezes in fine, settled weather in the summer. On warm, clear days the land becomes hotter than the adjacent sea, convection is set up in the warmed air above the land, and cooler air from over the sea is drawn-in to replace it, this onshore wind being called a sea breeze, and occurring from 6 to 8 hours after sunrise until shortly before sunset. A sea breeze can blow anything up to force 4 and extend up to 20 miles offshore. After sunset, the land becomes colder than the sea and the process is reversed, a land breeze blowing seawards from about midnight to sunrise, although it is never as strong as the sea breeze and rarely extends more than 5 miles seaward.

Winds approaching a strait or estuary tend to blow along the strait and increase in speed as the strait or estuary narrows, particularly in the afternoon, There is often no such tendency during the night and early morning.

 Cold, offshore winds (e.g. from snow-covered land) are usually squally off hilly coasts.

Near headlands or steep—sided islands there may be large changes in wind direction (up to 90°) and speed, because wind objects to rising over high ground so that much of it escapes round the ends.

Drizzle and very poor visibility are often experienced along coasts toward which a warm, moist air stream is blowing. Poor visibility is experienced to leeward of an industrial area on the land, from which smoke is blowing.

163. SINGLE OBSERVER WEATHER WATCH-KEEPING

Single observer weather watch-keeping is of vital importance to the small-boat navigator because it enables him to confirm whether or not the weather is conforming to the predicted pattern, as determined from the Shipping Forecast and his own weather map, and if it is not, to appreciate the change in the weather pattern which has occurred and to take the appropriate action for the safety of his vessel and his crew.

Probably the only meteorological instruments the small-boat owner will possess will be an aneroid barometer, a thermometer and his own senses, but these are quite sufficient for his purpose if he is alert and observant and can apply his knowledge of Meteorology. To keep a watch on the weather, the navigator should note the reading of his barometer, and the temperature from his thermometer, at regular intervals in his log book. He should also observe and record the wind direction and force, the amount of cloud cover, the extent of the visibility, and any other weather phenomena he may observe (such as rain, thunder and lightning, etc.). It is of no use whatsoever just making a “mental note” of these observations – they must be recorded in the Log book. This is because their significance is only rarely immediately apparent, but only after a period of time has elapsed, and the small-boat navigator has so many things to watch and attend to, he can never expect to remember with any accuracy what his observations were four or eight hours previously.

The average atmospheric pressure for the area in which the navigator is sailing is obtainable from the appropriate Admiralty Sailing Directions (Pilot Book), and from various meteorological charts published by the Met. Office. If the barometer reads 4mbs or more below average for the time of the year, and is steady or falling, the chance of fair weather continuing for the next 24 hours is small. A low barometer indicates unsettled weather, and if it is falling, increasing wind force. If the barometer is 4mbs or more above the normal and is steady or slowly rising, fair weather will probably continue for at least the next 12 hours, and if the pressure is 8mbs above normal and slowly rising, fair weather will continue for at least 24 hours. Generally speaking a rising “glass” (as the barometer is familiarly called) indicates improving weather, and a falling one a deterioration, but if the speed of rise or fall is rapid, the change is likely to be short-lived because you are probably near the path of a Low, behind it with a rapid rise and in front of it with a rapid fall. On the other hand, a slow, steady fall indicates the approach of a Low and a warm front. The speed at which a barometer falls is not a reliable guide to wind strength as the pressure will fall more rapidly if you are sailing towards a Low than if you are stopped or sailing away from it and the Low is overtaking you, and also because you may be sailing along the line of an isobar with steep gradients on either side of you.

In taking air temperatures, the navigator should ensure that his thermometer is so placed as to be on the windward side of his vessel, out of direct sunlight, and where it will not be affected by heat radiation from nearby objects, or by warm air from any source on board. If he is in doubt about the type of air mass in which he i is situated, the observer can take a sea temperature: if the air temperature is higher than the sea temperature, he is in tropical air, and if the air temperature is lower than the sea temperature, he is in polar air.

By watching, and recording, his own weather in this way, the small-boat navigator can check that the weather is following the pattern predicted either in an official forecast or by himself from a weather map, and thus receive advance warning of any significant change which might endanger him and his craft. But, as in all things, proficiency in doing this will only come with practice, and the navigator should take every opportunity, both ashore and afloat, of recording Shipping Forecasts, constructing weather maps, observing the weather systematically and as accurately as his instruments will permit, and checking the validity of his own forecasts and those of the Met. Office. It may happen some day that his life may depend on this ability.