Plan ahead for weather changes

Waimarino Bulletin, Volume 4, Issue 27, 2 December 1986, Page 14

Plan ahead for weather changes

The following article from the NZ Water Safety Council is the second in the series which the Bulletin intends to publish in the lead up to summer and the boating season. Today the subject is 'Weather'.

Marine forecasts broadcast on radio stations at set times give the relevant information for boaties. but the predictions of the general forecast for the land area do not apply the same at sea. For a longer trip it pays to watch weather maps for several days ahead to note development of high and low pressure areas and the location of fronts. When a high and a low are together, creating a close spacing of isobars resulting in high winds and big waves, it is a bad time to be at sea. A copy of the latest weather map can be updated with information supplied by forecasts on the

radk>, or from the nearest MeteorologicalOffice. Information can be had from some local airports, aero clubs, harbourmasters and even the Ministry of Agriculture and Fisheries. Organisations that rely on up-to-date forecasts may be willing to share their information. such as the Maui Oil rig which has details of conditions about sea and wind off Cape Egmont. A barometer will indicate how a high or low pressure area is moving, and if a low is intensifying this will be shown by a more rapid fall in pressure than is on the weather map. Cloud developing in the south or west indicates fronts approaching, and any thickening of cloud should be taken as a warning. New Zealand's rugged land mass creates local conditions that are quite different from those shown on the weather map. The most obvious is the funneling effect of a range of hills, and one of the best known is the concentration through the Cook Strait caused by the Tararua Ranges when the wind is from the west. When the wind is blowing off land masses it will be felt as just a light breeze on the beach. but becomes stronge: further out to sea or the middle of a lake.

When wind blows parallel to the shoreline it will be concentrated by a headland. so when a boat rounds the headland it meets strong gusts — these can be avoided by keeping further out from the land. It is not always easy to work out exactly how the land will affect the wind in any particular place but local fishermen and yachters can often provide advice. In the summer sea breezes can bring big changes in the wind direction and strength. In some districts the sea breeze is known as the "eleven o'clock chop" because it usually springs up before noon and lasts for several hours, often causing a lumpy sea to develop. Sea breezes are caused by air rising above land as it is warmed by the sun and cooler sea air blowing towards the land. The hotter the day — the stronger the wind and, if it is blowing in the same direction as the prevailing wind, quite strong gusts can develop by midafternoon. If the sea breeze is blowing against the prevailing wind it can cause light winds. The stronger the wind — the»higher the waves. Winds of over 15 knots can create choppy conditons for small open boats. Winds of 25 knots create difficult waves for even larger launches. A 5-knot wind blowing over water flowing in the opposite direction at 10 knots will create waves as large as those created by a 1 5-knot wind on still water.