Extract from "SOUTHERN AFRICAN CRUISING NOTES"
Cruising the Southern African coast (east to west) from Richards Bay to Durban to Cape Town and beyond. (The Cape of Good Hope Cruising Route)
The sailing route from Durban to Cape Town is governed by three major factors:
- THE VARIABLE WEATHER PATTERNS: These patterns are governed by low and high pressure systems moving across from the Atlantic Coast of South America and travelling eastwards up the eastern seaboard of the South African coast.
- THE AGULHAS CURRENT: This current is one of the great ocean currents of the world, running mainly from northeast to southwest, following the two hundred meter contour of the continental shelf and dissipating over the Agulhas Bank south of Mossel Bay. The main axis of the current is on or near the two hundred meter line and can run at up to 6 knots at it's fastest point. (Full details in the relevant sections of the book).
- SHELTERED HARBOURS AND ANCHORAGES: The third factor is the lack of sheltered harbours and anchorages, particularly between Durban and Port Elizabeth.
WEATHER RELATED SAILING CONSIDERATIONS:
- The duration of an individual frontal low is usually from three to five days. Cold fronts tend to travel faster than the associated warm fronts, and an occlusion occurs when they merge. In these conditions the warmer air rises above the surface, and the system decays rapidly.
- The closer the isobars are to one another around such highs or lows, the greater the pressure gradient, and consequently the stronger the associated winds. Latitude also affects wind strength, with the winds being weaker closer to the poles for a given pressure gradient. However, because of the balance of forces in the two systems, anticyclones tend to be large, with the strongest winds near the perimeter and light variable winds near the centre. In contrast, for cyclones the strongest winds are associated with small, deep systems.
- The air above particular regions may acquire reasonable uniform properties while passing over these regions. Temperature is one such property that can be used to identify air masses. But what is of importance are the rapid changes that can occur when one air mass is replaced by another at a particular place.
