Melbourne Osaka Double-Handed race  NE Trade Winds 11/4/13

by Robin Hewitt
The yachts have, or are about to enter the North-East trade winds zone characterized by a steady flow of wind in a roughly steady direction. These are caused by outflowing air from the sub-tropical ridge of high pressure systems usually about 30 degrees latitude north or south and migrating according to the season. The air from the highs travels towards the uplift from the Inter Tropical Convergence Zone (ITCZ) which in turn travels in the very upper atmosphere toward the poles until descending as the high pressure systems and thus forming a vertical circulation. The cloud formation in the trades is quite different usually having a base of about 1 km height and seldom above 10 km. These cumulus clouds are vertical or have a ‘lean’ according to the strength of the trade winds. They are capped in height due to the descending air of the highs and may be quite patterned. With the consistent winds of the trades, we can expect the yachts to make rapid progress until they again meet calms being the center of high pressure systems. Some differences can be expected due to just where their path and the highs appear. Following the calms they will again be in a variable westerlies region similar to when leaving Melbourne and some strong winds are possible. As shown in this weather map, large areas of fog may also provide some interesting experiences, -Robin Hewitt