Question 1

Why is the wind at sea surface level often different from the gradient wind (wind shown on synoptic chart)?

Accepted Answer

Surface friction slows the wind and turns it towards lower pressure, backing in the Northern Hemisphere and veering in the Southern Hemisphere.. Gradient wind is the theoretical friction-free wind flowing along curved isobars, with the pressure gradient and Coriolis forces providing the centripetal acceleration required for the curved flow. At the sea surface, friction slows the wind and causes it to cross the isobars towards lower pressure. Compared with the gradient wind, surface wind backs (turns anticlockwise) in the Northern Hemisphere and veers (turns clockwise) in the Southern Hemisphere. A speed reduction of 10 to 30% is an approximate guide, with the actual reduction depending on conditions. Frictional effects are generally greater over land, helping explain differences between winds offshore and those reported by coastal stations.

Question 2

You are at sea and the water surface shows numerous white horses with some spray. What Beaufort force does this suggest?

Accepted Answer

Force 5-6. Beaufort visual indicators: F4 (11-16kt): small waves, frequent white horses. F5 (17-21kt): moderate waves, many white horses. F6 (22-27kt): large waves, extensive white foam crests, spray. 'Numerous white horses with spray' fits F5-6. For a Day Skipper, Force 5-6 is challenging, consider whether to continue or seek shelter. Force 7+ (near gale), sea heaps up, foam in streaks.

Question 3

The wind has been SW and is now shifting to W and then NW. Is this a veer or a back?

Accepted Answer

A veer, the wind is shifting clockwise (SW → W → NW). VEERING: wind shifting clockwise in direction (in Northern Hemisphere). SW → W → NW is clockwise, a veer. BACKING: anticlockwise (NW → W → SW). A veering wind in the NH is often associated with a cold front passage or improving weather. A backing wind often indicates deteriorating conditions. In sailing: a veer is usually favourable for a vessel on starboard tack heading NW.

Wind, questions and explanations

Why is the wind at sea surface level often different from the gradient wind (wind shown on synoptic chart)?

You are at sea and the water surface shows numerous white horses with some spray. What Beaufort force does this suggest?

The wind has been SW and is now shifting to W and then NW. Is this a veer or a back?

Practise this properly

Wind, questions and explanations

Questions

Why is the wind at sea surface level often different from the gradient wind (wind shown on synoptic chart)?

You are at sea and the water surface shows numerous white horses with some spray. What Beaufort force does this suggest?

The wind has been SW and is now shifting to W and then NW. Is this a veer or a back?

Practise this properly