Wind Quiz

In the Northern Hemisphere, the Coriolis effect and pressure gradient force combine so that wind flows roughly parallel to isobars with low pressure to the left. A slight inward deflection toward low pressure occurs at the surface due to friction. This is Buys Ballot's Law applied to isobars.

Over open water, reduced surface friction means surface winds are approximately 60–70% of the gradient wind (the pressure-gradient driven wind above the friction layer). This is higher than over rough terrain where friction is greater and the ratio drops to 50–60%.

In the Northern Hemisphere, winds back (shift counterclockwise) ahead of an approaching low — the counterclockwise circulation of the low pulls the wind direction counterclockwise as the low approaches. Backing winds combined with falling pressure is the classic deteriorating weather signal.

A deepening low is in the process of intensifying — pressure at its center is still falling. The pressure gradient is actively steepening, meaning winds will be stronger than what the current chart (which was analyzed at a fixed time) shows. Always account for the direction of change in addition to the current snapshot.

A halo is caused by refraction of light through hexagonal ice crystals in cirrostratus cloud. Cirrostratus is the second cloud type in the warm front approach sequence, appearing after cirrus. A halo typically indicates a warm front 12–24 hours away and should prompt a barometer check and forecast review.

Dark patches (cat's paws) are caused by wind ruffling the surface in localized areas. They are visible before the wind reaches your boat and show exactly where a puff is coming from and where it will arrive next. Tracking cat's paws is a key skill in light-air sailing — they tell you where the wind is and where it's going.

Widely spaced isobars mean pressure is changing slowly over horizontal distance — a shallow gradient. Wind speed is proportional to the pressure gradient, so widely spaced isobars mean light winds. The reverse is true: closely spaced isobars = steep gradient = strong winds.

Cirrus clouds reveal the direction of upper-level winds. If they are coming from the southwest, the upper atmosphere is already responding to an approaching system from that direction even before the surface front arrives. Combined with the surface easterly, this implies a warm front approach — expect cloud thickening and eventually rain.

A 5 mb fall in 3 hours is a rapid pressure drop indicating significant weather deterioration within 6–12 hours. The appropriate response is to reduce sail, note current position, review forecast and route options, and prepare the vessel. Waiting to see what happens is not the right approach.

Over open ocean with relatively low surface friction, surface winds are approximately 60–70% of the gradient wind. This is higher than over rough terrain (50–60%) due to lower friction over water.

Long-period swell from an unusual direction with no local wind is generated by a distant storm. Swell propagates outward from the storm source in all directions, traveling faster than most storm systems. This can be an early warning of a storm approaching from that direction 12–36 hours before official warnings reach you.

As the sea breeze deepens and strengthens through the day, the Coriolis effect increasingly deflects the onshore flow to the right (in the Northern Hemisphere). A southwest sea breeze in the morning on a north-south coast may become westerly by afternoon. This veering pattern is consistent and predictable on good sea breeze days.

The sea breeze front is visible as a line of advancing ruffled, darker water — the boundary between the calm conditions ahead and the incoming sea breeze behind it. On a calm morning, this line advancing from the horizon toward the coast is one of the most satisfying sights in coastal sailing.

Sea breezes are thermally driven phenomena that typically require synoptic wind below about 15 knots to develop fully. A 20-knot offshore synoptic flow has enough momentum to suppress the onshore sea breeze development entirely — the thermal pressure differential is insufficient to overcome the opposing flow.

Daytime solar heating creates a large temperature differential between sun-heated land and relatively cool ocean — driving a strong sea breeze. At night, radiative cooling of the land creates a smaller differential — land cools perhaps 10–15°F while the sea changes very little. The smaller pressure gradient drives a weaker wind.

The pattern of departing pre-dawn on the land breeze, working the fishing grounds during the calm of early morning, and returning home on the afternoon sea breeze is one of the most ancient and consistent coastal fishing practices worldwide. It uses both halves of the thermal cycle efficiently.

A light 5-knot offshore synoptic wind is not strong enough to suppress a sea breeze on a good heating day. The thermally driven onshore flow will build through the morning, overcome the light opposing flow, and by early afternoon will be the dominant wind. The transition from offshore to onshore may be marked by a brief calm period.

As solar heating begins and the sea breeze starts to develop, it drives turbulent mixing in the boundary layer. This mixing brings drier, warmer air down into the fog layer, raising the temperature above the dew point and dissipating the fog. The sea breeze's arrival in the morning often coincides with fog clearing — another reason to watch for the morning breeze.

The strongest sea breeze develops with maximum solar heating (clear sky), maximum land-sea temperature differential (cold ocean water means cooler baseline, but the key is hot land vs. cool sea), and no suppressing synoptic flow (light offshore is overcome, not reinforced). Cold upwelling coastal water creates a larger temperature contrast with sun-heated land, driving a stronger thermal circulation.

As the sea breeze deepens and the circulation cell grows taller through the day, the Coriolis effect acts more strongly on it, deflecting the flow to the right. This produces the characteristic afternoon veering — a southwest sea breeze in the morning becoming westerly by late afternoon on many mid-latitude coasts.

The land breeze peaks in the hours before dawn, providing a gentle offshore push. Traditional sailors worldwide have used this pattern for pre-dawn departures. It is particularly useful for leaving anchorages inside reefs or protected channels, where the afternoon sea breeze may create too much chop for a comfortable departure.

The Southern Ocean at 40–60°S has essentially no land interrupting the Westerly wind circulation. Wind can blow unimpeded around the globe, and waves can build to enormous heights over unlimited fetch. The same latitude band in the Northern Hemisphere is interrupted by continents that break up the circulation and limit wave fetch.

The Bermuda High (Azores/North Atlantic High) is a semi-permanent subtropical high-pressure cell whose position and strength directly control the wind pattern for the East Coast and western North Atlantic. Passage planning in this region requires understanding the current position and forecast movement of the Bermuda High.

Williwaws are violent katabatic winds in Patagonia, the Falklands, and the Straits of Magellan — cold, dense air that has pooled over elevated terrain and suddenly accelerates downslope. They can arrive in clear weather with minimal warning and reach hurricane force.

Land heats faster than water in sunlight. Warm air over land becomes less dense and rises, creating lower pressure at the surface. Cooler, denser marine air flows in to fill the gap — creating the sea breeze. The mechanism reverses at night when land cools faster than water, creating the offshore land breeze.

Wind funneling through a constriction must speed up to maintain the same mass flow rate — analogous to water accelerating through a narrower pipe. This orographic effect can significantly amplify wind speed in straits and channels compared to the surrounding open water.

Wind flowing around a headland is forced into a narrower path and accelerates — orographic acceleration. This effect can be dramatic: a bay that seems sheltered in 15 knots can have violent gusts at its headlands. Always check conditions beyond the protection zone before committing to a departure.

Sea breezes typically begin mid-morning as land heating gets underway and peak in early to mid-afternoon when the temperature differential between land and sea is greatest. They typically die near sunset as land begins to cool.

The Mistral is a cold, dry, strong wind funneling down the Rhône Valley into the Gulf of Lion (western Mediterranean). It can reach 40–60 knots and blows in multiples of roughly 3 days. It is one of the most powerful and consistent regional winds in European sailing waters.

The ITCZ is a variable band of converging Trade Winds near the equator. It migrates north and south with the seasons, and its width varies. Crossing it may mean days of frustrating calms interspersed with violent squalls, or a relatively quick passage if the band is narrow. Timing to narrow ITCZ width and passage planning to cross quickly minimize exposure.

At night, land radiates heat more quickly than the sea and cools faster. The cooler, denser air over land flows toward the warmer, lower-pressure surface over the water — creating the land breeze. It is typically lighter than the daytime sea breeze and is most pronounced in calm synoptic conditions.

The surface boundary layer is dominated by friction between moving air and the Earth's surface. This friction slows the wind near the surface. Moving higher in the friction layer reduces the frictional drag and wind speed increases, approaching the gradient wind speed above the boundary layer.

Sail twist opens the leech of the sail, allowing the top of the sail to fall off to a wider angle than the bottom. This compensates for the difference in wind angle between the deck (lower, slower wind, slightly backed) and the masthead (faster wind, slightly lifted). Under-twisted sails stall at the head in a shear environment.

Convective gusts occur when thermal convection drives downdrafts that carry faster-moving upper-level air to the surface. They are associated with cumulus development and squalls and are often stronger and more erratic than mechanical turbulence-driven gusts.

A sudden gust increases aerodynamic force on the sails, causing heel and weather helm — the boat's tendency to turn toward the wind. The helmsperson must actively counter this with helm or depower the sails to maintain course. An over-canvassed boat in strong gusts risks a broach.

Heading up (feathering) in a gust spills some wind pressure from the sails as the apparent wind angle widens, keeping the boat flatter and speed more constant. Bearing away in the lull rebuilds speed. This technique maintains momentum through the gust sequence rather than absorbing each gust as a knockdown.

A preventer rigged from the boom forward prevents the boom from swinging across in an accidental gybe. Without a preventer, a sudden wind shift or momentary inattention downwind can result in a violent uncontrolled gybe — one of the most dangerous situations aboard, especially with crew in the cockpit or on deck.

In convective conditions, gusts can be significantly above the mean — a mean of 15 knots with gusts to 28–30 is common near squalls. The gust determines the maximum stress on sails, rigging, and crew. Reefing for the mean while being over-canvassed in the gusts is where most knockdowns and broaches happen.

A temperature inversion caps convective mixing, preventing the blending of air at different heights. Wind below the inversion may be calm while above it the conditions are very different — or the inversion may trap a local wind pattern that doesn't match the regional synoptic situation.

Squalls arrive with a rain curtain, temperature drop, and sudden intense wind lasting 5–15 minutes before passing. Frontal wind increases build more gradually and persist for hours. The response differs: squalls are survived by depowering briefly; frontal increases require a proper reef and a plan for sustained conditions.

Sailing a broad reach rather than dead downwind keeps the wind on one quarter consistently, making accidental gybes much less likely. It also typically increases boat speed enough that VMG (actual progress toward the destination) is equal to or better than slow dead running — and the motion is more comfortable and controlled.

Due to wind shear, the apparent wind at the masthead is from a slightly different angle than at the deck. Sail twist — opening the leech of the sail — allows the top to fall off to a wider angle that matches the masthead apparent wind. Without twist, the head of the sail is over-trimmed and stalls.