Global Meteorology and Climatology - Hadley Cell, ITCZ & Monsoons

Prerequisites

This lesson covers the "worldwide meteorology" and "tropical revolving storms" elements of the RYA/MCA Yachtmaster Ocean syllabus; these are also tested in the Ocean written paper. Before starting you should:

• Hold, or be working towards, Yachtmaster Offshore (or MCA OOW Yachts).
• Be competent with the meteorology taught at Day Skipper, Coastal Skipper and Offshore level: air masses, pressure systems, warm and cold fronts and occlusions, the life cycle of a depression, Buys Ballot's law, sea and land breezes, fog types, reading synoptic charts and GRIB files, and the Beaufort scale.
• Understand basic geography: latitude bands, the tropics, the main oceans and continents.

Learning Objectives

By the end of this lesson you will be able to:

• Meet the RYA Yachtmaster Ocean syllabus items for worldwide meteorology and tropical revolving storms (see the Syllabus Map).
• Choose a passage season and route using pilot charts, routeing charts and Ocean Passages for the World.
• Explain the three-cell model of global circulation and the resulting pressure belts and wind systems.
• Describe the trade winds, the ITCZ (doldrums), the subtropical highs (horse latitudes), the westerlies, the polar front and the polar easterlies, and their seasonal movement.
• Explain the monsoon mechanism and the Indian Ocean monsoon seasons.
• Interpret ocean synoptic charts: isobar spacing, the geostrophic wind and surface wind, and barometric trends.
• Describe the formation conditions, life cycle, structure and intensity scales of tropical revolving storms (TRS).
• Know where TRS form, in which seasons, and which centres issue warnings.
• Recognise the warning signs of an approaching TRS, determine its bearing using Buys Ballot's law, decide which semicircle you are in, and take the correct avoiding action in either hemisphere.

Syllabus Map

The RYA Yachtmaster Ocean course and exam (oral plus written paper) require "concepts of worldwide meteorology" and "tropical revolving storms". The table shows where each is taught.

In the Ocean exam you must be able to apply this knowledge to a passage you have planned: say why you chose your season and route, and what you would do if a named storm appeared on the forecast. The written paper typically asks for Buys Ballot bearings, semicircle identification and the correct avoiding action for either hemisphere.

Why the Ocean Skipper Needs Global Meteorology

On a coastal passage you can wait in port for a weather window. On an ocean passage you are committed for weeks, beyond the range of reliable forecasts. Your safety and comfort depend on choosing the right season and route from the climatology (the long-term average weather), and then on reading the actual weather at sea. The examiner will expect you to explain the big picture in your own words and to explain what you would do if a tropical storm threatened.

The Global Circulation

The sun heats the equator far more than the poles. The atmosphere (and ocean) carries that heat poleward. If the Earth did not rotate, a single giant cell in each hemisphere would do the job: air rising at the equator, flowing to the pole aloft, sinking and returning at the surface. Because the Earth rotates, the Coriolis effect deflects moving air to the right in the Northern Hemisphere and to the left in the Southern, and the circulation breaks into three cells in each hemisphere.

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The pressure and wind belts

From the equator to the pole:

1. ITCZ / doldrums (near the equator, moving seasonally): low pressure, rising air, calms or light variable winds interrupted by squalls and thunderstorms, heavy rain.
2. Trade winds (roughly 5 to 25 or 30 degrees): north-east trades in the Northern Hemisphere, south-east trades in the Southern; typically Force 4 to 5 (15 to 20 knots), very steady in direction, with occasional "reinforced trades" of Force 6 to 7 and squalls.
3. Subtropical highs / horse latitudes (around 30 to 35 degrees): large semi-permanent anticyclones such as the Azores (Bermuda) High, the North and South Pacific Highs, the St Helena High and the Mascarene High. Light winds, fine weather, calms in the centre.
4. Westerlies (35 to 60 degrees): prevailing westerly winds but highly variable, dominated by travelling depressions with their fronts and the ridges between them. Stronger in winter, and much stronger and more consistent in the Southern Hemisphere (the Roaring Forties).
5. Polar front and sub-polar lows (around 50 to 60 degrees): where cold polar air meets warmer mid-latitude air; the breeding ground of the mid-latitude depressions, e.g. the Icelandic and Aleutian Lows.
6. Polar easterlies and polar high: cold, often strong easterly winds near the ice.

The trade winds

The trades are the reason sailing ships could cross oceans reliably, and they still shape the main yacht routes. They blow from the subtropical highs towards the ITCZ and are deflected west by the Coriolis effect, becoming north-easterly north of the equator and south-easterly south of it.

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Trade wind weather: fair-weather cumulus, a steady breeze, a long swell, and frequent squalls, especially at night. A squall is a line or cluster of cumulonimbus and may bring a sudden wind increase of 15 to 25 knots above the trade wind, a shift in direction, torrential rain and then a calm. Reef early at night and watch on radar.

The ITCZ

The Intertropical Convergence Zone is where the two trade wind systems meet. It follows the sun, lagging it by a month or two, and lies mostly north of the equator in the Atlantic and eastern Pacific (because there is more land, and warmer water, in the Northern Hemisphere). Over the Atlantic it lies at about 5 N in February and moves to about 10 to 12 N in August, varying in width and activity from day to day. Over the Indian Ocean and the western Pacific its seasonal swing is much larger, linked to the monsoons.

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Image: NASA Earth Observatory, public domain, via Wikimedia Commons. Downscaled to 1600 px.

The satellite image shows the ITCZ as a band of towering cloud across the tropical oceans. The band is ragged, with squall clusters that are the main hazard to a yacht crossing it.

The westerlies and depressions

In the temperate zones the weather is a procession of depressions and ridges moving generally eastward. You met these at Coastal and Offshore level: the warm front with its lowering cloud and steady rain, the warm sector, the cold front with its veer (in the Northern Hemisphere), clearance and showers, and the occlusion. Ocean depressions can be very large and deep, and in the Southern Ocean they follow one another with little respite.

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Remember the Southern Hemisphere differences: depressions rotate clockwise, highs anticlockwise; a cold front passage brings a backing wind shift (for example north-west to south-west); and with your back to the wind the low is on your right.

Monsoons

A monsoon is a seasonal reversal of the wind caused by the different heating of land and sea. In summer a continent heats far more than the ocean, forming a thermal low that draws in moist ocean air. In winter the continent cools and a cold high forms, pushing dry air out over the sea.

The Indian Ocean monsoon is the most powerful in the world, driven by the Asian continent:

• South-west monsoon (June to September). The low over Asia draws the south-east trades of the Southern Hemisphere across the equator; the Coriolis effect turns them to south-westerly. Strong (often Force 6 to 7 in the Arabian Sea, with heavy seas), wet and squally. Generally avoided by yachts.

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• North-east monsoon (November to March). A high over Asia drives dry north-easterlies across the Arabian Sea and Bay of Bengal, generally Force 3 to 5 with fine weather. The yacht season for westbound passages (Thailand or Sri Lanka to the Red Sea or East Africa).

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• Transition periods (April to May and October to November) bring variable winds and the peaks of the North Indian Ocean cyclone season.

Similar, weaker monsoon effects occur over West Africa, northern Australia (north-west monsoon December to March) and the Gulf of Mexico/Central America.

Large-Scale Oscillations

• ENSO (El Niño Southern Oscillation). In El Niño, warm water spreads east across the tropical Pacific, the Pacific trades weaken, rainfall shifts east and South Pacific cyclones occur further east than usual. La Niña is the reverse. The Southern Oscillation Index (Tahiti minus Darwin pressure) tracks it: persistently negative in El Niño, positive in La Niña.
• NAO (North Atlantic Oscillation). The pressure difference between the Azores High and the Icelandic Low. A positive NAO means stronger westerlies and a more active storm track into northern Europe; a negative NAO means weaker westerlies and more blocking.
• Effects for the skipper. In El Nino years the Pacific trades are weaker and cyclones form further east and closer to the Marquesas and Tuamotus than usual, while the Atlantic hurricane season tends to be quieter (stronger wind shear). In La Nina years the Atlantic season is often more active. The NOAA and Australian Bureau of Meteorology ENSO outlooks are worth checking before a Pacific season. A positive NAO can mean a more northerly storm track and more gales on the Atlantic route home from the Azores, while a negative NAO brings weaker westerlies and a southward shifted track.
• Southern Ocean. Between 40 and 60 degrees south there is almost no land to slow the westerlies. The Roaring Forties, Furious Fifties and Screaming Sixties carry a procession of deep depressions, with swell periods of 12 to 16 seconds, wave heights of 8 m or more, and ice limits to watch for. Yachts passing Cape Horn or the Cape of Good Hope pass the Southern Ocean in the southern summer, with enough sea room and a clear heavy-weather plan.

Reading the Weather at Sea

Synoptic charts

Ocean synoptic charts (analysis and forecast, received by HF weatherfax, satellite email or internet) show isobars, fronts and pressure centres, often with forecast tracks of the lows. Look for:

• The position and movement of highs, lows and fronts relative to your route over the next 48 to 96 hours.
• Isobar spacing: close isobars mean strong winds.
• Troughs and secondary lows that can deepen quickly.

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Geostrophic wind and surface wind

Above the friction layer the wind blows parallel to the isobars at a speed determined by the pressure gradient and latitude: the geostrophic wind, measured with the geostrophic wind scale printed on the chart. At the surface, friction slows the wind and turns it in towards low pressure. Over the sea:

• Surface wind is about two-thirds to three-quarters (roughly 70 percent) of the geostrophic wind.
• It is backed by about 10 to 15 degrees from the isobars (Northern Hemisphere; veered in the Southern), i.e. it blows inward towards the low.

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Note that for the same isobar spacing the geostrophic wind is stronger at lower latitudes (the Coriolis effect is weaker), which is why the scale must be used at the correct latitude. Near the equator the geostrophic relationship breaks down entirely; in the tropics, look instead at streamline analyses and satellite images.

Curvature matters too: around a low the wind is a little less than geostrophic; around a high, a little more (though highs rarely have tight isobars).

The barometer

On an ocean passage the barometer is your most important instrument. Log it every hour (or at least every watch) and plot the trend.

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In mid-latitudes the classic rule is that a fall of more than about 3 hPa in 3 hours warns of strong wind (often Force 6 or more within hours), and a fall of more than about 6 hPa in 3 hours indicates a gale. In the tropics the barometer normally shows a regular diurnal variation of about 2 to 3 hPa: peaks at about 1000 and 2200 local time, troughs at about 0400 and 1600. Before deciding pressure is "falling" in the tropics, correct for this diurnal swing and compare with the normal pressure for the place and season (from the routeing chart or Pilot).

Climatology and Planning Sources

Before the passage, climatology tells you what to expect on average. At sea, forecasts tell you what is actually happening. Use both.

Read the pilot chart for your month, for the whole length of the route, and for the month after you plan to arrive. The rose on each chart square shows how many days in a hundred the wind blows from each direction: the longer the arrow, the more often. The numbers in the centre show calms and the proportion of gales (Force 8 and above). A leg with 10 percent gales in the chart month is a very different risk from one with 1 percent.

Passage Seasons

Tropical Storm Seasons

An insurer will often require the yacht to be outside a named zone during the season. Read the policy and do not assume a good forecast makes up for being in the wrong place.

Named and Regional Winds

Some regional winds can produce dangerous conditions within the otherwise settled belts.

Doldrums, Squalls and Calms

The ITCZ can be crossed in a day or in a week. In the Atlantic the doldrums are narrowest in February and March, when they lie close to the equator, and widest in the northern summer. Tactics:

1. Cross at the narrowest part, ideally at right angles, by a route chosen from the pilot chart and recent satellite images.
2. In the zone, sail towards the next area of wind rather than waiting. Use the engine for a few hours if you have the fuel, but keep a reserve for the rest of the passage.
3. Watch the sky and radar. Squalls are cumulonimbus clusters 5 to 20 miles across, travelling with the upper winds, so they can often be avoided by changing course by 30 to 60 degrees. A squall that looks like a dark curtain with a clear, rolled cloud edge ahead of it will usually bring a gust front 20 to 30 knots stronger than the pre-squall wind, with the wind shifting about 90 degrees.
4. At night shorten sail early. The first sign of a squall is usually the wind dropping, then the line of cloud appears on the radar. Reef or drop the sails before it arrives.
5. Collect rainwater from the squalls. The first few minutes wash salt off the sails, then the water is usable.

Worked Example: Geostrophic Wind Scale at Two Latitudes

An ocean chart shows isobars at 4 hPa spacing. At a point on the chart the isobars are 100 nautical miles apart.

• At 50 degrees north the geostrophic wind scale gives about 30 knots. The surface wind over the sea is about 70 percent of that, 21 knots (Force 5), blowing 10 to 15 degrees across the isobars towards low pressure.
• The same spacing at 30 degrees north gives about 47 knots, so the surface wind would be about 33 knots (Force 7) there.
• At 15 degrees north the scale would give about 90 knots, which shows why the method fails in low latitudes. The geostrophic balance requires a large enough Coriolis effect and little curvature, so use satellite wind data and observed winds instead.

This illustrates the exam point: the isobar spacing means little unless you use the right latitude on the scale.

Tropical Revolving Storms

A tropical revolving storm (TRS) is an intense, compact low that forms over warm tropical oceans. The same phenomenon is called a hurricane (North Atlantic, eastern and central North Pacific), a typhoon (western North Pacific) and a cyclone or tropical cyclone (Indian Ocean, South Pacific, Australia). It is the greatest weather hazard a yacht can meet. The aim is never to be caught by one; the skill is in planning the season and recognising the warning signs early enough to avoid it.

Formation conditions

All of these are needed:

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• Sea surface temperature of at least about 26.5 C to a depth of around 50 m: the warm water provides the energy, through evaporation and the release of latent heat in the clouds.
• A pre-existing disturbance: an easterly (tropical) wave, a cluster of thunderstorms in the ITCZ, or a monsoon trough.
• Low vertical wind shear: the winds aloft must not tear the developing storm's top away from its base.
• Moist mid-level atmosphere.
• Enough Coriolis effect to start rotation: TRS almost never form within about 5 degrees of the equator, and never cross it.

These conditions occur mainly in late summer and autumn of each hemisphere, when sea temperatures are highest. That is why the seasons are as they are.

Life cycle and classification

In the Atlantic, hurricanes are further classified by the Saffir-Simpson scale:

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Other regions use their own scales (for example the Australian 1 to 5 scale, which is based on gusts, and Japanese typhoon categories). Rapid intensification, an increase of 30 knots or more in 24 hours, can turn a tropical storm into a major hurricane within a day or two.

Where tropical storms form

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Image: Nilfanion with NASA background, public domain, via Wikimedia Commons. Downscaled to 1600 px.

The map shows every tropical cyclone track for 1985 to 2005. Notice the gap along the equator and the near-absence in the South Atlantic and in the eastern South Pacific. Tracks start between about 5 and 20 degrees of latitude, run west with the trade winds, then curve poleward and eastward, as described below. The strongest storms (the red and purple tracks) are concentrated in the western North Pacific, the Caribbean and western Atlantic, the Bay of Bengal, and the south-west Indian Ocean and Australian regions.

Tracks

In the tropics a TRS generally moves westward with the trade wind flow at 10 to 15 knots, drifting poleward. Many then recurve around the western edge of the subtropical high, turning north then north-east (Southern Hemisphere: south then south-east) into the westerlies, often speeding up to 20 to 30 knots and eventually becoming extratropical depressions. Tracks are, however, often erratic: storms can stall, loop or move in unexpected directions, especially near recurvature.

Structure

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Image: Mike Trenchard / Ed Lu, NASA Johnson Space Center, public domain, via Wikimedia Commons. Downscaled to 1600 px.

The astronaut photograph of Hurricane Isabel (2003) shows the clear eye, the eyewall of towering cloud and the outer cloud bands described below.

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• Eye: typically 10 to 30 miles across (20 to 60 km), light winds, sometimes clear sky, the lowest pressure, but a chaotic, pyramidal sea from all directions.
• Eyewall: the ring of towering cumulonimbus around the eye, with the strongest winds, heaviest rain and worst seas. After the eye passes, the wind returns abruptly from the opposite direction at full strength.
• Spiral rainbands: bands of squalls spiralling inward over hundreds of miles, with lulls between.
• The overall storm may be 100 to 500 miles across; gale-force winds may extend 100 to 250 miles from the centre, more on the poleward and dangerous side.
• Pressure may be 950 hPa or lower at the centre of a strong hurricane; the storm surge and wave heights near the centre can be extreme.

Where and When: Warning Centres

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Warnings reach yachts at sea by Inmarsat SafetyNET (via Inmarsat-C or a FleetBroadband/Iridium service with enhanced group calling), NAVTEX near the coast, HF weatherfax and radio broadcasts, and satellite email or internet. Advisories are usually issued every 6 hours, with a forecast track and an uncertainty cone. The cone shows the probable track of the centre; the storm's winds extend well outside it.

The mariner's 1-2-3 rule

Forecast track errors grow with time. The 1-2-3 rule adds a margin to the forecast radius of 34-knot winds: 100 miles at 24 hours, 200 miles at 48 hours and 300 miles at 72 hours. Plot this "danger area" and stay out of it. Yachts, being slow, should aim to keep well beyond this, typically at least 200 to 300 miles from the centre.

Warning Signs at Sea

If you are without forecasts, or the forecasts have not yet picked up a developing storm, the signs are:

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1. Swell: a long, low swell from an unusual direction, often felt a day or two (sometimes more) before anything else. The swell comes from roughly the direction of the storm.
2. Cloud: cirrus in bands radiating from a point on the horizon (the storm's direction), thickening to cirrostratus and altostratus, then the dark wall of the bar of the storm.
3. Pressure: in the tropics, a barometer 3 hPa or more below the normal for the date and place (after correcting for diurnal variation) is grounds for suspicion; 5 hPa or more below normal means a TRS is almost certainly within about 200 miles.
4. Wind: a change in direction and strength of the trade wind; increasingly squally and gusty.
5. Visibility: often unusually good before the storm, then very poor in rain.

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The pressure thresholds in the diagram above are practical triggers for action; the key is the departure from normal and the rate of fall. A fall of 1 hPa per hour or more in the tropics demands immediate action.

Finding the Centre: Buys Ballot's Law

Face the true wind. In the Northern Hemisphere the centre of the low lies on your right, between about 90 and 130 degrees from straight ahead (more than 90 because the surface wind blows inward across the isobars; nearer 90 degrees close to the centre, 120 degrees or more at a distance). In the Southern Hemisphere it lies on your left.

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Example (Northern Hemisphere): the wind is from 060 T. Face 060; the centre is 90 to 130 degrees to the right, bearing 150 to 190 T.

Dangerous and Navigable Semicircles

Divide the storm along its track. In the Northern Hemisphere the right-hand semicircle (looking along the direction of travel) is the dangerous semicircle:

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• The storm's forward speed adds to the rotational wind, so winds and seas are strongest.
• The wind tends to blow a vessel into the path of the storm, ahead of the centre.
• If the storm recurves (to the right in the Northern Hemisphere), it tends to curve towards vessels in this semicircle.

The left-hand semicircle is the navigable semicircle: the forward speed subtracts from the rotational wind, and the wind tends to blow you away from the track and astern of the storm. "Navigable" is relative; it is still extremely dangerous.

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In the Southern Hemisphere everything is mirrored: the left-hand semicircle is dangerous and the right-hand is navigable.

Which semicircle am I in?

Watch the change of wind direction over a few hours, while staying put (hove-to) if necessary so that your own motion does not confuse the picture:

Avoiding Action

The basic principle is to get as far from the centre as possible, as early as possible, and above all never to cross ahead of the storm's track.

Northern Hemisphere

Southern Hemisphere

Everything is mirrored: port replaces starboard.

The reasoning behind the dangerous-semicircle rule: with the wind on the bow you are heading away from the centre and forward of the track's beam, beating out of the storm's path rather than being swept into it. A yacht may not be able to point high enough or make enough way in rising seas, which is why early action is essential: a yacht that acts at 300 miles has options; at 100 miles it has almost none.

If you are caught

When the wind and sea exceed the yacht's ability to make way, survival tactics take over: heave-to (if the yacht does so well) on the correct tack for the hemisphere, lie to a sea anchor, or run under bare poles with a drogue (a series drogue is considered most effective by many). Secure everything below, close all openings, rig jacklines and harness on, prepare the grab bag and EPIRB, and keep the crew rested and hydrated. Remember the eye: the wind will return from the opposite direction with full force.

In harbour

If in the cyclone area in season, know your nearest hurricane hole, how long it takes to get there and how quickly it fills with other boats. Leave early. A crowded anchorage in a hurricane is a dangerous place; some skippers prefer to put to sea well ahead of the storm.

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Worked Example: TRS Avoidance

A yacht is at 15 N, 55 W in early September, bound west. Over 6 hours the barometer (corrected for diurnal variation) has fallen from 1012 to 1007 hPa, while the normal for the area is 1013. A long swell is arriving from the south-east, and cirrus bands are radiating from the south-east. The wind, normally east-north-east Force 4, is now east Force 5 to 6 and has veered from 070 to 090 in 3 hours. No forecast has been received.

Analysis.

1. Pressure is 6 hPa below normal: a TRS is almost certainly within about 200 miles.
2. Buys Ballot: facing the wind from 090, the centre lies 90 to 130 degrees to the right: bearing 180 to 220 T, consistent with the swell and cirrus from the south-east to south.
3. The wind is veering in the Northern Hemisphere: the yacht is in the dangerous semicircle, probably ahead and to the right of a storm moving west-north-west.

Action. Get the latest advisory (SafetyNET, satellite) immediately. Put the wind on the starboard bow: with the wind from 090, a course of about 045 to 060 T (wind 30 to 45 degrees on the starboard bow), making maximum speed, motor-sailing if necessary. As the wind veers, alter to starboard to keep it on the bow. Prepare the yacht for storm conditions, reduce sail early, and log pressure and wind every 30 minutes. Do not run off west, which would take the yacht across the storm's path ahead of it.

Worked Example: Calculating a Danger Area with the 1-2-3 Rule

A 48-hour forecast gives the centre of a hurricane at 24 N, 70 W, with 34-knot winds extending 120 nautical miles from the centre. Your yacht is at 21 N, 64 W (about 3 degrees of latitude south and 6 degrees of longitude east of the centre, roughly 375 nautical miles away), making 6 knots (144 nm per day), heading west towards the Bahamas.

1. Danger radius for the 48-hour forecast is 120 nm plus 200 nm, which is 320 nm. Today you are outside that circle, but in 48 hours at 6 knots you would have sailed about 288 nm west to roughly 21 N, 69 W, only about 180 nm south of the forecast centre: well inside the danger circle.
2. The decision is clear: do not continue west. Options are to turn south-east or south, away from the centre, to open the distance, or to stop in a protected anchorage if one can be reached well before the storm is within 300 nm.
3. Check the next advisory in 6 hours. Re-plot the circle every time. If the track shifts towards you, the radius may grow.
4. Record the plot and the decision times in the log.

The lesson: a slow yacht cannot out-run a storm by speed, only by early decisions and large distances.

Worked Example: Southern Hemisphere Storm

A yacht is at 18 S, 60 E in the south-west Indian Ocean in late January. The barometer has fallen 6 hPa below normal after correcting for diurnal variation. A long swell from the north-east has been building. The wind is from 120 T, Force 6, and has been backing over three hours to 105 T.

Analysis.

1. Pressure is more than 5 hPa below normal: assume a tropical cyclone within about 200 nm.
2. Buys Ballot in the Southern Hemisphere: face the wind (120 T), the centre is 90 to 130 degrees to the left, so it lies between 350 T and 030 T. This agrees with the swell from the north-east.
3. The wind is backing in the Southern Hemisphere, which means the dangerous (left-hand) semicircle.

Action. The wind should be kept on the port bow, about 30 to 45 degrees off, and as much speed made as possible. A course of about 150 T gives wind 30 degrees on the port bow. As the wind backs, alter to port to keep it there. If the yacht cannot make way, heave-to on the port tack and prepare for survival conditions. Get the latest advisory from Meteo-France La Reunion immediately.

Why the Centre Is Hard to Pinpoint, and Other Pitfalls

• Your own course and speed change the apparent wind. When you work out wind shifts for semicircle identification, use true wind, or stop (heave-to) for an hour to check.
• Close to the centre the bearing is nearer 90 degrees from the wind, and at 300 miles it is nearer 120 to 130. Using 90 degrees at long range gives a bearing error of up to 40 degrees.
• A recurving or accelerating storm makes the dangerous semicircle larger than expected, and storm tracks can stall or loop, especially near recurvature.
• Several systems can exist at once (for instance a tropical cyclone and the monsoon trough). Check the full advisory, not just one line.
• The tropical cyclone can undergo rapid intensification and also an eyewall replacement cycle, which causes a temporary weakening and then a bigger storm with a larger wind field.

Heavy Weather Survival Reminders in a Tropical Cyclone

The deeper heavy weather tactics are covered in the heavy weather lessons, but remember the main points when you plan to be at sea in the season:

• Secure the yacht for storm conditions early: below-decks stowage, deck lashings, storm sails ready, a well-practised drogue or sea anchor in its bag at the stern.
• Know the lee shore: the yacht needs sea room. Land downwind of a storm is the most dangerous place.
• Tropical cyclone seas are confused. The wave pattern changes by quadrant, with the largest waves in the dangerous semicircle to the right of the track in the Northern Hemisphere.
• Crew rest and food come before bravery. Keep a watch rota with few people on deck, always clipped on.
• Keep the EPIRB, grab bag and liferaft ready, and the radio and satellite phone charged.

Common Mistakes

• Treating the "navigable" semicircle as safe.
• Running off downwind in the Northern Hemisphere dangerous semicircle, straight into the path of the storm.
• Trying to cross ahead of a TRS to reach a destination.
• Confusing hemispheres: in the Southern Hemisphere the rules are mirrored (port for starboard; the low is on the left facing the wind).
• Ignoring the tropical barometer's diurnal variation, or reading "falling pressure" without comparing with the seasonal normal.
• Relying on the forecast cone: it is the centre's probable track, not the extent of dangerous winds.
• Leaving the decision to seek shelter too late.
• Using the geostrophic wind scale at the wrong latitude, or near the equator where it does not apply.

Summary

• Global circulation: Hadley, Ferrel and Polar cells give the ITCZ, trades, subtropical highs, westerlies, polar front and polar easterlies. All move north and south with the seasons.
• The ITCZ is mostly north of the equator in the Atlantic and Pacific; trades are steady but squally; the horse latitudes are calm; the westerlies are stormy.
• Monsoons reverse seasonally: Indian Ocean SW monsoon June to September (rough), NE monsoon November to March (the westbound season).
• Surface wind over the sea is about 70 percent of geostrophic, blowing about 15 degrees across the isobars towards the low. Watch the barometer: in the tropics correct for diurnal variation.
• TRS need sea temperatures of at least 26.5 C, a disturbance, low shear, moisture and at least 5 degrees of latitude. Hurricane force is 64 knots or more.
• Warning signs: unusual swell, radiating cirrus, pressure 3 hPa (suspect) or 5 hPa (certain) below normal, changing wind.
• Buys Ballot: face the wind, centre to the right in the NH (left in the SH), 90 to 130 degrees.
• NH: veering means the dangerous (right) semicircle: wind on the starboard bow; backing means the navigable semicircle: wind on the starboard quarter; in the path ahead: wind on the starboard quarter. SH: mirror, port for starboard.
• The best tactic is avoidance: correct season, forecasts, the 1-2-3 rule and early action.

Check Your Understanding

1. Explain how the Hadley cell produces both the trade winds and the subtropical highs.

Answer: Air heated near the equator rises (low pressure, the ITCZ), flows poleward aloft, cools and sinks at about 30 degrees latitude, forming the subtropical high-pressure belt. At the surface the air flows back towards the equator and is deflected west by the Coriolis effect, becoming the north-east trades in the Northern Hemisphere and the south-east trades in the Southern.

1. Where is the Atlantic ITCZ in February and in August, and why is it mostly north of the equator?

Answer: About 5 N in February and about 10 to 12 N in August. It follows the sun with a lag, and lies mostly north of the equator because the Northern Hemisphere has more land and warmer surface water, shifting the zone of maximum heating north.

1. The geostrophic wind scale gives 30 knots. What surface wind would you expect over the open sea, and in what direction relative to the isobars?

Answer: About 70 percent, roughly 20 knots, blowing about 10 to 15 degrees across the isobars towards the low pressure (backed from the isobars in the Northern Hemisphere, veered in the Southern).

1. Why is the south-west monsoon generally avoided by yachts in the Arabian Sea, and when do yachts usually sail westward across the North Indian Ocean?

Answer: The south-west monsoon (June to September) is strong, wet and squally with heavy seas. Westbound passages are made in the north-east monsoon, roughly November to March, when winds are moderate and the weather fair.

1. List the conditions needed for a tropical revolving storm to form.

Answer: Sea surface temperature of at least about 26.5 C to about 50 m depth; a pre-existing disturbance; low vertical wind shear; a moist mid-level atmosphere; and sufficient Coriolis effect, so at least about 5 degrees from the equator.

1. In the tropics your barometer, corrected for diurnal variation, reads 1006 hPa where the normal is 1012 hPa. What does this tell you?

Answer: The pressure is 6 hPa below normal, more than the 5 hPa threshold, so a tropical revolving storm is almost certainly within about 200 miles. Take avoiding action and obtain the latest warnings immediately.

1. In the Northern Hemisphere the wind is from 030 T. Where is the centre of the storm?

Answer: Facing the wind (030), the centre is 90 to 130 degrees to the right, bearing about 120 to 160 T.

1. In the Northern Hemisphere the wind is steadily veering. Which semicircle are you in, and what is the correct action?

Answer: The dangerous (right-hand) semicircle. Put the wind on the starboard bow (about 10 to 45 degrees) and make as much way as possible, altering to starboard as the wind veers; if unable to make way, heave-to on the starboard tack.

1. In the Southern Hemisphere the wind direction is steady, the wind is increasing and the barometer falling. What does this mean and what should you do?

Answer: You are in the path of the storm, ahead of the centre. Put the wind on the port quarter and run to get into the navigable (right-hand in the Southern Hemisphere) semicircle, then continue as for the navigable semicircle.

1. What is the mariner's 1-2-3 rule?

Answer: A margin added to the forecast radius of 34-knot winds to allow for forecast track error: 100 miles at 24 hours, 200 miles at 48 hours and 300 miles at 72 hours. The resulting danger area should be avoided.

1. Give three sources of climatological information for planning an ocean passage, and say what each provides.

Answer: Pilot charts show monthly wind roses, gale frequency, currents and storm tracks. Admiralty Routeing Charts and Ocean Passages for the World give recommended routes and seasons. The Admiralty Sailing Directions give regional climate, local winds and cyclone seasons.

1. In the Southern Hemisphere, at 20 S, the wind is from 100 T and has been backing. Where is the centre, and what should you do?

Answer: Facing 100 T, the centre is 90 to 130 degrees to the left, between 330 T and 010 T. Backing in the Southern Hemisphere means the dangerous (left-hand) semicircle, so keep the wind on the port bow (about 30 to 45 degrees) with as much way as possible, altering to port as the wind backs. If unable to make way, heave-to on the port tack.