A little about hurricanes in the North Atlantic

The season for hurricanes in the North Atlantic is generally reckoned to be 1st July to 31st October. However hurricanes can and do occur outside these dates.

Simply explained, hurricanes in this area start with tropical waves (weather waves, not sea waves) which come off the coast of Africa. At this time of year water temperatures are high (often over 30C) and this coupled with the wave can lead to convection. If the wave starts developing an anticyclonic motion then a tropical depression can form, and this then can lead to a tropical storm and finally a full blown hurricane, which becomes self-fuelling as it sucks more heat and water out of the sea. A wave moving slowly (at a few knots) is more likely to develop these tendencies and once it is pumped up to a hurricane it can travel at up to 30 knots.

Hurricanes are classified from cateory 1 to category 5 (the worst). When Ivan approached Grenada it was already a category 3, and became a category 4 as it hit Grenada. (See below for the Saffir-Simpson hurricane scale). Note that many believe that small tornados form inside hurricanes which create even stronger winds than those calculated by the Saffir-Simpson scale. Most boat anemometers pack up at around 100kts.

Typically cruisers get plenty of notice of an impending hurricane. We have wonderful weather services so we know straight away if there is an 'iffy' tropical wave that has hurricane potential. Mostly they start between the Azores and Cape Verde islands and track North West. Hence the islands to the South of the Caribbean, such as Barbados, Grenada, and Trinidad, experience hurricanes rarely and are considered 'safe' for the summer. Islands further North such as Antigua or the Virgin Islands are regarded as in hurricane alley, and most insurers will not give you cover there during the 'season'.

Sometimes we get less notice of potential trouble. Tropical Storm Earl, for instance, popped up just a few hundred miles away from the Caribbean; but hurricanes don't develop overnight. It was only some days later, after passing Grenada, that Earl became a hurricane.

The dangerous quadrant of a North Atlantic hurricane is to the North West. This is because this is the predicted direction of travel, and also because strong winds and seas extend far further in this direction than elsewhere. With Ivan for example tropical storm force winds extended at one time over 100 miles from the centre to the North West, but only half, or less than that, to the South and East. That's why you need to get below them - also if you are out at sea you have far greater risk of being sucked in towards the centre if you are in the dangerous quadrant than if you are elsewhere.

The problem with forecasting hurricanes is that the met services are still struggling accurately to predict growth in intensity, speed and direction. Ivan was always a big system and going to be dangerous, but right until the last moment it was predicted to pass North of Grenada - when in fact the eye passed right over. And also at that time Ivan increased in approach speed to 28kts, reducing the time people had to prepare or clear out. There is a lot of chance with hurricanes. Ivan ended up passing directly over the main yachting centres in Grenada, whereas 30 miles North in Carriacou (closer to the expected track) boats sheltering in the mangroves came out really well. Ivan could just as easily passed right over there. We ourselves were in Grenada when Tropical Storm Earl passed through.  Right up to the last minute Earl was due to pass North of Grenada and it was only afterwards, puzzled by the wind directions, that we learned that it had passed to the South.

Saffir Simpson Hurricane Intensity Scale

Category One - A Minimal Hurricane

Winds: 74-95 mph, 64-83 kts, 119-153 km/h

Minimum surface pressure: higher than 980 mbar

Storm surge: 3-5 ft, 1.0-1.7 m

Damage primarily to shrubbery, trees, foliage, and unanchored homes. No real damage to other structures. Some damage to poorly constructed signs. Low-lying coastal roads inundated, minor pier damage, some small craft in exposed anchorage torn from moorings. Example: Hurricane Jerry (1989)

Category Two - A Moderate Hurricane

Winds: 96-110 mph, 84-96 kts, 154-177 km/h

Minimum surface pressure: 979-965 mbar

Storm surge: 6-8 ft, 1.8-2.6 m

Considerable damage to shrubbery and tree foliage; some trees blown down. Major damage to exposed mobile homes. Extensive damage to poorly constructed signs. Some damage to roofing materials of buildings; some window and door damage. No major damage to buildings. Coast roads and low-lying escape routes inland cut by rising water 2 to 4 hours before arrival of hurricane center. Considerable damage to piers. Marinas flooded. Small craft in unprotected anchorages torn from moorings. Evacuation of some shoreline residences and low-lying areas required. Example: Hurricane Bob (1991)

Category Three - An Extensive Hurricane

Winds: 111-130 mph, 97-113 kts, 178-209 km/h

Minimum surface pressure: 964-945 mbar

Storm surge: 9-12 ft, 2.7-3.8 m

Foliage torn from trees; large trees blown down. Practically all poorly constructed signs blown down. Some damage to roofing materials of buildings; some wind and door damage. Some structural damage to small buildings. Mobile homes destroyed. Serious flooding at coast and many smaller structures near coast destroyed; larger structures near coast damaged by battering waves and floating debris. Low-lying escape routes inland cut by rising water 3 to 5 hours before hurricane center arrives. Flat terrain 5 feet of less above sea level flooded inland 8 miles or more. Evacuation of lowlying residences within several blocks of shoreline possibly required. Example: Hurricane Gloria (1985)

Category Four - An Extreme Hurricane

Winds 131-155 mph, 114-135 kts, 210-249 km/h

Minimum surface pressure: 944-920 mbar

Storm surge: 13-18 ft, 3.9-5.6 m

Shrubs and trees blown down; all signs down. Extensive damage to roofing materials, windows and doors. Complete failures of roofs on many small residences. Complete destruction of mobile homes. Flat terrain 10 feet of less above sea level flooded inland as far as 6 miles. Major damage to lower floors of structures near shore due to flooding and battering by waves and floating debris. Low-lying escape routes inland cut by rising water 3 to 5 hours before hurricane center arrives. Major erosion of beaches. Massive evacuation of all residences within 500 yards of shore possibly required, and of singlestory residences within 2 miles of shore. Example: Hurricane Andrew (1992).

Category Five - A Catastrophic Hurricane

Winds: greater than 155 mph, 135 kts, 249 km/h

Minimum surface pressure: lower than 920 mbar

Storm surge: higher than 18 ft, 5.6 m

Shrubs and trees blown down; considerable damage to roofs of buildings; all signs down. Very severe and extensive damage to windows and doors. Complete failure of roofs on many residences and industrial buildings. Extensive shattering of glass in windows and doors. Some complete building failures. Small buildings overturned or blown away. Complete destruction of mobile homes. Major damage to lower floors of all structures less than 15 feet above sea level within 500 yards of shore. Low-lying escape routes inland cut by rising water 3 to 5 hours before hurricane center arrives. Massive evacuation of residential areas on low ground within 5 to 10 miles of shore possibly required. Example: Hurricane Camille (1969)

Click here for pics of Ivan

Click here for zigzags about our time with Ivan