By Dick Beaumont
Offshore satellite communications have improved beyond recognition over recent years. Time was when skippers had to rely on HF radio transmissions or weather fax, which were based on the same communication system, to access weather. Most radio operators, for that’s what you needed to be, would agree that the vagaries and complications of managing the equipment and the airwaves was a dark art that only dedicated radio enthusiasts could master. What’s more, most skippers accepted that once they were over three or four days out, they had to rely on their own short term weather forecasting from barometer and hydrometer measurements, combined with an assessment of cloud formation.
Whilst this provided skippers with a heightened sense of awareness, the deductions made were uncertain. The crew of an independent ocean cruising yacht simply had to take what weather was coming.
The advent of affordable and reliable satellite communications has changed that. Skippers, anywhere on the oceans, can now very easily download weather forecast grib files to keep themselves better informed about the weather, wind and sea conditions they can expect. Even so, it’s as well to remember the following:
- It’s a forecast and however sophisticated the computerised models become, occasionally the computers get it wrong and forecasters are surprised by an ‘off the grid’ weather system development.
- Add ten knots to the wind forecast if you’re going upwind, to convert the true wind forecast to apparent, which is what it will feel like.
- These worldwide computer analytics will not show local weather variances.
For example, if you are sailing in the tropics, particularly as the season moves towards the summer, forecasts will not pick up nasty, little thunderstorms that can turn a lovely calm evening with 10kts of wind, into a 40kt white squall maelstrom in just minutes, yet be gone again 15 mins later, leaving blown out sails and carnage in its wake, so keep a weather eye out.
In my experience. the forecast for the first 24 hours is likely to be 90-95% accurate, 48 hours 70-80%, 72 hours 55-60%, at 96 hours its 50/50 and at 120 hours, five days, 30-40 % but it does depend where you are in the world and how intense the weather systems are. In certain places around the world once a weather system has formed, the certainty of it, and it’s associated weather fronts, moving in a given direction, can be very high.
Generally, the accuracy of forecasts beyond five days is questionable and can only be used as a guide of what might occur.
Furnished with good weather info, you may not be able to completely avoid heavy weather, but you can know where and when it’s coming. That gives modern skippers a huge safety advantage compared to their counterparts of just a few years ago.
Please don’t misunderstand, I am not falling in line with the mistaken belief that with good forecasting or a fast boat, you’ll never have to sail in heavy weather. I have heard some shockingly foolish statements made by salesmen at boat shows. One Ocean Sailor follower, Ivan Clarke from New Zealand, asked a Jeanneau salesman at the Sydney Boat Show how the yacht would fare in heavy weather. He was told: ‘You’ll never know, sir, this yacht is so fast she’ll out-run the weather.’!
Nevertheless, with a good understanding of the weather charts that are derived from grib files, one can ‘load the dice’ in your favour, so that, rather than head straight into the heart of a weather system, you can make a course change and avoid the worst of it. That can make the difference between a tough sail and a survival situation.
I believe it is necessary for any skipper that is intending to go on offshore passages of more than three or four days to be able to read and understand weather charts. It is not sufficient to rely on weather forecasts that predict at xyz place next Tuesday there will be 15kts of wind from the south-west, or to pick up a digital animated graphic forecast from a website such as Windy or Predict Wind.
You need to know what’s going on.
These animated forecasts are helpful while you are learning to understand a weather chart, but weather systems are so fluid that you need to understand how weather systems are building, deepening, declining and moving, so you can form a probability in your own mind as to what might happen.
In this series, I will explain the rudiments of understanding weather, and how to read the weather charts. The article ‘Systems, Satellites and Software’ on page 12 written by Tom Copper, sheds some light on the confusing array of satellite communication systems and software programs that are available to enable sailors to communicate to the web, where weather information can be accessed.
Firstly, we need to understand the weather systems, and particular, the wind and sea state that these systems might generate in given circumstances.
If you want to understand meteorology in more detail, there are many very good books written from a yachtsman’s perspective. In this month’s Mariners Bookshelf section Dick Durham has recommended his favourite.
In simplistic terms, weather is the result of the uneven heating of the earth’s atmosphere and the movement of the air as it cools or warms up during summer and winter, and day and night, combined with the Coriolis Effect which throws the weather systems off of a fixed position.
The Coriolis Effect (above) is created by the rotation of the earth as it spins around its axis.
There are two main types of weathar systems, high pressure and low pressure:
These are generally more stable systems created by cool air from the higher atmosphere dropping down onto the earth’s surface. The winds generated are caused by the dropping air spreading as it comes to the earth’s surface.
These systems generally create lighter winds, are mostly larger than low-pressure systems, and move around slower than low-pressure systems.
These are however, circumstances in which two high-pressure systems in proximity of one another will create a pressure squeeze between them that then generates strong winds. Also, otherwise lighter winds from a high-pressure system may become exacerbated in the proximity of a low-pressure system, particularly a cyclone, when effectively the cyclone sucks air in from the weather systems around it. This effect can be felt many hundreds or even a thousand miles away from the cyclone itself.
Whilst sailors generally welcome the onset of a high-pressure system, it must also be understood that as one moves to the centre of the high-pressure system, slack, or no wind at all, may be experienced. The need to ‘choreograph’ your route around the systems as they move, to maintain good wind, is essential for the blue water cruiser.
In simplistic terms, these systems are created by the convection of warm air.
As air is warmed up over land or sea and rises, ambient air is pulled in to replace the air that has risen.
As the name suggests, as lower atmospheric air ascends it creates low pressure at its base, which ambient air rushes in to fill. The Coriolis Effect then starts the weather system spinning, and the rotation of the earth throws the weather system off across the earth’s surface.
Low-pressure systems generally create more wind than high-pressure systems and can be much more violent. Cyclones, hurricanes and typhoons, which are all the same things, are intense low-pressure systems.
Low-pressure systems often generate both warm, and cold fronts as they drag in air from surrounding areas. It’s important that sailors understand the severe wind and sea state conditions that may occur as these fronts, particularly cold fronts, affect the area they are in.
In the account I’ve written on a voyage from Madagascar to Cape Town alongside this article, I have explained the potential consequences of running into the extreme weather that such low-pressure systems can produce.
Look at the weather charts below and you can see the huge cold front generated by the remainder of Hurricane Teddy, which stretches right across the Atlantic east
to west.
If you were heading for Europe from Boston or Annapolis when Teddy formed, way down in the Atlantic, level with the Caribbean, it’s difficult to see how you could avoid going through the cold front. Mitigating the wind generated by the hurricane a thousand miles away would be fine, but outrunning the front wouldn’t be an option.
I want to stress that gaining a better understanding of weather is complementary to sailing in a well-founded seaworthy yacht and to good seamanship. It can never be an alternative to either.
Next Month
How the weather systems actually move over a 20-day voyage compared to forecasts and what that means to our on route passage plan.
Out-running the storm
On one nine-day passage from Nosy Bee, Mozambique to Durban, South Africa, we had planned a stop of 12 hours at an atoll reef called Bassa Da India, to dive the surrounding deep-sea pinnacles. All of the crew were experienced divers and we were all very excited at the prospect of diving this reef, which is hundreds of miles from the nearest land.
After three days sailing, as we began to approach the atoll, to my great concern when I downloaded the weather charts for the next five days, I saw that a ‘weather bomb’ ( an intense but small depression ) was developing down at The Cape of Good Hope.
I had been warned about this kind of weather system and had witnessed first hand the dramatic effects these ‘weather bombs’ can generate: While planning a departure from Nelson in New Zealand to Sydney, a similar weather system had developed in Tasmania, throwing weather fronts across seven hundred miles of the Tasman Sea.
We stayed in port for a few days to let it blow through.
Indeed it was a ‘weather bomb’ depression that wreaked havoc in the ill-fated Sydney-Hobart Race in 1998.
I said nothing, but rechecked the forecast six hours later, only to find, to my consternation, the storm was following precisely the direction and pattern I had been warned about, it was deepening and following a curving track hugging the coast around the Cape going east.
I checked the plotter, I had 750nm to run to Durban and so did the storm,
it was moving at about 5.5kts. I calculate my daily run distances based on 7.5kts, we normally do more, but it’s best to be conservative.
It became clear that if we stopped at Bassas Da India it might be a close call.
With a heavy heart, I told the guys we couldn’t stop and that we needed to get to Durban before the storm hit. The charts in the area carry a warning to mariners that waves in excess of 20m (66ft) have been reported in this area!
We cracked on making good progress and covered just under 200nm in the following 24 hours. I began to think I had been overcautious, maybe we could have stopped and dived for the day, but the next weather download removed any complacency, the storm was speeding up, fuelled by the warm waters flowing down from the Mozambique Channel. It was now moving at just under 10kts and deepening. It had also begun to throw out a cold front which was already more than 200nm long.
It still looked like we would beat it to Durban, where most of the crew were due to fly out back to the UK, but I began to consider that Richards Bay, some 80nm nearer, might be a handy fall back plan.
With less than two days to run, Richards Bay became the primary target, as the storm continued to increase its speed. It was gobbling up the miles at over 12kts an hour and the vicious front now stretched over 500nm from the east coast of South Africa out into the Indian Ocean. There would be no going around it, we either got to port or were going to have to go through it. The storm had passed Port Elizabeth and had 400nm to run to Richards Bay, we had 300nm to go. If we kept up 10kts we would beat it to Richards Bay by three hours.
We ran on under sail and power, covering 245nm in the following 24 hours and now had just under six hours and only 55nm to run. The storm however had sped up to 14kts. It would be a very close call.
There was no longer any talk from the crew of whether we still could have dived at Bassas Da Indian. The storm, our Nemesis, was now only 80nm from our safe haven.
With three hours to go the wind died, the proverbial calm before the storm. There was an eerie silence that spread over the greasy flat swell of the sea, and my crew. We furled all sails and I cranked up the iron tops’l (engine) to 2600rpm, well above the engine’s cruising speed, to keep up to 10kts.
As the entrance to Richards Bay came in distant sight, we could see the monstrous front stretching across the horizon into the distance, like something out of a Steven Spielberg movie. We could see it was south of the port, but by how far no one could tell.
The barometer was now dropping like a stone.
We powered through the heads at the entrance to Richards Bay just as the wind began to pick up to 15kts, and we could smell the storm in the air. We had made it with an hour or less to spare. No one spoke.
We tied up in the main port alongside a big old fishing vessel and the skipper came out and said ‘you better put extra mooring lines on guys, there’s a bit of a storm coming’, everyone started laughing. I told the bemused gnarly old salt ‘yes I know, we have just run 750nm over five days and beaten it here by an hour’.’ Good job’ he said, ‘I reckon this one will be real nasty’. He was right.
By the time we had finished tying White Dragon up, the wind was 25kts, it hit 50kts as the barman served the beer up, and blew at 65-75kts until dawn. We stayed in the pub for a while that night.
