Bouncy bouncy… a small lesson on met.

Bouncy Bouncy
Oh what a good time!
Bouncy bouncy….  Hmm, enough with the Boosh references, but you should have got the idea, it’s been rough out there today! If you watch the UK weather forecast you will have gathered that there’s been a big old low pressure going over us today, and this one was particularly vile. For those of you who’ve done met at college or just know this stuff, please feel free to skip this one, I don’t want to teach grandmother to suck eggs, but for those of you who’ve not yet had that pleasure:

Here’s the surface pressure analysis for midnight this morning:

And then the forecast for 1200 midday:
 The thin black lines are called isobars, they are basically contours, like you get on an OS map, only these show pressure. A low pressure system moves in an anti-clockwise direction and as it moves, so the wind goes too. Wind generally moves along the isobars, and on a low pressure system the wind circles inwards. The closer the lines of equal pressure, the faster the wind will be. (Think of it as a funnel, the closer the isobars, the steeper the slope of the funnel and the faster a ball will roll down the inside). The red lines with semi-circles on are warm fronts, and the blue lines with triangles on are cold fronts. Our part of the world is a mixing bowl of sorts; to the north we have cold air around the pole, and to the south we have warm air around the equator. If the earth didn’t spin and butterflies didn’t flap their wings (bit of chaos theory there for you) then these two masses of air would sit side by side quite happily, but if anything stirs things up a bit then things get interesting. When warm air and cold air meet and mix, they don’t want to mix, so one body of air will actually slide under the other. This makes air rise, and when air rises it cools, and the water vapour in it condenses, condensed water vapour is generally know as clouds, and you know what they bring! So that cold front is a big ass wedge of cold air, pushing up warm wet air (warm air can hold more water than cold air) from the south into the atmosphere, which is why a cold front brings with it rain in buckets.
So there you go, that’s not the whole met syllabus, there are further complications and possible permutations of the scenario, but those are the basics. High pressure systems of course go clockwise (think of the two types of system as cogs) and are gentler beasts. Then when you head to the other side of the equator things flip and low pressure systems rotate clockwise and high pressure systems rotate anti-clockwise. The direction in which they move depends on what latitude you are at, this is is due to something called the Coriolis Effect, caused by the spin of the earth. Google it, if you’re bored!

Not going to St John

It’s official, I am the queen of procrastination! When I have work to do I will do laundry, hoover, cook large and time consuming meals, clean the bathroom, re-do the silicone seal around the bath, buy stuff I shouldn’t on the internet, blog, edit the photos I’ve been meaning to get around to editing for two years…. anything but the thing in hand! And so, in that spirit….

Before we didn’t go to St John, we went to Halifax, my nav work book starts to get a little more technical here, with tide print outs and lists of instructions on the use and set up of radar and echo sounders and steering and propulsion tests; I was getting a bit more comfortable in my surroundings on the bridge by then. I got ashore in Halifax too, we didn’t go far, just down the wooden boardwalk to the first decent looking restaurant we saw, where I recall eating a very good lobster sandwich.

Next day we were due to go to St John, which we had actually been to before as a 3 day cruise from New York, but I failed to mention it when I was writing about early September (my excuse is that my brain was fried at the time of writing, having only recently got home. It wasn’t a big event anyway; I did get ashore, but only as far as the nearest restaurant where I devoured as much lobster as I could!) Anyway, I was rather looking forward to getting to see a bit more of it, as we were due to spend a longer time than normal there, due to the tides and depth of water at the entrance to the harbour.

The weather wasn’t good, and there had been much discussion on the bridge over the days preceding as to whether we would be able to make it in, but the final decision was only made when we were a few miles off the pilot station. We would have been able to get in, but the forecast for when we were due to be leaving was simply too bad and it would have been too dangerous to try it, which left us with the chance of getting in but not getting out and thereby screwing up a lot of people’s holidays by making them late back to NYC, or not going in. That’s it in a nutshell, but for the more technically minded I thought I’d put in what I wrote in my nav workbook here, I’m afraid you’ll have to do without the charts and diagrams that should accompany it, but I hope it’s vaguely interesting (If it’s not, just skip to where the italics end!)

The ship was due to call into the port of St John today, arriving on the flood at high tide and departing on the flood of the next hight tide. The ship is only able to get into St John at high tide and has a very narrow window of opportunity to get up and down the channel. The port authority will only allow ships to enter or leave between 2hrs before High water and 2hrs after high water, and they must have at least 5meters clearance over the bank at the entrance. Chart datum at the bar is 8.8m, QM2 has a draft of 10.3m and also has to take squat into account. Squat occurs in shallow waters when a ship is travelling at speed, water is displaced more than normal and so the depth of water is reduced (though the ship’s draft stays the same). For the channel at St John squat is estimated at 1.5m, the ship therefore has a clearance of 0mover the bar at datum [datum being the height of water the lowest astronomical tide] and so needs a Height of Tide of at least 5m to be allowed into the harbour.

Looking at the tidal graph for St John that day it was apparent that the ship could make it from the pilot station to the berth by slack water (when the tide turns) but there was not much margin for error or contingency, even without the bad weather that was developing. The forecast for our due time of departure showed a large depression right over St John, bringing winds of 35-40kts. As a depression passes over, the cold front comes first, as this happens the wind tends to alter direction suddenly, first veering slightly and then backing dramatically. If this was to happen when the ship was in the narrow channel the ship could be pushed off course and grounded very easily.

The forecast showed that this bad weather would be likely to happen when the ship was leaving the port, so while we would have been able to get in to St John, we could have been stuck there for much longer than the itinerary allowed and therefore all the passengers who had to catch flights from New York would have missed them, so the Commodore took the decision not to to in. It was not a lighthearted decision, and was only taken at the last moment, when the most up-to-date information was at hand and much advice from shoreside support had been listened to. At the end of the day, it’s the master’s responsibility to keep the ship safe first, and being on schedule comes after, and we could not have done both.

To fill the time we should have spent in St John, we took a slow scenic cruise down the Grand Manan Channel, heading for New York. However, it wasn’t quite as simple as that and the navigator had to work out some extra legs for the ship to sail so that she could go fast enough to be able to keep the water making plant working. The ship uses approximately 1000 tonnes of water a day, and there’s no way she could store that much! She has three evaporators, which, when making a speed of 26kts can produce 1500 tonnes of water a day. To be able to run one evaporator the ship needs to be going at a minimum of 13kts so the navigator had to extend our passage to enable the ship to travel at that speed and not reach New York too early.