Rough Guide to the Weather.

Transcription

1 Rough Guide to the Weather. A presentation by John Baldock for Mylor Yacht Club, Jan 2012 Suppose we are intending to cruise from Mylor over to the Isles of Scilly and hoping to stay for a couple of weeks. Let us look at the charts of the cruising area. As far as the Lizard (almost 20nm.) we are heading basically south and south west. From the Lizard to Isles of Scilly it is just over 40 nm.due west. If the wind is S we shall be close hauled as far as the Lizard but then we shall have a good reach across to the Isles of Scilly. A wind from NW round to SE would give us a good reach or an easy run all the way. If it is anywhere between SW and NW we shall be close hauled all the way across from the Lizard and may need to tack. If it is W we may decide to put in to Newlyn overnight as the direct passage will involve beating, sailing much further and arriving very late. Once arrived in the islands, we must choose an anchorage in a location shielded from the swell and the wind-driven waves for a comfortable stay and as the wind shifts from day to day, we must

2 reassess the wind and swell forecast from day to day and be prepared to move to secure a comfortable berth. We must look ahead to see how the wind speed and direction will change, so as to plan which islands we could visit as well as for any signs of a marked deterioration justifying our swift return to the mainland. Finally, we will be looking for a window in the weather in the days ahead for our return passage. Obviously, we need a forecast. This available from a variety of sources. Met Office Shipping and Inshore forecasts. From radio or Navtext sources we get regular updates which we certainly need to record and to take notice of. The shipping forecasts are based on synoptic charts, but for the sake of brevity, they are lacking in detail. American forecasts for Europe. We also have numerous sources of weather information on the Internet, but most of the forecasts are based on an American model (GFS) which covers the whole world. The computer models start with analysis of current observations and lead to a synoptic chart (with isobars showing lows and highs with frontal systems). They then run several American computer models through an enormous number of calculations to try to predict how the pressure systems will develop over the next few days. Experts then choose the most likely outcomes. The output is a series of forecast synoptic charts at 12 hour intervals over coming days. These American synoptic charts can be found on many websites. From these American synoptic charts, the many forecasters (again mainly Americans) forecast wind and weather (and swell) over the next week or so, usually with more computer programs. The output is often in the form of a table for your chosen location (or sometimes coloured maps) showing wind direction, speed, gusts, swell etc. One useful site is and of course, you can change to whichever area you require. European forecasts. In Europe, the UK Met Office, in conjunction with DWD the German weather service and some other European centres carry out a similar analysis of current observations, and predictions using European based computer models which have been carefully developed by our scientists over more than 50 years. The European modes concentrate on the north Atlantic and Europe. The output of these European models are also scrutinised by European experts and the most likely outcomes selected to furnish forecast synoptic charts at 12 hour intervals over coming days. These charts are excellent. Sometimes there is a great deal of similarity between the European and the American forecast synoptic charts, but often they differ significantly, particularly over coming days, in ways that considerably affect the wind and weather to be expected. The Met Office in the UK and the Germans, Spanish and others now make the same European charts freely available over the internet. However, there is not the same amount of readily available wind, weather and swell forecasts available from these European charts.

3

4 One forecast can be found at displayed in graphics of some use to the cruising sailor. Using the synoptic charts. As the excellent European charts are themselves available to us, we should try to learn to interpret them ourselves, to get our own forecast for the particular sailing area of interest. The first step is to download the up to date charts for the next few days (how, I will leave til later). Putting the charts in date and time order, we start with the analysis of the most recent conditions (dated today and timed up to 12 hours ago, actually these are updated every 6 hours, but are not all are made readily available). Find the spot on the map where you are, or intend to be and examine the isobars running either side of this position. Generally they are plotted at 4mB intervals. Windspeed. Measure the shortest distance between the spanning isobars at the required position. (I make a print out and measure it with dividers. If you are working on a screen, make two marks on the edge of a

5 piece of scrap paper to record the distance) Transfer this measurement to the little graph at the top left-hand side of the chart. You can zoom in to make the measurements easier, but do be sure not to alter the zoom when going from the isobars to the graph. The curves on the graph (Geostrophic wind scale) indicate a wind speed in knots (80, 40, 25, 15 and 10) at various latitudes (40 to 70 N) plotted against the spacing between the isobars. I am at about 50 N, so I transfer my measurement onto the horizontal line at 50, measuring off the distance from the vertical axis of the graph and mark the distance along the 50 line. Now if one of the curves (say 15kts) passes through this point, the geostrophic windspeed would be 15 knots. If the mark falls between two curves, we must interpolate (judge and estimate a windspeed in between, leaning towards the higher or lower speed depending on where the mark falls between the curves.) The geostrophic windspeed relates to winds at 2000ft and corresponds to the strength of the gusts. At sea level, the winds are slowed by friction as they blow over the sea (and slowed even more where they flow over the land though topographical factors, valleys, hills, high trees etc have a great influence, making estimates inside an estuary for example, much more difficult. In landlocked seas and in hot places like the Mediterranean, special effects (mistral, meltemi, katabatics etc.) often take precedence I haven t sailed there so have no experience.). Lets concentrate on the winds a mile or two offshore around UK and nearby continent, where the upper level winds can generate the flow at sea level more predictably. Roughly, at sea level, the wind speed is reduced to 2/3rds of the upper level winds. Actually, it is a bit stronger at mast top level than at deck level and slightly skewed hence the need for some twist in your sails.

6 So we take 2/3rds of the Geostrophic wind speed (18 kts in the following example) and arrive at 12 kts (Beaufort force 4) for the wind at sea level. We write this steady windspeed down on the bottom of the chart. Wind direction. W. 4 At 2000ft the geostrophic wind is basically parallel to the isobars. This initially seems odd, since there is a 4mB fall in pressure between the adjacent isobars and perhaps we might expect the wind to blow straight in from the outer higher pressure towards the inner lower pressure. No, the geostrophic wind is more like a whirlpool, with the winds blowing along the isobars and circling round the low (also known as a depression or cyclone). Instead of sinking down as water does in a whirpool, they rise up in the centre of the low (twisters and tornados are extreme examples). However, closer to sea level they slow, and start to angle in about 15 degrees away from the isobars towards the low. Which way do the winds flow? Buys Ballot s law says (in the Northern hemisphere) with your back to the wind the low is on your left. They circulate anticlockwise around the low. Over land wind is slowed even further and angles nearer 30. So back to the chart, we draw in an arrow at the nearest isobar to our position, tilted in towards the low at about 15 degrees. We read off the direction by comparison with the adjacent meridian of longitude and parallel of latitude and write the wind direction against the windspeed at the bottom of the chart. Your forecast. You now have a windspeed and direction for the steady wind at your location and a windspeed for the gusts. The gusts don t have time to slow and turn in as they come down in turbulence from on

7 high, so these are going to be parallel to the nearest isobar, which means the gusts will be veered about 20 around a low in the northern hemisphere. All this takes only a very short time once you have had a bit of practice. You can rattle through the whole set of forecast charts, out to 5 days or so in a few minutes. Now you have your forecast for the various charted times at your chosen locations for some days into the future, as well as a measurement of what it should have been a few hours ago where you started. I usually make a list of days and times (in date order) with the forecast windspeeds and directions. I note the day when the analysis of observations was made at the top. Comparing with other forecasts. You can now inspect any chosen forecast from the experts to see if they agree with you. You will soon find out what reliance to place on those experts. You can also see if your nowcast corresponds to current conditions. Look up at the clouds overhead and observe their speed and course against a stationary object (telegraph pole, tall building etc.). You are taking a spot check on the geostrophic wind. This is also a way of finding out what the conditions are like outside a sheltered marina or anchorage, without having to put to sea. In between times. Your forecast is for specific times (as shown on the relevant chart). You may experience weather in the intervals, which may be sufficiently different to worry you. To be safe you need to look at how the lows are moving from one 12 hour chart to the next to see if you are likely to experience a band of much closer isobars passing through (greatly stronger winds) or more widely spaced isobars (less wind). If this is the case, you should try to estimate which part of the system will pass through where you are and check what the wind will do then. The lows in the Atlantic tend to come over from America, taking several days to reach us. During their passage they often fill quite a lot, ie. the central low pressure often slowly increases from day to day and they lose some of their ferocity. Highs near Spain often tend to slow their progress and turn them northwards a bit, so that we in the SW of the UK experience the southern part of these systems with their SW winds, veering W then NW as the centre of the low passes in a broadly NE direction. Rain etc. On the charts you will also see weather fronts. When a front passes over you will get wet. If it is a warm front (black semicircles) it will have been raining for a couple of hours and getting worse. If it is a cold front (black triangles) showers can be expected for a few hours. If it is an occluded front (alternate semicircles and triangles) it will be grey and drizzly on and off for a couple of hours as it goes through. In the sky, you will see the rain clouds at these fronts. If you happen to be in the warm sector between a warm front and a cold front and within a couple of hundred miles of the point where they meet, it will be raining heavily. If there is no front near you may have clear skies, though you really need to look at a satellite image to see what is coming your way. A curved short dark line with no semicircles or triangles is a squall line or trough. When this passes over it will rain heavily (even hail) for a short time, with strong gusts. You will see the dark band of cloud in the sky as the squall approaches.

8 High pressure weather. If we are lucky, we get some spells of warm settled weather in the summer. These are associated with high pressure systems (anticyclones). You can apply the same analysis as above to find windspeed and direction. By Ballot s law applies just the same, (wind from behind with low on your left N hemisphere) so you can find the direction of the Geostrophic wind. As it slows (to 2/3rds) near sea level it angles again towards the low 15. You will see that this means the arrows slant outwards from the high and go round clockwise. Generally the winds are much lighter around a high and the isobars are more widely spaced. You will find if you are anywhere near the centre of the high that the Met office will say variable which gives you no idea of direction. When you are looking for a calm anchorage even in Beaufort 2 or 3 you will have a quite uncomfortable swell if it is blowing in from the sea, so as always your forecast is important to your comfort and safety. A ridge of high pressure almost always occurs between successive lows and is accompanied by dry sunny weather and winds gradually backing from W or NW through W to SW.

9 E. 3 Getting the European synoptic charts whilst afloat. You will see today s and tomorrow s pinned up outside the Harbourmaster s or Marina office. They will usually give you a copy if you ask. In a marina (or close to someone s unlocked wireless modem) with a wi-fi enabled laptop you can download the European charts from the internet. I always use as it comes down with absolutely no frills and loads in a few seconds. You get the latest analysis chart and forecast synoptic charts for 5 days ahead. You can get synoptic charts for the next ten days from the European Centre for medium range weather forecasting %20850%20and%20mslp!144!Europe!pop!od!oper!public_plots! !!/.

10 With a smart phone, within range of a coastal signal mast you can access the same website with a 3G connection. Once on screen you can try to zoom in and pan around, though it is a bit like looking through a keyhole, but it is possible. Places like the Isles of Scilly, you have to carry your phone to where you can see the TV mast to get a signal from the cellphone mast next to it. Better, if you have a laptop, and a smartphone with a wire or wireless/bluetooth/ir connection or a mobile broadband modem you can download the smartphone file onto your laptop back at the boat or carry the laptop up to see the TV mast (for a 3G signal) and download direct from the website. Whilst you are connected, you can download any weather forecasts of your choice from the Internet. If you start with Ant Veal s weather site you can follow links to just about every forecast that exists. Make your choices at home and note the URL of the required website pages. Some places don t have 3G only 2G coverage. For these, I can connect with the mobile phone by a dial-up connection to a different ISP and download albeit very much more slowly over 2G. This is when the fancy images and moving pictures on a website are definitely not desirable. You will pay for a call lasting several minutes even with my basic website s data. Don t forget to end the call as soon as the file has come down. Anything prettier will never finish loading. Finally, wherever you are, even if you are out of signal perhaps 2 days out across the Bay of Biscay, you can receive the European synoptic charts free of charge by weatherfax using a laptop connected to any short-wave radio receiver having SSB (single side band). These have been provided for shipping at certain times every day since long before the internet. If anyone wants to know about this, please contact me. John Baldock Jan 2012 jbaldo7679@aol.com