Posts Tagged ‘weather’

This little blog is simply to act as a reminder for symbols you may commonly see on weather charts. As we know weather forecasting is not an exact science but a science it is, the information given below is a very simple summary of the common symbols used with the typical weather associated.

warm front

Warm Front – This symbol depicts the leading edge of a warmer air mass. The air ahead of this front will be cooler than the air behind it. Typical weather ahead of the warm front will be fine then heavier rain, as the warm front passes the rain will stop and become drizzle in the warm sector.


Cold Front

Cold Front – This symbol depicts the leading edge of a colder air mass. The air ahead of this front will be warmer than the air behind it. Typical weather just before and at the front itself will be heavy rain, hail, thunder, lightning, behind the front there will be sunny periods with scattered showers.


Occluded FrontOccluded Front – An occluded front is formed when the cold front catches up the warm front, the symbol combines the shapes of the warm and the cold front. Typically underneath an occluded front there will be rain as the warm air is being pushed aloft and condenses.


Frontal System Northern Hemisphere

Frontal System – This diagram shows a frontal system and the relationship between the fronts described above. This is a typical frontal system in the Northern Hemisphere and can be seen on good quality weather forecasts on television. The system is formed around an area of low pressure which will circulate in an anti-clockwise direction. This example already shows an occluded front where the cold front has already caught up with the warm front, the cold air in front of the warm front and behind the cold front with the area of warm air being squeezed in between.


Stationary FrontStationary Front – A stationary front is a boundary between two air masses neither of which is strong enough to replace the other. Cloud and rain are usually associated with a stationary front.


Developing Warm FrontDeveloping Cold Front Developing Warm / Cold Front (Frontogenesis) – This symbol depicts the formation or intensification of a meteorological front caused by the difference in temperature of adjacent air masses. No significant weather associated.


Weakening Warm FrontWeakening Cold Front Weakening Warm / Cold Front (Frontolysis) – This symbol represents a front which is loosing its identity usually due to rising pressure and where air masses are diverging. Cloud and precipitation becomes increasingly fragmented.


High and Low Pressure Isobars

High & Low Pressure Systems – This diagram depicts pressure systems in the Northern hemisphere, the arrows are showing us the wind direction, the wind moves anti-clockwise around an area of low pressure and clockwise around an area of high pressure. You will also note that the arrows around the low pressure are pointing slightly inwards toward the centre this is due to the wind converging around an area of low pressure, the opposite is true of high pressure, the wind diverges and therefore points slightly away from the centre.


Geostrophic wind scale

Surface Pressure Chart – This surface pressure charts shows areas of high and low pressure depicted by the H and the L on the chart. You can also see lines all over the chart which are called isobars, these connect areas of the same pressure, some of these lines have numbers on them which is giving you the pressure at that particular isobar. Isobars are like contour lines on an ordnance survey map, the closer they are together the steeper the hill, with isobars the closer they are together the stronger the wind. With this type of chart you can actually measure the wind speed by measuring the space between the isobars and using the scale in the top left hand corner.


Wind Strength symbols

Wind symbols – If you use grib files or sites like Passage Weather you will see lines such as the ones on the left here showing you the wind direction and the wind speed in the area you are looking at. Wind is always described in relation to the direction it is coming from, in the UK our prevailing wind and weather comes from the South West and in wind terms would be described as a South Westerly. On the bars on the left there are a number of different lines or barbs, the short lines represent 5 knots and the long lines 10 knots of wind. You can now see combinations of these making up 15, 20, 25 etc until you reach the triangular shape which represents 50 knots. These lines will always be on the back of the arrow and this is where the wind is coming from so in all the examples on the diagram the wind is coming from the north east.


If you found this blog useful please let us know and if there are any specific topics you would like us to cover in future blogs, again please let us know, you can contact us directly at info@nomadsailing.co.uk

Weather Predictions

Posted: 29 May 2013 by Kirsty Elliott in Round the Island Race
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The Nomad 1 and Fortyniner crews have been avidly watching the long range forecasts for Saturday in the lead up to the race.  The conditions for the last two years have been very windy and not allowed for the boats to get their spinnakers/cruising chutes up, so the big question is ‘Will we manage it this year?’  Would also be nice if we had some nice sunshine on the way round too!  Although I’ll settle for no rain.

We have also been watching the official forecast information as it comes out from Race HQ.

All the forecast information at the moment is pointing to the wind staying predominately from the north west/north north west for the majority of the time it is likely to take us to get round.

In the last 24 hours windguru has changed its predictions in terms of gust strength and also the predicated strength of the wind appears to have dropped slightly as well with forecasts of between 9-11 knots.  Gust strengths are forecast to be lower in the afternoon.

We are starting the race two hours earlier than last year and an hour earlier than in 2011 to take best advantage of the tide – particularly on the way down to the Needles which will be our first major check point on the way.

The teams will be meeting on Thursday evening for race planning which will be updated again the following evening based on the latest weather available.

Don’t forget on Saturday you’ll be able to check which boats plans are working best using the race tracker!  And look out for updates from each boat on the day!

Fog Blog – Navigating in Restricted Visibilty

Posted: 26 October 2012 by Jim Barden in Navigation
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Of all the different types of weather we may encounter at sea,  fog must be one of the most daunting and one which carries the most risk ; especially in crowded European waters which can be alive with commercial shipping, and where the risk of collision is high.

The best advice, offered by Des Sleightholme,for when fog is forecast might be :  ‘Stay home. Keep Hens’

However  if we’re already underway when the fog creeps in,  then some strategy to get safe and keep safe must employed.  As confidence with the various techiniques grows you should be able to navigate blind, if the ground permits, to most destinations.

Get prepared early.  If visibility is noticeably  deteriorating, get ready for it.  Get a reliable fix on the chart by conventional means – visible bearings, GPS etc.  If necessary work up an EP from you last known position (this shouldn’t be any more than an hour old on your chart).  If you happen to be near a recognisable mark get to it and ask the helm to hold station while you figure what to do next.

We are going to consider later the navigational aspects of dealing with fog,  but here are some basic precautions you should consider in immediately :

  • Fog Horn Ready – sound appropriate signal ( one long – under power, or one long and two short – sailing : every two minutes.
  • Lights On – make the vessel more visible with the appropriate lights for your category
  • All Crew in Lifejacket and on-deck – the collision you fear could happen very suddenly resulting potentially in all hands in the water.  On deck they also provide extra eyes and ears.
  • Radar Reflector Up – if it’s not already, hoist your reflector high.  If you can’t find it, string your pots and pans together on a halyard !
  • Monitor VHF – and contact relevant Port Traffic Control in necessary
  • Liferaft / Tender Ready
  • Engines on or ready
  • Navigate away from potential danger of collision.

If fog, shallow is safe, away from commercial shipping and also the most likely place to find a reliable contour of depth to follow (or a safe place to anchor)

With reference to your position and any shipping channels indicated on the chart ,measure up a course to steer to the nearest strong contour of depth and away from shipping channels.  This should be one which is has a fairly consistant direction, if possible close and parallel to the shore. ( the odd mark on it or near it along the way wouldn’t go amiss. )

Calculate the height of tide.  Add this to the value of the contour (and subtract draught if your echo sounder reads depth below keel) —  this is the displayed  echo-sounder depth we’re looking for. For example if we decide to follow the 5m contour, and calculate the height of tide to be 2.3m – we are actually looking for a depth of 7.3m to follow.  With a draught of 1.8m and an echo sounder reading below the keel, this would be 5.5m displayed.

If there is no strong contour to follow nearby then just head shallow and anchor.

Otherwise measure the rough direction of the contour you intend to follow, this will be the heading we turn onto when the required displayed depth is reached.  No adjustment to the heading is usually required for tidal stream here as in general the tides follow the contours. (but make note of exceptions to this where the contour might veer away from the tidal stream)

This information should be passed to the helm and crew as a clear and concise set of instructions :

“Head 030°(M) , maintain speed of 4kts. Depth should fall.  When we get to 5.5m dispayed (10 mins or so)  turn starboard onto a mean course of  095(M)  ± 20°  and  maintain depth between 5 and 6m.  Feed me depth readings at every 0.5m of change and let me know when you get to 5.5m”

When the boat hits the desired depth and turns to follow the contour make a note of the time / or start your stopwatch and mark this on the chart of this where you think you have joined the contour.

Now every six minutes  ( 1/10 hr) you can plot your estimated position along this line.  This should be fairly straight forward as you will cover one tenth of your boat speed in knots (± any estimated tidal stream) every six minutes.   Our arrival to or near to any marks on or near to the contour can now be predicted and the crew told when , where and what to keep a look out for.  On sighting and identifying each mark we can fix our postion again and make a note of time.  This also allows us to recalculate our actual ground speed rather using an estimate of tide along with a possibly inaccurate log/speedo.

Changes or trends in the contour’s direction should also be noted and this information passed onto the crew before it happens  eg :

“ in the next few minutes the contour will start curving to the north onto a rough heading of 005°(M) ±10°.  Alter course as nessessary to follow it and maintain depth 5.0 -6.0 “

Sometimes it might be useful to sketch the shape of the contour and mark it up with rough courses for the helm to look at in the cockpit.  Also don’t forget that every half hour (or at least every hour) the tidal height may have changed significantly , so the indicated depth you wish to maintain will need to be recalculated.

It may be necessary or desirable to move between contours to avoid hazards or find marks to help you fix position. Do this early.

When crossing a channel and potential traffic , check local VTS on the VHF for any ships in the area and whether its safe to cross and then cross at heading at right angle to the shallow contour of depth on the other side, then turning onto a new heading.

Have a look at this chart and consider how you might find the refuge of  Stokes Bay to sit out the fog at anchor. Imagine we’re just approaching the exit of North Channel heading east when the Fog starts to fall. An east cardinal mark can be seen 200m ahead …..

We’ll give a possible solution next week and a consider how we might then continue on to Portmouth harbour.  Or come on one of our training courses and where we complete simulated fog exercises on the water