About this page
A miscellany of questions that I have received over the years. If you have a question, why not try me?
Page under construction.
Questions
Q. The higher the clouds the better the weather? Useful?.
Q. Do halo rings the moon or sun mean that rain's approaching on the run?
Q. Are upper air charts, such as 500 hPa and jet stream level, are useful for making sailing decisions?
Q. Why do gusts often seem to occur when the sun goes behind a cloud? Is this a fact or myth?
Q. Why does a sudden rain shower often seem to kill the breeze?
Q. Why do we not have probability forecasts/ They have them in the USA. Surely, they would be useful.
Q. Do I need a computer on board? If so, what?
Answers
Q. A backing wind says storms are nigh, Veering winds will clear the sky. Any truth in this? If so, why?
A. Ahead of a warm front, as you probably learned in your RYA Yachtmaster course, the wind will back from W or NW to SW, S and even SE. So, this can be a good predictor although not all backing winds will presage a warm front. Like much weather lore it can help amplify a forecast rather than be a forecasting tool in its own right. There is no indication of how strong winds will become, nor when.
As a cold front passes, winds will veer from a SW’ly point ahead of the front to NW behind. So, in this case, the wind veer will be as or after the front has passed. In other words, the cold front clearance will be happening or will have happened. Further, behind the cold front showers are likely to develop so that the clearing skies will be interrupted by showers, perhaps with thunder. Not such a reliable indicator.
Q.The higher the clouds the better the weather. Is that a useful rule?
A. Hmm! It all depends on what type of cloud. If there is cumulus cloud, those nice little woolpacks, and the bases are high – around 4000 feet or more, then rain is unlikely. Similarly, if the cloud is of a mackerel appearance with no mare’s tails, then again, it looks set fair.
However, ahead of a warm front you will almost always see high cirrus cloud spreading across the sky. It will be high but be a fore-runner of rain some hours later.
Q. Do halo rings the moon or sun mean that rain's approaching on the run?
A. As the cirrus ahead of a warm front spreads across the sky it often becomes a high uniform layer of cloud known in the trade as cirro-stratus. Such cloud consists of ice crystals, mostly hexagonal plates. In a similar way to a rainbow being formed as light passes through water drops, light passing through these ice particles is split into the colours of the spectrum.
Because of the different geometry, haloes almost always subtend an angle of 22.5 degrees while rainbows have an angle of 45 degrees, ie rainbows are twice as big as haloes. Cirrostratus can create several varieties of halo and these sometime intersect to create “mock suns”. These will be either side of the real sun on the 22.5 degree halo.
As ever, weather lore or such rules of thumb have limited value. Read more at my Single Observer Forecasting page.
Q. Are upper air charts, such as 500 hPa and jet stream level, are useful for making sailing decisions?
A. In a word, NO. That may seem a rather sweeping statement, but let me explain.
First, a little background. Upper air charts refer to fixed pressure levels. Partly, this is because aircraft fly at constant pressures. Heights where the pressure is 900, 500, 300 and 200 hPa are approximately 3, 6, 9 and 12 km above sea level.
Relative topography or “Thickness” charts refer to the difference in height between two pressure levels. Usually, this is between 1000 and 500 hPa and is a measure of the average temperature of the air. Think of it as a volume and remember that volume varies with temperature.
Before and in the early days of numerical weather prediction, forecasters used to study upper level charts and try to understand how the atmosphere worked in three dimensions. They developed various rules of thumb to help in the construction of forecast charts.
It was all very subjective and those days of scientifically inspired guesswork have long gone; numerical weather prediction is a far more scientific approach to the problem. The computer produces an internally consistent set of charts at all levels from the surface up to about 60 km (40 miles). Forecasters look at all the data available to them and, using experience can modify the forecast if they think fit.
Anyone with a good background in meteorology, such as an airline pilot can look at upper air charts and get a good idea about the general weather situation. However, numerical prediction has reached such a level that improvement on the overall pattern is unlikely. For weather detail, there is simply not enough data and insufficient detail in upper air forecasts for use by a professional, let alone us weather amateurs.
Q. Why do gusts often seem to occur when the sun goes behind a cloud? Is this a fact or myth?
A. My first instincts are to say that it is a myth, but myths usually have some basis in fact, albeit not always sufficient to mean that there is a useful rule. One of the ways that gusts occur is when the sun heats the earth’s surface and bubbles of air rise with inflow to the heat source; nature abhors a vacuum. The inflow is associated with descending air that has a greater speed than surface air slowed down by friction. I would expect these gusts to be around rather than under the cloud.
Strong gusts occur when there are thundery showers; then the falling rain or hail drags air down with it to create strong down-draughts which spread out in all directions to give strong gusts. If near a large cloud, you could be getting the effect of the gusts caused in this way.
There used to be a saying that “The big ones come against the wind.” This is because, before a shower falls, there can be strong up-draughts in a big convective cloud. Even though the cloud may be drifting towards you, driven by the winds at cloud height, the surface wind will be towards the cloud due to the inflow. When you feel a sudden cold blast of air it is a sign that the rain or, more likely, hail, is falling and you will shortly get wet.
I have dined out on such short term predictions for many years!
Q. Why does a sudden rain shower often seem to kill the breeze?
A. Really heavy showers often fall when the general wind is quite light. However, around the shower itself, there are likely to be stronger winds due to heavy rain and hail dragging air down at great speeds, as much as 30 knots or more. This air spread out around the cloud in all directions away from the shower. Once right under the shower you are in an area where the air is all moving away from you so that your local wind will be pretty light.
Very heavy rain can flatten even quite a lumpy sea. On one occasion, off the south coast near Torbay, we experienced a really unpleasantly rough sea becoming as smooth as a millpond under a very heavy shower.
Q. In the event that thunder was not forecast What weather factors may lead one to suspect that thunder may occur when considering the risk before going out for a sail?
A. It is rare for thunderstorms to occur and not be mentioned in forecasts. They can be extremely dangerous with the energy of a single storm equating to several A-bombs, even reaching H-bomb level. The Met Office walks a tightrope by trying not to cry wolf unnecessarily. Also, there is a random, chaotic element to where thunderstorms occur even when conditions are right for them.
Thunderstorms result from vigorous convection in cumulo-nimbus clouds, with their trade mark anvils, giving heavy showers. This is usually when cold air is blowing over warmer sea or land. Such storms are often transitory and, individually, short lived. A very short term predictor might be a strong cold blast of air from the downdraft caused by heavy rain falling within the cloud.
Rather less common but a source of heavy, prolonged thundery weather is when storms drift slowly across the Channel from France and then linger over southern parts of England and Wales, depending on their source area and track. Typically, this is after a spell of hot, sultry weather over southern Britain. An indicator of such weather might be the grandiose sounding alto-cumulus castellanus – turreted looking cloud fairly high up.
Another reason for thunderstorms can be an active front. Warm moist air in the warm sector of a low can be lifted rapidly as the cold air cuts underneath. A vigorous cold front or occlusion – ones with heavy rain – are the likely suspects.
So, think thunderstorms in these situations; keep an even more than usually careful ear open for warnings. Perhaps keep a watch on the radar for particularly heavy shower echoes. In some areas, for example near the Isle of Wight, keep an eye open for water spouts near heavy thunderstorms.
If there is lightning around, protect your handheld GPS and VHF, laptop or hand held computers, cell phones, iPods etc, by putting them in the oven. The Faraday cage effect can save you pain and grief later.
Consider fitting one of those “bottle brush” devices known as Lightning Master Static Dissipaters. On the basis of one unnervingly close encounter with a thunderstorm off Sicily, I think that they may well work!
This large convective cloud over the Marano Lagoon, in the northern Adriatic, is just starting to from an anvil. A thunderstorm is inevitable, although this one was some distance away.
Attach:ac-cast.jpg Δ Δ This turreted cloud, alto-cumulus castellanus, is a harbinger of thunderstorms. This was seen near la Ciotat.
Q I saw some high streaky cloud clouds while crosssing Biscay and approaching the Spanish coast - about 100 NM from La Coruńa.. Could they have been noctilucent clouds?
A. While not totally impossible, it is unlikely that noctilucent clouds were seen when heading south and approaching Spain. These clouds form at heights of around 80 km where there is a temperature inversion.
As at the tropopause and low level (anticyclonic) inversions, dust particles get trapped; at this height, they can be especially noticeable after meteor showers. Conditions for viewing are when the sun illuminates the inversion level but not the atmosphere lower down. That means that the sun is between about 4 and 10 degrees below the horizon. I saw them over central Sweden many years ago at the time of the Perseids meteor shower, late July/early August, a very good time for optimum solar illumination in those latitudes.
There might be condensation on the dust particles but, if so, this is at very low temperatures of around -80 °C or even lower. It is unusual to see these clouds below 50° latitude simply because the sun is not at the necessary elevations long enough; when they are seen it will be only for short periods and usually looking polewards. They can be seen quite commonly in northern latitudes, say 60 to 70° at times of the year when the sun does not go too far down and stays between those elevations for some hours. The light from noctilucent clouds is strongly polarised indicating a particle size of 10-5 m (0.1µm). Being polarized like the blue sky they will disappear if you rotate Polaroid sunglasses while cirrus cloud will stay white with greater contrast against the sky.
Q. Does cold air have a greatyer effect on a boat than warm air? Any implications for siting of wind farms?
A. The weight of the wind is an old saying that, perhaps unusually, has more than a grain of truth, although it is a fairly small grain. A simple question in meteorology rarely has a simple answer and it is all too easy to fall into the trap of making simplistic statements.
The starting point is that the force on a sail or windmill blade varies directly with air density and as the square of the wind speed. Fairly obviously, cold air is denser than warm air and cold air will create a greater force than warm air.
It is easy to quantify this using Boyle’s law. For those without long memories, this simply says that, for a given pressure, the density of a gas varies inversely with the temperature; in other words (density at 0°C)/ (density at 15°C) = 288/273. The temperature has to be in degrees Kelvin ie 0°C = 273°K. Therefore, with an air temperature of 0°C there will be 5% more force than with air at 15°C.
The effect will be compounded by the fact that moist air is less dense than dry air. This is because water vapour is a relatively light gas compared to oxygen and nitrogen - the main constituents of the air. When water vapour content increases, the amount of oxygen and nitrogen in a given volume of air decreases. The density of the air as a whole decreases because the mass decreases.
Anyone wishing to do the sums can use Google ("air density water vapour" will probably do the trick) to find some rather frightening looking equations. For a fairly extreme example, in the tropics with a temperature of 32°C and 90% humidity, the air density is 1.14 kg/m3.
In a northerly airstream around the UK, we might have temperatures of 10°C and 10% humidity giving an air density of 1.25 kg/m3. So with cold dry air, the force of the wind on a sail would be about 10% higher than with warm humid air at the same wind speed. For dry air at both temperatures there would be a 7% increase; the water vapour effect is fairly small.
However, the expectation may lead to some misinterpretation of what is actually happening. Remember that an increase in speed from 14 to 15 knots will give an increase in force on a sail of about 15%. In cold and unstable air, convection will result in mixing in the vertical up to heights of anything from 1000 to around 4000 metres. There will be stronger gusts than with warm stable air in which the mixing is much more restricted in depth. Compare the steady winds in the warm sector of a depression with the much more variable wind behind the cold front.
So, the reality is that there is “more weight” in cold dry air than in warm moist air. However, around the UK during the sailing season in one area the variation in the “weight” of the wind must be fairly small and masked by larger effects due to variations in wind speed and especially when there are convective gusts. The effect may only be detectable to a far better helmsman than I ever was. A practical corollary of the above is that wind turbines can generate more power at a given wind speed in cold climates than in warm ones.
A. The answer is YES - sometimes.
The reason for what many of have experienced relates to convective weather when heating causes air to rise. There has to be descending air to compensate. Air above the surface of the earth is less affected by friction and moves more along the isobars at gradient wind level. As this air comes down, and before it is slowed down by friction, it will be veered relative to friction affected air. Sometimes, vigorous convection can cause air from even higher up to come down to the surface, perhaps from 3000 metres (10,000 feet) or more. Gusts in these conditions can be very strong indeed, perhaps twice the average wind speed.
That all sounds and is very sensible, but is only a part of the story. First, when there are large shower clouds, cumulo-nimbus, then rain or hail falling within the cloud, literally, drags air down with it often at great speed - as fast as the rain falls. This descending air spreads out in all directions and can give very strong gusts. You may feel a cold blast of wind a few minutes before the rain hits you. Additionally, particularly over land, objects such as houses and trees can cause turbulence with local wind increases from random directions.
So, what advice can I give? On a nice sunny day with convective, cumulus, cloud look for gusts to veer. If there are showers around, expect the gusts to be spreading out and away from the nearest shower. In strong winds, near land, be on the lookout for gusts from any direction.
Q. Why do we not have probability forecasts/ They have them in the USA. Surely, they would be useful.
A. Probability forecast are not new. For very many years they have been used in aviation landing forecasts. However, they have always been purely subjective guesswork. There are now ways to do better using numerical weather prediction models. Considering a specific area, say the UK, it is possible to determine that part of the current weather chart that is the most critical. For example, a small error in the analysis over northern Canada might cause a substantial error in the forecast over the UK for tomorrow. The numerical prediction programs can be run several times with random variations in the source area and get a range of possibilities for the outcome.
Perhaps the important point, really, is how do we use that information? Do we cross the Channel on a forecast that may have a 70% probability attached? We would have to know the range of outcomes possible. Imagine the length of such a forecast.
My own way of working, and one that I do suggest to others, is to keep a continual watch on the forecasts. Intending going somewhere at the weekend, start looking at the forecasts at least 5 days ahead. This can be by forecast charts from any of the major centres, UK, DWD or ECMWF. Usefully it can also be the (free) global GRIB coded forecasts. Use two sources eg the UK charts and the US GRIBs. See how the weekend forecasts vary from day to day. Consistency implies that the forecasts are on the right lines, inconsistency the reverse. During the 24 hours prior to departure, look at each of the shipping and inshore waters forecasts and, again, see how they vary from one to the next. Look for trends in the forecasts. Look at the inshore forecasts for the area that you are going to eg France, Ireland etc.
Using such an approach you will be better able to make an informed judgement on whether to go or not and where to. Of course, your deliberations should not just be for today’s weather for the passage, but what is likely thereafter. Should you do the crossing to Cherbourg or might it be wiser to go on to St Peter Port?
Never go to see on the basis of one forecast, perhaps hurriedly and half heard in the car on the way down to the boat or picked off the marina notice board. Yachtie mags are full of stories about poor forecasts. More often than not, the stories should have been of misheard or incorrectly used forecasts.
Q. Do I need a computer on board? If so, what?
A. Every day life is becoming dominated by the Internet and sailing is no exception. To many, that means the use of computers, anathema to some although most of us have some form of computer hardware on our boats, even if it is only a basic Garmin GPS.
So what do we really need as aids to getting and using weather forecasts? What would be useful? Depending on what you want to access from the Internet and what are your other onboard computing needs, if any, there are several options.
The Rolls Royce solution is to have a built in marinised computer running off the 12 or 24 volt boat supply, obviating the need for a power hungry inverter. It gives maximum flexibility as such a system can have many uses such as for navigation, radar and AIS displays, as a controller for radios including for NAVTEX. It is the ideal solution for and adopted by some liveaboards.
For the rest of us, the choices are a laptop, a handheld computer, a Blackberry or, quite simply, a mobile phone with the facility to view Internet pages and text emails. The laptop is the most flexible of these and is likely to be able to run any software needed for weather centred and other purposes. A big disadvantage is that it does not like being bounced around and hates getting wet. (I know from personal experience just how disastrous water on a keyboard can be.). At sea, it may well have to be kept dry in a nice cushioned environment An inverter or one of those cheap DC voltage converters from Maplin’s will be needed; both are fairly power hungry. The main use of a laptop is likely to be before you go to sea, it may well be of little use when at sea.
Handheld computers are more robust, less demanding of amp-hours but are likely to be restricted in the software that they can host. From discussions with others, it seems that few can handle the very useful GRIB files. They are more likely to be useable when at sea. Screen size may be a disadvantage for graphics, such as weather charts. Quite rightly, they have their adherents. A Blackberry can access web pages, handle email and has a screen that is considerably larger than a mobile phone. However, unless you use one in your everyday life, it may not be cost effective.
For those with small boats, perhaps sailing mainly at weekends or on occasional short cruises, and with consequently limited weather needs, the cell phone with a mini browser could be a very effective solution. Opera Mini, available free of charge, lets you look at web pages of text on your mobile phone screen. It can send and receive emails.
Cost of Internet access with them all will be the same. All will use 2G or 3G mobile phones and GPRS or one of the newer derivatives for downloading data efficiently. If GPRS is a mystery to you, speak to your phone company. They will help, but make it clear that you want GPRS for web and email use. Speed of access will be identical. For our extended cruising, on our HR 34, not a big boat, I like the laptop option because that is what I have always used. I use it for weather information before we sail; at sea I rely on my NAVTEX and the VHF. It handles my quite voluminous email, I can update my website, and I can draft articles and other documents as easily as at home. I would like to have a built in computer but, for me, and my kind of sailing, it would be overkill and not cost effective.
Others will have different needs and priorities. However, there will always be a solution that meets your needs. The internet is not just for computer buffs; it is available to all as an additional means of getting weather information to supplement and complement the GMDSS which is our main safety lifeline.