Wind maps and grib files, how to make the most of them

The interpretation of wind maps / grib files and routing

Wind maps derived from grib files are nothing more than one of the possible representations of the development of the meteorological situation. The attention is focused on the wind but also on the wind maps derived from grib files, with a little bit of practice it is possible to draw many other very important conclusions. This is why they deserve an in-depth study, to understand the wind maps deribed from grib files and especially the limitations. This will make them more useful by remembering a few important considerations that we are now going to discover.

The sky before the warm front
The sky before the warm front 

The life cycle of a depression

To understand a wind map or grib well, we must immediately talk about the most important of the limitations of this representation of forecasts. Wind maps or gribs do not report cloud cover, warm, cold and occluded fronts. Nor do they indicate which are the centers of high or low pressures. The less experienced user may find it difficult to make any assessment. It is of little use to observe many colored arrows and hope that reality matches the prediction.

From the wind maps or gribs we can deduce by logical steps much more information than is represented. First of all we could find cyclones and anticyclones or areas of high and low pressure. High pressures will be characterized by weak winds blowing in a clockwise rotation. The depressions will be recognised by the classic whirlwinds of intense wind in counterclockwise rotation.

The sky as the warm front approaches
The sky as the warm front approaches 

Warm fronts and cold fronts

The second logical step is to remember that fronts, warm and cold, are exclusively associated with depressions. Understanding the birth of a depression, its development and dissolution is the first step in understanding what is happening when you analyse the weather situation. The topic can be addressed by studying a text on meteorology or by enrolling in a course or by referring to the many online resources.

The matter is undoubtedly complex and often we end up having even more confused ideas. Even worse, those who think they have understood everything and suffer from the know-it-all syndrome. These people, pointing the finger at clouds imagine that with a quick glance you can understand everything. Sometimes they get it right and sometimes they are completely wrong, showing you that their brilliant intuition and interpretation was not much more than a coincidence. You shouldn’t fall into this category and rely on chance!

Knowing the overall situation, on the other hand, will allow you to interpret local phenomena. The reverse is not necessarily true and the ability to make logical deductions by observing a wind map or gribs will help you to really understand what is going on.

Between the warm front and the cold front
Between the warm front and the cold front 

Going through a depression

In transatlantic navigation, where the weather systems are not disturbed by land masses it is possible to clearly experience the life cycle of a depression. Even observing a wind map or grib files you can understand a lot, but you must learn to read between the lines, or rather, between the arrows. After identifying the centres of high and low pressures by observing the wind distribution, you can try to deduce the fronts associated with each low pressure.

Warm front

If we are south of a depression when a warm front arrives, the wind initially rotates from west-southwest to south. Afterwards, with a progressive rotation over time, it will return to blow from the southwest. You can look for the areas where from right to left the wind follows a rotation from south to south-west, that’s where the warm front is. In the area where the wind blows from the south you will probably still have clear skies with high temperatures brought by the wind from the south.

As the warm front approaches you will begin to observe the famous candy floss clouds in the sky and slowly the sun will be covered by a halo. Lower and darker clouds will follow, it will initially begin to drizzle. The wind will not necessarily have increased much and it will not be particularly unstable or gusty. As the warm front passes, you can expect some showers, not particularly violent. Even the wind will not be excessively violent nor it will undergo a significant rotation. After the rains of the warm front, there will be a period of smooth wind, very regular, typically from the south-west. The sky will be overcast with the possibility of continuous Irish countryside style drizzle or scattered rains.

These conditions, although gloomy and unpleasant, are the ones where records are made in races. In fact, the wind is stable and distributed and the gusts usually do not deviate more than 30% from the average. In round the world records on large multi-hulls, the navigator looks for this situation. Thanks to the very high speeds of the boats, the record-men try to stay in this area of the depression for as long as possible. By doing so, they will move at approximately the same speed as the weather system.

Passage of the cold front
Passage of the cold front 

Cold front

For us mortals on slower boats, however, after this zone of stable winds, we will have to expect the arrival of the cold front. This is because we will not be able to keep up with the depression which on average moves at 25-35 knots. The distance between the warm front and the cold front is greater the farther we are from the centre of the depression. Near the centre these changes occur within hours while hundreds of miles further south we will hear like a distant echo of the same conditions. Everything will happen over a longer time frame and both wind and cloud cover will be less significant.

The arrival of the cold front is one of the crucial moments in the life cycle of a depression. When we talk about storms we often refer to the passage of this front. It is often associated with violent showers and gusts of wind followed by a rapid improvement in conditions. After the strongest winds, thunderstorms and storms, good weather often follows.

The approach of the front does not always take place according to the same pattern, therefore caution is required. Sometimes there will be a progressive reinforcement of both wind and cloud cover and the wind will make a slight rotation from southwest to west. In some cases, however, the wind may drop slightly before the actual front arrives. It is very important to recognise this step in order not to be deceived by this temporary wind drop.

Violent showers will follow, thunderstorms especially in the summer with gusts of wind that may even be double compared to the average wind up to that moment. After an intensification and a succession of showers and gusts of wind, the sky will suddenly open. The wind will make a rapid rotation of up to 90 degrees, blowing from the north west.

Crossed seas associated with the passage of the cold front
Crossed seas associated with the passage of the cold front 

Crossed seas

In open ocean navigation, the arrival of the cold front is also anticipated by the progressive deterioration of the state of the sea. In fact, the new wind still far from us sends forward a wave train crossed with respect to the one we had encountered so far. A crossed sea is certainly a strong sign that things are about to change. Behind the front there is often a zone of weaker winds and the sun will appear. After the blows taken so far by the crew, the rays in the reassuring clouds will give everyone a sudden sense of security. But, beware, that’s not all, the relief in the crew is enormous but it is not yet the time to let our guard down.

After this temporary drop the wind will strengthen again and will now be very unstable, cold and with very violent gusts and with other thunderstorms or showers. The sea will still be crossed and only in the course of many hours will it begin to realign with the wind which will also stabilise and gradually decrease. If it was an isolated depression then a phase of good weather should follow. However, depressions often travel in trains and after a short time the wind will start to turn south again. This is a clear sign that the cycle is about to repeat itself with a secondary depression.

Of course this is a simplification, especially in the Mediterranean where the depressions that arrive are already at an advanced stage of their life cycle. With the maturing of a depression, the masses of warm and cold air begin to overlap, forming the so-called occluded fronts. The situation is much less clear and predictable than previously stated.

Tiredness after the wind blow near the cold front
Tiredness after the wind blow near the cold front 

The limitations of the wind maps and grib files

This long introduction should now allow us to observe a wind map or grib files and make far more meaningful deductions.

Light winds

So let’s move on to specific examples and the limitations of wind maps and grib files with light winds. When it comes to light winds, the model and reality can be very different. It is very likely that local phenomena such as breezes will prevail over the wind created by the pressure gradient. If the wind field indicates light winds, you should sail with what you find without worrying too much about what the model said. Especially when you see confusing arrows pointing in all directions, even in opposite directions from one to the other. In this situation it is clear that the model is guessing. This is typical near the centres of high pressures and alas a very common situation in the Mediterranean in summer. Here large areas of high pressure make the wind maps and grib files very unreliable.

Medium and strong winds

It is only when the gradient coefficient increases that the models underlying the wind map or grib file begin to have good reliability. In the range between 8 and 15 knots the rows of wind maps or grib files are quite reliable but a warning is imperative. When winds are above 15 knots, some additional calculations need to be done with caution. Sailors tends to remember the highest of the gusts and not the hourly average, so when encountering gusts will think the figure in the wind map or grib file was under-estimated. It must be understood that 15 knots (on an hourly average) can vary quite a lot near the fronts.

Hourly Averages

The wind maps or grib files report the average hourly wind to a height of 10 meters above the water surface. A first consideration is that due to the friction that the wind encounters on the surface of the sea, the wind is less intense than when we rise in altitude. For a sailboat with a 15m mast you will probably need to add a good 10% to the estimated wind. For a typical 20 metres mast of 40-45 feet boat, the figure is a good 20% off. The further you are from the surface of the water, the less the wind is slowed down and is therefore stronger.

Instantaneous wind

The second important consideration is that the wind field reports hourly averages, not gusts. As we’ve seen a small arrow indicating 15 knots of wind on a 45 foot boat must be read as an hourly average of 18 knots (+ 20%). If you are sailing in a stable mass of air you will have to allow for 30% more in gusts. In an area of unstable air such as the passage of a cold front you can find gusts of up to 50% more. On some occasions, with particularly active fronts even 100% more than indicated by the forecast.

So, a harmless 15 knot arrow can actually mean 20 knots of knots with gusts of up to 30 knots. Maybe even with a gust of wind up to 40 knots and more at the passage of the front. As you can see coming out of the harbour having only taken a quick look at a wind map or grib file can lead to nasty surprises. Especially if you have not crossed this information with the general synoptic situation and the presence of passing fronts. During the Global Ocean Race many wrote to us from home saying “you should have 25 knots of wind”. If you repeat the calculations made above we had about 30 knots average wind gusting 40 knots or more. The few times we saw 60 knots or more the wind maps or gribs where showing us in an area of arrows around 30-35 knots of wind.

Torn spinnaker and typical post cold front clouds
Torn spinnaker and typical post cold front clouds 

The need to pay attention to the passage of fronts

The most classic example is that of looking at a wind field that during the day indicates a wind rotation from southwest to northwest. In the life cycle of a depression, it is a clear sign of the passage of a cold front. The model inevitably carries out an interpolation between one point and another and with the passage of the front it will indicate a gradual rotation of the wind. In reality, things are very different and the wind could suddenly turn 90 degrees. The model instead will show you a nice breeze of 15 knots in progressive rotation of 15 degrees every hour over 6 hours. In reality there will be nothing progressive, the wind will rotate in a flash while you are knocked about by a very annoying cross sea!

Boating accidents happen when the cold front passes

This is the point in the lifecycle of a depression where spinnakers are torn, booms broken and boats dismasted. First of all, the rotation of the wind brings a cross sea which makes everything more complicated. After the front, in that moment of calm before reinforcement, it is a classic mistake to hoist everything up thinking it’s all over. Full mainsail and light masthead spinnaker up, only to finding yourself a few minutes later with the boat knocked down and the spinnaker in shreds.

Route optimisation using wind maps or grib files

Here I am making just a few hints on the subject which is very complex and of particular interest only for long race sailors. The wind maps data can be downloaded to your computer with GRIBs that contain the vectorial representation of the current and expected wind. This file can be viewed with programs grib viewers or on a navigation software. Among these the most popular are Maxsea (Timezero), Expedition and Adrena, the latter growing rapidly as a software used by professionals. There is an open source free software called Qtvlm growing in popularity.

In addition to GRIB files, to perform our routing we need to know the predicted boat speed for each possible wind angle and intensity. This information grid is referred to as the boat polars.

The routing software

The navigation software, with a GRIB file, polar data and a route can perform a routing simulation. From the starting point it will calculate a very high number of possible routes in successive iterations. At the end of the simulation, the software will present the route that takes the shortest time to reach the destination. This, in addition to the range of sub-optimal routes for all successive steps along the so-called isochrones. These also allow us to understand how far this route differs from alternative routes.

This is obviously an extreme simplification of the process and is worth a consideration applicable to any type of simulation. The result is as reliable as the quality of the data input. If your polars are not indicative of the averages that you can actually sail, the model will make calculations with validity that will be almost nil. As for the wind data, we have already seen how there are many limitations of the wind maps and grib files. Unreliable in light winds, gribs underestimate gusts and perform interpolations by presenting us with incorrect data. Such as for example the progressive rotation of the wind rather than a violent jump in correspondence with the passage of a cold front.

Weather improving but still strong wind after the cold front
Weather improving but still strong wind after the cold front 


Wind maps or grib files are an increasingly popular tool among racers and yachtsmen but you need to understand their limitations in order to make good use of them. The representation of the wind in the form of grids with vector data has opened new horizons for the competitor.

Instead of analysing the synoptic maps to make his own considerations on the optimal course, the sailor will soon succumb to the temptation to rely on data processing on their computer. With the help of a navigation software the sailor will delegate this difficult calculation to the computer with the risk of getting a wrong answer. This is because it will suffer from all the limitations that afflict the wind maps and grib files.

The result will have little validity if it is not interpreted with an expert eye.