The Weather

Weather, the first guarantee of safety at sea

A guarantee of prudence, the first thing to do before deciding to go out to sea is of course to check the marine weather forecast for anyone who cares about safety in the practice of their passion, sailing. And since it is not an exact science, it is better to cross-check information sources in order to form the most accurate possible idea of the weather to come and its evolution.

For example, it can be useful to compare forecasts over fairly large areas offered by sites such as Météo France with very localized forecasts provided by sites such as Wind Guru. The aim is therefore to know as precisely as possible what to expect once at sea.

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Another good habit to adopt is to compare theoretical forecasts with the actual evolution of the weather in the field. Observing clouds, atmospheric pressure and temperature makes it easier to anticipate the different phenomena and therefore the best options to optimize the route, as well as the safety of the vessel and its passengers.

La Rochelle: an ideal climate for recreational boating

Who said you should save the best for last? According to data collected by Météo France, the amount of sunshine in the fine city of La Rochelle approaches 2,100 hours over the 30-year period from 1981 to 2010.

These statistics, which have remained consistent to this day, make the Charente coast one of the sunniest in France after, of course, the Mediterranean coast, where annual maxima approach 2,900 hours in Marseille. The famous La Rochelle microclimate is therefore not a legend but a reality for tourists and recreational sailors, often quick to reach for sunscreen for much of the year.

While sailing is more pleasant in sunshine, it also requires wind, but not too much. On this point, the Charente coast far outperforms the Mediterranean, where the sun shines more but where the mistral and tramontane, which blow throughout the year, tend to push Mr. Beaufort higher up his scale. Violent winds are much less frequent in La Rochelle, with an annual average below force 4 and predominantly westerly winds. Note that sunshine does not mean the absence of rain.

With an annual average rainfall below the national average, marine weather in La Rochelle remains very attractive. It is also milder than south of the Gironde, where strong winds are more frequent and rainfall heavier due to the proximity of the Pyrenean reliefs.

The limits of the La Rochelle microclimate extend roughly from north of the Gironde to south of Vendée. A fine playground that largely includes the islands of Ré, Aix and Oléron, offering excellent sailing opportunities.

Finally, the La Rochelle sounds (the areas between the islands and the mainland) provide good natural shelter from offshore swell on windy days.

Where to get the marine weather forecast in La Rochelle?

The “marine weather” bulletins from Météo France are the official forecasts for France.

The north of the island of Ré lies at the northern boundary of the coastal zone of the Anse de l’Aiguillon, at the Spanish border.

These coastal forecasts are accessible via the Météo France website or mobile application.

Marine weather is displayed in all marina harbormaster’s offices, although sources may vary (Météo Consult bulletins are often found there).

At sea

Outside mobile phone coverage areas, official Météo France coastal bulletins for the 20-mile band are continuously broadcast on VHF channel 63 by the Chassiron semaphore.

Good to know

The anemometer of the Minimes harbor weather station can be consulted in real time on the harbormaster’s website:

https://www.portlarochelle.com/meteo/

Thermal breezes

Thermal breezes are not uncommon during the summer season. As their name suggests, these phenomena are caused by the temperature contrast between land and sea. The greater the thermal contrast, the stronger the wind. These very localized phenomena are rarely mentioned in official weather bulletins, yet they can have a significant influence on the general wind.

Reminder of the principle of thermal breezes

-Daytime thermal breeze: the wind blows from sea to land.

When the sun is high enough in the sky, the land heated by the sun warms the lower layers of the atmosphere by refraction. Air particles expand and begin to rise vertically. The missing air layer at ground level is replaced by cooler marine air, which in turn warms up. Upon reaching a certain altitude, the air particles cool and contract, causing them to descend. A circular air movement is then created above the coastal strip.

-Nighttime thermal breeze: the wind blows from land to sea.

After sunset, air temperature drops in contact with the land, which cools rapidly. The air above the sea is then warmer. This thermal contrast, even if weak, is enough to trigger a nighttime thermal breeze, following the opposite pattern of the daytime thermal breeze.

Conditions for the establishment of thermal breezes in La Rochelle

-good sunshine
-presence of cumulus clouds
-synoptic wind (wind due to the general circulation of air masses) below 18 knots
-air temperature higher than sea temperature

Limiting factors:

-low sunshine
-synoptic wind too strong or opposed to the direction of the thermal breeze
-overly strong anticyclone tending to keep air in the lower layers of the atmosphere and slow the formation of thermal breezes
-absence of cumulus clouds

The breeze gradually sets in around midday (between 11 a.m. and 2 p.m. depending on conditions) and can reach 5 Beaufort if the synoptic wind is weak or favorable in direction.

Wind guru

Originally designed for surfers, Wind Guru is a weather forecast website increasingly consulted by users of all kinds of floating craft. This American site provides precise information on the evolution of weather conditions at a specific location. Many sailors use it to supplement the more general forecasts from sites such as Météo France.

Wind Guru information is presented in the form of tables (see below) with many parameters measured in three-hour intervals: wind strength and direction, wave direction, height and period, temperature, cloud cover percentage, precipitation, etc.

Wind Guru weather data is automatically updated every 6 hours in connection with the National Oceanic and Atmospheric Administration, the U.S. national weather forecasting service.

The site offers several types of mesh for the spots covered. Most European spots are analyzed using the global forecast model GFS 27 km (Global Forecast System). This model corresponds to a division of the area into cubes with edges measuring 27 kilometers, providing relatively broad forecasts well suited to coastal navigation.

https://www.windguru.cz/157