{"id":148226,"date":"2023-09-21T06:00:10","date_gmt":"2023-09-21T05:00:10","guid":{"rendered":"https:\/\/www.yachtingworld.com\/?p=148226"},"modified":"2023-09-21T09:39:19","modified_gmt":"2023-09-21T08:39:19","slug":"how-to-find-the-right-weather-window","status":"publish","type":"post","link":"https:\/\/www.yachtingworld.com\/cruising\/how-to-find-the-right-weather-window-148226","title":{"rendered":"How to find the right weather window"},"content":"Radar and AIS help to make navigating in fog safer, but it is still an unnerving experience to sail in visibility of less than 1,000m. Foghorns are hard to pinpoint and the rumble of large engines seem closer than they actually are. It usually feels as if you are sailing around in circles.\r\n\r\nAt some stage all of us will encounter fog and it will have been generated by one of three processes.\r\n\r\nhttps:\/\/youtu.be\/E9jlBF00w4w\r\n

What is advection fog?\r\n<\/strong><\/h3>\r\nAdvection fog \u2013 the widespread fog that covers large sea areas \u2013 is caused when a warm moist air mass moves over a cold sea. The cold sea cools the air above to below its dew-point, causing moisture in the air to condense. Fog is formed from the numerous water droplets.\r\n\r\nThis happens over a large area and will persist until there is a change in the air mass. This can take anything from hours to days. In one recent Round Britain Race, we started from Calais in fog and returned down the North Sea in even thicker fog; it did lift a little for a while in between. It was a fairly memorable race, but not for the right reasons.\r\n\r\nVarious locations are prone to fog with a given weather pattern. Indeed, it can be so common that the fog receives a local name \u2013 the Haar or sea fret along the east coast of England and Scotland, for example. In an east wind, warm continental air moves over the cooler North Sea. By the time it reaches the UK dense fog has formed. Sea fret doesn\u2019t penetrate far inland, but it means the east coast can struggle with low temperatures and poor visibility while the rest of the UK basks in sunshine.\r\n

Grand Banks fog\r\n<\/strong><\/h3>\r\nThe best known fog in the Atlantic is that found over the Grand Banks. The prevailing wind direction is south-westerly, which brings warm moist air from the Tropics and the Gulf Stream. Cold water moving south with the Labrador Current provides a steep temperature gradient between the currents, causing thick fog banks to form quickly as the warm air moves over the colder waters.\r\n\r\nWhen considering a passage along the northern route from the US to Europe, staying north or south of the Gulf Stream will usually make all the difference in temperature and visibility.\r\n\r\nThis passage can be quite testing. We found ourselves surrounded by radar echoes on one occasion and although most of these were fishing boats, some may have been ice. We were pushing hard, so we all slept feet forward just in case.\r\n\r\nFog and ice are not great sailing companions and in the days of the Whitbread Round the World Race (see some great images of EF Education from the 1997-98 race here<\/a>), before the use of ice gates in the Southern Ocean, poor visibility was a source of concern owing to the increased chance of ice.\r\n\r\nThe fog indicated cold water and bergs or \u2018bergy bits\u2019 (small ice that doesn\u2019t show on radar, but is big enough to do damage) could be around; spectacular sailing, but also somewhat on the scary side.\r\n

What is radiation fog or coastal fog?<\/strong><\/h3>\r\nWhat we often see around coasts of the UK is radiation fog, generated from the radiative cooling of the land at night, which in turn cools the air above. Any moisture condenses and forms fog. This is most common in the early morning after a cold, clear, still night.\r\n\r\nAlthough this is a land fog, it does drift down valleys and estuaries to the sea, and morning fog can spoil those early departures. We used to keep our boat up the Tamar river, between Devon and Cornwall, and it was always a great feeling to slip away at first light. But this plan could be ruined by radiation fog which reduced visibility to a few metres. Radiation fog will burn off quickly as the sun intensifies and is limited to coastal waters as the warmer sea will disperse it.\r\n\r\n[caption id=\"attachment_98081\" align=\"alignnone\" width=\"630\"]\"Early Early morning fog on the River Tamar.[\/caption]\r\n\r\nThe final main process that generates fog is known as frontal fog. This forms when rain falls into cooler, drier air and some of the drops evaporate into water vapour. As the air becomes saturated some of the vapour condenses into fog.\r\n

How to navigate a yacht in fog\r\n<\/strong><\/h3>\r\nKnowing the different processes of the development of fog helps you to decide your tactics. With advection fog<\/strong> you either need a change in the wind direction to shift the air mass to a drier, cooler source region, or you need to sail to warmer water.\r\n\r\nThis may be feasible if you\u2019re close to the Gulf Stream, but generally it is impossible. The tide may introduce a change in the water temperature, but it\u2019s not usually enough to clear fog.\r\n\r\nSo, to get better visibility you will have to wait for a change in wind direction or speed \u2013 in most cases once the wind increases to over around 15 knots, fog lifts to low stratus clouds and visibility improves. That said, it will linger even with a strong wind in locations where there is a large difference between the water and air temperatures.\r\n\r\nFrontal fog<\/strong> will quickly pass and radiation fog<\/strong> will burn off \u2013 a bit of patience really does go a long way.\r\n

What is Dewpoint?<\/strong><\/h3>\r\nUnless you have a means to measure humidity on board it will be hard to estimate the dew point of the air, although some weather reports on the internet will provide this.\r\n\r\nHowever, marine forecasts do give an indication of expected visibility. While GRIB files increase in popularity on board, they should be viewed as adding to the marine forecast rather than replacing it.\r\n\r\nChris Tibbs<\/strong> is a meteorologist and weather router, as well as a professional sailor and navigator, forecasting for Olympic teams and the ARC rally","excerpt":{"rendered":"

How do you pick the right departure date? One of the most critical aspects of planning for an offshore passage is choosing the right weather window for departure. A weather window refers to a period of favourable weather conditions that allow for safe and efficient passage \u2013 for either the beginning, or majority, of your passage. Here, we take a look at the process of selecting the ideal window. We\u2019re particularly focussing on using PredictWind, though the same principles apply to other sources of weather data. Plan your route The first step is to plan your route. Take into account seasonal weather patterns and look for past weather data, using tools like Pilot Charts or Historical Weather in PredictWind. These provide years of averaged monthly data for wind speed, wind direction and waves, along with other atmospheric data. Other sources of information include forums, cruising associations, or rally organisers. Jimmy Cornell\u2019s World Voyage Planner and Ocean Atlas includes historical data on average wind directions and strengths on many popular cruising routes. One essential planning consideration is ensuring you have enough time and avoiding hard deadlines. If you have less than a week to find a weather window with tough conditions, it\u2019s likely to force you to make a bad decision. Monitor the big picture Once you know approximately when you\u2019re planning to depart, it\u2019s important to monitor the big picture. Look at the global view to understand the larger weather patterns that may affect your passage, paying particular attention to wind, currents, and wave states. For an ocean crossing this may involve studying weather patterns over a longer period of time and running routing simulations to see how the weather develops in different scenarios. Ocean currents can be caused by many factors such as wind, density differences in water masses caused by temperature and salinity variations, gravity, and events such as earthquakes or storms. PredictWind offers three different ocean current models \u2013 RTOFS, HyCom and Mercator \u2013 on a global scale along with high resolution tidal currents for popular coastal areas around the world. The ocean and tidal currents are used in the Weather Routing and Departure planning calculations and data outputs, with warnings showing in the routing when you have a situation such as wind against current. Different current models can also be compared in the maps with your routes overlaid. Local weather observations can also help identify trends and provide valuable insights into likely weather conditions that will be encountered. These can be found through a variety of resources, such as The National Weather Service in the US, weather buoys, marinas and airports, and ship reports, as well as apps and GRIB files. If you\u2019re doing coastal passages, high-resolution modelling in shorter time frames can make a big difference to the accuracy of the forecast conditions. High-resolution modelling gives more detail and accuracy around land formations from having a much higher number of grid points; they\u2019ll also forecast thermal activity (sea breezes) which you don\u2019t see in lower …Continue reading »<\/a><\/p>\n","protected":false},"author":447,"featured_media":143611,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[139,975,977,1068],"tags":[2447,1633,723],"review_manufacturer":[],"acf":[],"introduction":null,"_links":{"self":[{"href":"https:\/\/www.yachtingworld.com\/wp-json\/wp\/v2\/posts\/148226"}],"collection":[{"href":"https:\/\/www.yachtingworld.com\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.yachtingworld.com\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.yachtingworld.com\/wp-json\/wp\/v2\/users\/447"}],"replies":[{"embeddable":true,"href":"https:\/\/www.yachtingworld.com\/wp-json\/wp\/v2\/comments?post=148226"}],"version-history":[{"count":3,"href":"https:\/\/www.yachtingworld.com\/wp-json\/wp\/v2\/posts\/148226\/revisions"}],"predecessor-version":[{"id":148373,"href":"https:\/\/www.yachtingworld.com\/wp-json\/wp\/v2\/posts\/148226\/revisions\/148373"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.yachtingworld.com\/wp-json\/wp\/v2\/media\/143611"}],"wp:attachment":[{"href":"https:\/\/www.yachtingworld.com\/wp-json\/wp\/v2\/media?parent=148226"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.yachtingworld.com\/wp-json\/wp\/v2\/categories?post=148226"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.yachtingworld.com\/wp-json\/wp\/v2\/tags?post=148226"},{"taxonomy":"review_manufacturer","embeddable":true,"href":"https:\/\/www.yachtingworld.com\/wp-json\/wp\/v2\/review_manufacturer?post=148226"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}