Sailors have always needed to watch the weather so it should be no surprise that we have lots of weather related tools and books on board to understand weather at sea before we set sail. Maybe ‘understand’ the weather is too literal. Maybe ‘some idea’ of the weather is better.
As we leave the US of A we leave a lot of what I and others have taken for granted: marine weather in our language, an accessible VHF weather broadcast, coastal forecasts, NOAA. In Mexico, more than the US have to to be our own forecasters, although I don’t look or talk like any of those TV weatherpeople and I can’t point and look in different directions at the same time. Some of the coasts we traverse have no safe harbor for many miles, no place to rest, no place to get out of the wind and waves, translating to many days of discomfort if we guess wrong. So we try as best as we can to forecast our own weather for some period of time usually one to four days.
Aboard Cetacean we use GRIB files a lot. GRIB files are the output of global computer models run four times daily. A few computers around the world run these models, all with slightly different goals, so the results are slightly different; one may be better closer to shore the others better out at sea, etc. We usually use the GFS operated by NOAA. Because these are global software models, wind/barometric pressure information available for any location on earth, with detail only limited by how much data one wishes to download. These files, are (generally) small and easily sent over low bandwidth communication systems. We mostly use the ham radio and or SSB (single side band) and wifi when in port. Atmospheric interference usually limits download speeds to 200-600 bits per second, compared to the average Comcast connected house at 100 million bits per second we are 500,000 time slower. We also occasionally use our Satellite phone but it’s not much faster and a lot more expensive to use. Now we have a Mexican TELCEL data dongle, it will be interesting to get data from cell phone. The SSB/HAM radio works with a network of stations all over the world. It’s a really wonderful service. There is usually a station that’s optimally located for our location and time of day; there are dozens scattered around the globe.
Unlike National Weather Service products, GRIB forecasts come directly off the computer, untouched by human eyes; they don’t get checked for obvious errors (to a meteorologist), or that they even make sense. We usually use some sort of check to verify the output makes sense. The easest for us is compare our on-board barometer to the isobars expected on the GRIB, then we have at least a crude sense of correctness. On the other hand, in the USA, the NWS issues maps that are checked against real time reports from buoys, ships and weather balloons and are adjusted as necessary.
GRIB files give us a peep show hole into the weather picture. The subject is so complex, it’s like looking at the Mona Lisa through a pin hole, it looks different every time we put our eye up to the cardboard and you can never see the whole thing at one time. There is a lot to learn.
As we sit here in waiting for a weather window, yesterday, the wind was blowing through the marina from the west at between 18 and 25 knots, that’s about 21 and 28 mph.
Last afternoons winds were mostly sea breezes. Meaning they are the result of land heating faster than the sea during the day and by the time 4 pm rolls around the temperature difference has caused a pressure difference and therefore a wind from the sea toward the land. The wind then turns a bit so it end up north westerly. The turn is from the Coriolis force, well it’s not really a force but rather a result of the us trying to understand a spherical and rotating environment (remember we live on a sphere and it turns) world in our two (or three) dimensional forecasts. The result is the wind veers (clockwise) and becomes northwest. These winds die in the evening.
Most of the (non sea breeze) wind in this part of the world results from major pressure differences. There is a semi permanent high pressure that sits off the US west coast. Located somewhere between San Francisco and Hawai’I it moves, especially with the season, but stays in the general vicinity. A low pressure area usually sits over Yuma Arizona in the spring and summer; because it’s hot and hot air is not as dense, a low pressure results. When these two regions get close the wind blows from the high to the low ( and then twists because of the aforementioned Coriolis effect) and you get wind, sometime large, like 30-40mph.
We can see these pressure differences on weather maps provided by the National Weather Service and on the GRIB files. The closer they get; the closer the isobars (lines of constant barometric pressure), we can calculate the wind if we also know our latitude.
Cruisers love to share what they know,especially about weather, so there are organized daily SSB and or HAM band discussions among cruisers in geographic regions. There are usually a weather forecasts given by the cruiser or a knowledgeable person at there house, like Gary in Bahia Conception, Baja California, that know more than the rest of us. In Baja California there is the Chubasco on the HAM bands and Sonrisa on the SSB bands. These network have been active, continuously for many, many years. The late, Don Anderson evaluated the weather from his home near Santa Barbra, and broadcast much needed weather information to cruisers on many networks. He will be missed.
Between the ‘nets’ and our on board weather ‘guesses’ we do OK. We are leaving this morning for a multi-day trip down the Pacific Baja Coast, GRIBS look good.