How to tell how thick the fog is... - SailNet Community
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post #1 of 8 Old 09-24-2012 Thread Starter
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How to tell how thick the fog is...

Right now I'm reading SailNet and listening to the sound of fog horns from ships. In our area we often get morning, or all-day fog, especially in july-october.

I've had times where I can see a wall of fog, and I wonder what my visibility will be after I sail into it. Once I sail into it, I continue to be able to see a mile. I sail on and my visibility remains a mile.I've had other times where I've sailed into a wall of fog and my visibility was 1/8 mile and stayed that way.

Is there any way to tell what your visibility will be once you sail into "that fog bank over there"? Will it be 1 mile once your in it or pea soup?

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post #2 of 8 Old 09-24-2012
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Re: How to tell how thick the fog is...

Tough one. Maybe contact boats in the fog via VHF. AIS could help here by identifying a boat in the area without having to call all boats.

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post #3 of 8 Old 09-24-2012 Thread Starter
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Re: How to tell how thick the fog is...

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Originally Posted by treilley View Post
Tough one. Maybe contact boats in the fog via VHF. AIS could help here by identifying a boat in the area without having to call all boats.
Actually, I kind of cheated and did just that. I don't have AIS (though I realised that it would be helpful here) and I called boats that I "assumed" would be out there where I was trying to go.

First I tried "Vessel in the northern end of the Rosario strait" -- No reply.

Then, since there are about 5 ferries that all go through this one area I called "Washington state ferry in the vicinity of Thatcher Pass." --Instant reply and fog report.

Strictly speaking, not how you are supposed to use your VHF though, calling ghosts in the fog.

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post #4 of 8 Old 09-24-2012
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Re: How to tell how thick the fog is...

Another way to look at it, if you sail into fog at 6 knots and someone is coming at you at 6 knots from the opposite direction, you have about 30 seconds to see each other before you collide if visibility is 1/8 mile (15 miles in 60 minutes or 1/4 mile in one minute closing rate,) not to mention fog can be denser in one spot than another ie visibility could get worse before it gets better. Makes radar (and knowing how to use it) or staying out of the fog seem like a good idea IMHO. Barges and other large vessels often travel much faster than 6 knots and the bigger they are the larger the radar shadow they have in front of them.
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post #5 of 8 Old 09-24-2012
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Re: How to tell how thick the fog is...

Quote:
Originally Posted by MedSailor View Post
Right now I'm reading SailNet and listening to the sound of fog horns from ships. In our area we often get morning, or all-day fog, especially in july-october.

I've had times where I can see a wall of fog, and I wonder what my visibility will be after I sail into it. Once I sail into it, I continue to be able to see a mile. I sail on and my visibility remains a mile.I've had other times where I've sailed into a wall of fog and my visibility was 1/8 mile and stayed that way.

Is there any way to tell what your visibility will be once you sail into "that fog bank over there"? Will it be 1 mile once your in it or pea soup?

MedSailor
You might want to try looking at the top edge of the fog bank as you approach it. You can see in the way the sky color/whiteness changes how much thickness of fog there is at different angles into the fog there. (I've never tried that, but when in the fog, I do look at the sky color straight up - and up at an angle - to determine the height of the fog bank. This as more for flying than for sailling.)

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post #6 of 8 Old 09-24-2012
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Re: How to tell how thick the fog is...

Once your in it look for land or a vessel that is just visible and then use your radar to measure the distance.

In my experience your depth perception goes all to heck in fog. And so can your sense of direction.

Current weather conditions on Wx often describe visibility. That is actually not that current.

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post #7 of 8 Old 09-24-2012
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Re: How to tell how thick the fog is...

I often use my plotter cursor to a known object I can barely see..

Like here where we could almost make out this island if we looked closely. As you can see we are at about 411 feet away so roughly 400 feet of vis.


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post #8 of 8 Old 09-24-2012
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Re: How to tell how thick the fog is...

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Originally Posted by Maine Sail View Post
I often use my plotter cursor to a known object I can barely see..

Like here where we could almost make out this island if we looked closely. As you can see we are at about 411 feet away so roughly 400 feet of vis.

That also shows how good it is to have the radar overlay on top of the chart platter. Very nice.

Regards,
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