when I see a marine weather report, the wind direction is given. for
example: wind from the southwest at 10-15 knots.

is the wind direction based on the true compass direction or magnetic?

So,, wind from the southwest = wind from 225* true

And if along the coast of Maine this would mean = wind from 242* magnetic

Do I have this right?

is the wind direction based on the true compass direction or magnetic?

True.*


So,, wind from the southwest = wind from 225* true
And if along the coast of Maine this would mean = wind from 242* magnetic
Do I have this right?

If the variation is 17 West where you're talking about then yes, you
do.

* Unless you're talking about true wind as measured by most on board
instrument packages, these tend to be magnetic and based solely on
magnetic heading, speed through the water and apparent wind speed and
direction ignoring set, drift and leeway.

-- Tom.

wrote in message
ps.com...
is the wind direction based on the true compass direction or magnetic?

True.*


So,, wind from the southwest = wind from 225* true
And if along the coast of Maine this would mean = wind from 242* magnetic
Do I have this right?

If the variation is 17 West where you're talking about then yes, you
do.

* Unless you're talking about true wind as measured by most on board
instrument packages, these tend to be magnetic and based solely on
magnetic heading, speed through the water and apparent wind speed and
direction ignoring set, drift and leeway.

-- Tom.


And then you can mostly discount the forecast anyway, since it's almost
unheard of for the wind to actually do what weathermen say it will.

On Wed, 22 Nov 2006 02:20:25 GMT, in message
JtO8h.3548$_x3.1618@trndny02
"NE Sailboat" wrote:

when I see a marine weather report, the wind direction is given. for
example: wind from the southwest at 10-15 knots.

is the wind direction based on the true compass direction or magnetic?

So,, wind from the southwest = wind from 225* true

And if along the coast of Maine this would mean = wind from 242* magnetic

I don't think any of the forecasters actually intend you to take a
bearing from their forecast. When I feel confident I interpret 10-15
from the southwest to mean 5 -20 from a direction somewhere between
south and west, so variation is moot.

SW is a 45 degree octant.

Ryk

Ryk wrote:
...I interpret 10-15
from the southwest to mean 5 -20 from a direction somewhere between
south and west, so variation is moot. ...

The OP says he has almost two points of variation which is nearly half
of your four points of expected error. Seems to me that the difference
between True and Magnetic is worth considering at least in a general
way in his case.

-- Tom.

wrote:
Ryk wrote:
...I interpret 10-15
from the southwest to mean 5 -20 from a direction somewhere between
south and west, so variation is moot. ...

The OP says he has almost two points of variation which is nearly half
of your four points of expected error. Seems to me that the
difference between True and Magnetic is worth considering at least in
a general way in his case.

-- Tom.

We don't all sail off the coast of Maine, so the variation will depend on
where you sail.

I would think that the reason weathermen use the cardinal points system
rather than actual degrees is that it gives them some leeway in their
estimates of what direction the wind will blow from, since the wind veers
and backs many times an in hour sometimes, as anyone who has raced will
testify.

NE Sailboat wrote:
when I see a marine weather report, the wind direction is given. for
example: wind from the southwest at 10-15 knots.

is the wind direction based on the true compass direction or magnetic?

So,, wind from the southwest = wind from 225* true

And if along the coast of Maine this would mean = wind from 242* magnetic

Do I have this right?


The report always assumes True, not Magnetic. Since many of the
recipients of the reports are land based, and not using a magnetic
compass, the reports would make no sense if not True.

While the forecast may not always be accurate enough to make a
difference, it now possible to get realtime reports for stations. In
fact for several years I've been picking up report on my cell phone
that include wind speed and direction. I can stay connected much of
the time on my normal travel routes, so if I used a PC for navigation
(instead of my 'berry for quick updates) I might be getting reports like:

http://tidesandcurrents.noaa.gov/dat...20Observations

....
I would think that the reason weathermen use the cardinal points system
rather than actual degrees is that it gives them some leeway in their
estimates of what direction the wind will blow from, ...

Good point. If you are using data from an automatic reporting station
or a digital model then you will get a report in degrees. Of course,
even in the precise area of the station or grid the wind will vary over
time and forecasts all have some level of error as well. WX reports
usually try to predict conditions over time and area which adds more
uncertainty. It all gets pretty fuzzy, pretty quickly.

-- Tom.