Dropping Barometer - when to run

[Dave_S]

I've been reading up on predicting the weather from what's going on around me. "watch for a big drop in barometric pressure in a short time frame" comes up every time - but what is big or rapidly

I've tried to find a scale or chart to indicate what might be coming based on rate of change but can't find what I'm looking for.

As a guide what sort of numbers do you run from (or more to the point what should I run from).

So, I'm out in my cat, cut off from the world and I observe the barometer dropping faster than usual. I can figure out what direction the low is and what direction to go in, but should I run or is it just going to be a bit bumpy.

How far out is the low when you see "Big" changes

[Lepke]

Get a copy of Bowditch - American Practical Navigator
There is a section on tropical cyclones that describes weather elements that indicate a coming typhoon/hurricane. Lot of other good info for a navigator.

You should be able to find a digital copy online. Or read it here:
https://en.wikisource.org/wiki/The_A...ical_Navigator


You can find a digital copy here:

https://msi.nga.mil/NGAPortal/MSI.po...2&pubCode=0002

[conachair]

Originally Posted by Dave_S

I've been reading up on predicting the weather from what's going on around me. "watch for a big drop in barometric pressure in a short time frame" comes up every time - but what is big or rapidly

I've tried to find a scale or chart to indicate what might be coming based on rate of change but can't find what I'm looking for.

As a guide what sort of numbers do you run from (or more to the point what should I run from).

So, I'm out in my cat, cut off from the world and I observe the barometer dropping faster than usual. I can figure out what direction the low is and what direction to go in, but should I run or is it just going to be a bit bumpy.

How far out is the low when you see "Big" changes

Masses of info in here - very good read >

https://www.amazon.com/Barometer-Han...meter+handbook

[cal40john]

Dashew's Mariner's Weather has some info on that -
Free books
https://www.setsail.com/free-books/

[El Pinguino]

Essentially ... in the tropics watch the diurnal change on your barometer.... ideally on your barograph or else log it every 2 hours.

The pressure will typically have a range of about 3mb...+/- 1.5 mb from 'mean'...... twice a day.... if it drops out of that band.... start paying very serious attention....

Then watch the wind and establish what quadrant you are in .. 'safe' or 'dangerous'... and then act accordingly...

[conachair]

Originally Posted by Dave_S

As a guide what sort of numbers do you run from (or more to the point what should I run from).

So, I'm out in my cat, cut off from the world and I observe the barometer dropping faster than usual. I can figure out what direction the low is and what direction to go in, but should I run or is it just going to be a bit bumpy.

How far out is the low when you see "Big" changes

From the barometer handbook >

[Dave_S]

OK, obviously I'm really crap at searching.

Thank you all, that will give me some good reading.

[JPA Cate]

Hi Dave, sorry we missed you in Manly.

Years ago, we got a book in NZ, "Weather for NZ Fisherman", or something like that. It will fill you in, in an easy to assimilate format.

Ann

[SVTwilight]

Originally Posted by Dave_S

OK, obviously I'm really crap at searching.

Thank you all, that will give me some good reading.

Some more good info when you google "Ocean Prediction Center".

You can get access to plenty of weather loops 24-96 hours that point out the location and probable speed and direction of the low causing the drop. Once you have that it is simply a risk and speed calculation for you to sort out.

The risk is: will it actually get to you and how bad is it?

The speed part is, if you don't want to be near it, do you run or hunker down? (Also with their own associated risks)

Large scale Sat images and prediction capabilities have come a long way since much of this conventional wisdom was penned and with more info you can be better armed to make the best choices for your situation.

We took a course from Lee Chesneau that really simplified a lot of this for us and showed us the value of less obvious resources like the 500Mb charts

[Lihuedooley77]

We add some other things into the equation.

For us here in socal, barometric pressure drop and and a southerly wind, usually indicates a low pressure storm is approaching, and we will get rain in 12 to 24 hrs. How many of today's sailors know that .

Back in the day....... , north of the equator.... barometic pressure drop. point your bones in into the wind, stick out your right arm.....that is where the center of the low pressure system is located. Watcha gonna do ?

Figure out a hurricane hole or protected anchorage, or square your vessel away and prepare to survive . Should it be september, What are you doing in those waters during hurricane season in the first place ?

You can also combo your barometric pressure drop with the information that you get off a weather fax, or what ever.

What is the safe side of the semi circle spinning around the eye. North and south of the equator.

Look that up if you wish...if it is still being published. Interesting old time sailing tips.

[Dave_S]

Originally Posted by JPA Cate

Hi Dave, sorry we missed you in Manly.

Years ago, we got a book in NZ, "Weather for NZ Fisherman", or something like that. It will fill you in, in an easy to assimilate format.

Ann

Thanks Ann

Could it have been this one

"Coastal Radio and Weather for New Zealand Fishermen"

I have found this but there is not much detail about it. Not even a pic of the cover.

Just want to make sure its the right one before I buy it.

[Dave_S]

Originally Posted by Lihuedooley77

We add some other things into the equation.

For us here in socal, barometric pressure drop and and a southerly wind, usually indicates a low pressure storm is approaching, and we will get rain in 12 to 24 hrs. How many of today's sailors know that .

Back in the day....... , north of the equator.... barometic pressure drop. point your bones in into the wind, stick out your right arm.....that is where the center of the low pressure system is located. Watcha gonna do ?

Figure out a hurricane hole or protected anchorage, or square your vessel away and prepare to survive . Should it be september, What are you doing in those waters during hurricane season in the first place ?

You can also combo your barometric pressure drop with the information that you get off a weather fax, or what ever.

What is the safe side of the semi circle spinning around the eye. North and south of the equator.

Look that up if you wish...if it is still being published. Interesting old time sailing tips.

You'll be pleased to hear they still do teach those techniques. Except down here it is your left hand + angle 20* foreword into the wind for the centre.

[AiniA]

There is a considerable difference from area to area. After many years sailing the Great Lakes I knew how the weather worked pretty well. When we started to cruise in the tropics and subtropics there was a lot of new stuff to learn. The resources are out there.

[jhulmer]

I don't know if this helps but - We rode out Hurricane Sandy in Baltimore Inner Harbor. I decided to mark the barometer (set the needle) and watch as it approached.

The second picture is the lowest we saw as Sandy passed us by on the way to NJ. Note the time.

I don't have the log with me but I remember this change over about 24 hrs with the biggest change in the last 10 hrs. It was kind of scary to watch because, at the time they had it coming up the Chesapeake but it made a turn later. We still clocked 70 mph winds (gusts).

If we didn't have any weather report and I saw this I wouldn't know which way to run. however, the cloud bands reached us early on giving some indication of from where the storm was approaching. I would probably start looking for a hole far inland to hide.

[Al Walker]

Originally Posted by Dave_S

Thanks Ann

Could it have been this one

"Coastal Radio and Weather for New Zealand Fishermen"

I have found this but there is not much detail about it. Not even a pic of the cover.

Just want to make sure its the right one before I buy it.

If you go onto the NZ Metservice website and have a ferret about you ought to be able to find a downloadable copy of Jim McDavitt's very good guide to understanding weather forecasting in this corner of the world. Excellent material. If you can't find it, let me know and I'll flick you over a copy.


Al