How many millibars in how many hours?

[burgundyben]

I was taught to watch the barometer, it was said that if it dropped X millibars in Y hours it would blow a gale.

I cant remember the numbers.

Was it 8 millibars in 2 hours?

 

[rickp]

I thought 6mb in 3 hours (ie. falling rapidly) always indicated a gale?

Rick

 

[bilbobaggins]

That must have been a memorable lesson! /forums/images/graemlins/wink.gif

The RYA's excellent 'Weather Handbook' by Chris Tibbs suggests that a change of 5-6mb ( oh, all right, hectoPascales ) in 3 hours predicts a Force 6. A change of 8hPa in 3 hours is likely to foretell an F8, and one needs to consider *any* steep change in baro-pressure as a warning.

You'll find that helpful reminder at the back of the RYA's 'Course Notes' booklets.

Do not forget that a steep *rise* in pressure is also likely to presage a 'blow' - for it is the Pressure Gradient that initiates the change in wind velocity.

Does that help? /forums/images/graemlins/smile.gif

 

[stephenh]

From weather course notes I did years ago - might have changed :

Change in last 3 hrs

Steady ........................up to 0.1 mb
Falling /rising slowly.........0.1 - 1.5 mb
Falling / rising...............1.6 - 3.5 = F6/7? - 12hrs?
Falling / rising quickly.......3.6 - 6.0 = F6/7/8? - 6hrs ?
Falling / rising very rapidly..6 mb + =F7/8/9? - 3hrs ?

Only just noticed that it goes from 'quickly' to 'very rapidly' - there doesn't appear to be 'rapidly'
Might be wrong ???

Sorry - can't get the colomns to line up......

 

[rickp]

Yes, you're right - "very rapidly" falling or rising. My mistake /forums/images/graemlins/wink.gif

Rick

 

[Birdseye]

[ QUOTE ]

The RYA's excellent 'Weather Handbook' by Chris Tibbs suggests that a change of 5-6mb ( oh, all right, hectoPascales ) in 3 hours predicts a Force 6.

[/ QUOTE ]

HectoPascales? Pascale was French so its either millibars or hectofrogs. /forums/images/graemlins/grin.gif

here in Cardiff its just fallen 12 mb in as many hours and its blowing old boots. Havent seen that in the RYA weather handbook!

 

[Jonny_H]

14mb drop in Newcastle since last night - barometer almost at the end of it's range! /forums/images/graemlins/shocked.gif

So far today we have had (in no particular order):

Rain
Snow
Sleet
Temp between -4 and +5
Sunshine
and VERY windy (circa 50mph avg according to local weather station).

Jonny

 

[tcm]

More brilliant things for ben to remeber

If a fire starts to burn
there's one thing that you should learn
something something - then you'll see
there'll be no catastrophee!


(H simpson)

 

[VicS]

[ QUOTE ]
A change of 8hPa in 3 hours is likely to foretell an F8,

[/ QUOTE ] Yes that is what it says in my notes except that it says a fall of 8mb in 3hours

 

[keelbolt]

Um, not if you're sailing with Sunsail. A crew member of mine was asked specifically to record the barometer readings hourly in the log. He objected, saying that he was taught that yachtsmen don't use barometers any more - its old fashoined, and they don't mean anything anyway. When asked who taught him that, he proudly told me all about his Sunsail sailing course earlier that year.

Happily, we were at that time in a steep pressure gradient. I told him to do it anyway, and a few hours later, as the wind got up to force 8 and he was looking decidedly green, I asked him to plot the pressure gradient he had logged. Enough said!

 

[webcraft]

Re: More brilliant things for ben to remeber

Sharp rise before low
There'll surely be a blow

Sharp fall before high
Massive gales are surely nigh

Low then sudden drop
The bloody wind will never stop

High then through the floor
Bank the fire and bar the door

Pressure normal, fine and steady
Here comes a gale, make your ship ready

(That's enough pressure sayings - ed)

 

[Cornishman]

As far as I recall there is no direct relationship between rate of rise or fall and predicted wind force. However, if you are skilful enough to draw up your own synoptic chart on the RYA/R Met Soc maps you can use the geostrophic scales to predict A. wind force B. speed of movement of a warm front and C. speed of movement of a cold or occluded front

 

[bilbobaggins]

[ QUOTE ]
Yes that is what it says in my notes

[/ QUOTE ]

"You say 'tomey-to' and I say 'tomah-to'..."

Isn't it just incredible what the pedants find to quibble about on 'ere?

How many actually USE a barometer on board?

Milllibars and hectopascals

and Aunty Beeb

 

[Twister_Ken]

"How many actually USE a barometer on board?"

I do. Looks nice next to the clock.

 

[Cornishman]

Just what do you write in your log book, then?

 

[jenku]

I do as well. Even write down the readings in the log daily. Reason is mainly I got the thing for christmas once so I feel it should have some use.

 

[bilbobaggins]

[ QUOTE ]

Just what do you write in your log book, then?

[/ QUOTE ]

Wrong end of the stick! I certainly pay close attention to the baro. I've learned the hard way. And I sail in your waters when I can, which reminds me...

[ QUOTE ]
West of The Lizard is no place for a small boat, come the end of August.... Jack Pender

[/ QUOTE ]

M'n thee agree, y'see.

/forums/images/graemlins/wink.gif

 

[DeeGee]

Here's a nice picture for you.... mull over it, you experts....

Slope of steepest part of the curve about 20mb in 8 hours.

 

[DeeGee]

Should have posted this one too, shows the drop far more dramatically than that gentle curve for the last 24 hrs.

 

[peteandthira]

I use mine too, and an electronic barograph (records the last 48 hours).

Not aimed at Bilbo at all, but it might be worth mentioning that gales are not always preceeded by a pressure change of any sort. A suitable weather pattern can travel such that the same isobar is on your location for quite some time, even though they are tightening up to the north and south of you.

In that situation you won't see a pressure drop on a barometer, but will have seen the tightening isobars on a synoptic chart.

BFN

Pops