jpcarter30
Well-Known Member
Does anybody have a guide in layman's terms on the purpose of a barometer and how to read one in layman's terms?
Much appreciated.
Much appreciated.
Barometers...a question
- Thread starter jpcarter30
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lenseman
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- 3 Jun 2006
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- South East Coast - United Kingdom
I will start the ball rolling with this one:
It is "the rate of change" of the 'glass' which tells you that the air masses are moving and how fast they are moving.
A steady barometer whether it reads at the high end or the low end of the glass basically means 'no change' (in the weather).
A rapid rise or fall in the needle warns you to shorten sail and run for a safe haven!
It is "the rate of change" of the 'glass' which tells you that the air masses are moving and how fast they are moving.
A steady barometer whether it reads at the high end or the low end of the glass basically means 'no change' (in the weather).
A rapid rise or fall in the needle warns you to shorten sail and run for a safe haven!
Skysail
Well-Known Member
One convention is that a rise or fall of 6 mb in 3 hours indicates a gale - if it is not blowing already!
More here:
http://weather.about.com/od/weatherinstruments/a/barometers.htm
As said below, a barograph or digital instrument gives the most useful picture of pressure over time, but on a boat you can do this by logging the pressure every hour.
Its worth studying the way that pressure falls and rises as a depression passes over, that will give a good indication of how the weather system is developing compared to the forecast.
More here:
http://weather.about.com/od/weatherinstruments/a/barometers.htm
As said below, a barograph or digital instrument gives the most useful picture of pressure over time, but on a boat you can do this by logging the pressure every hour.
Its worth studying the way that pressure falls and rises as a depression passes over, that will give a good indication of how the weather system is developing compared to the forecast.
Last edited:
johnalison
Well-Known Member
There are of course, endless books/articles/sites about weather. I suspect that most of us don't view our barometers with the same intensity as we used to, with more & better forecasts available.
I would propose that the chief use for a barometer today is to compare the true current reading with that predicted and thus to form an opinion as to whether the weather is progressing at the rate predicted, or if, say, a depression is approaching earlier and more deeply than forecast.
For this purpose a barograph is much easier to interpret than a barometer, and modern electronic ones are excellent - I even have one on my wrist, which I look at every time I want to know the time.
I would propose that the chief use for a barometer today is to compare the true current reading with that predicted and thus to form an opinion as to whether the weather is progressing at the rate predicted, or if, say, a depression is approaching earlier and more deeply than forecast.
For this purpose a barograph is much easier to interpret than a barometer, and modern electronic ones are excellent - I even have one on my wrist, which I look at every time I want to know the time.
jpcarter30
Well-Known Member
Thanks for the replies. What I failed to mention was another issue: how do I set one that I have in my cabin? The two hands, most confusing.
Moodyjim
Well-Known Member
You have to match them to the clock, make sure the battery is OK and then keep adjusting every hour!
or
The one that moves you set on top of the one that seems fixed and then see what the "fixed" one does.
Eze innit.
or
The one that moves you set on top of the one that seems fixed and then see what the "fixed" one does.
Eze innit.
Salty John
Well-Known Member
OK, the barometer simplified:
The barometer measures atmospheric pressure. There are areas of high pressure and areas of low pressure moving around the globe. These areas are in constant movement. In some sailing areas, like the UK and northern Europe, these areas move more rapidly and more frequently than in other parts of the globe, such as the more tropical climes.
Generally speaking, if you are in a low pressure area or in a high pressure area the wind is likely to be stable.
However, air flows from areas of high pressure to areas of low pressure.
Air is wind.
So, it’s the interface between these areas that is important to the sailor.
When a low pressure area approaches a high pressure area, the wind flows from the high pressure area to the low pressure area. Depending upon how rapidly the low pressure area approaches the high pressure area, and on how high and low the pressure in the two areas is, the wind flowing from one to the other will be either gentle or fearsome.
The barometer will tell you how rapidly the pressure is changing and therefore how quickly the areas are approaching each other and how steep is the gradient between them. Big gradient, big wind; rapid approach, big wind soon. Big drop in barometric pressure, big wind. Rapid change, big wind soon.
If the barometer reading drops rapidly, you are in for a big blow. If it drops less dramatically, you will see a less aggressive increase in wind. Similarly, when the pressure rises rapidly you can expect a big blow.
Typically, when an area of low pressure, a ‘low’, approaches the UK you will see the barometer start to fall. Rain and wind will arrive. The speed and severity of the barometer 'fall' (movement to left of the dial) in pressure will be a direct indicator of the wind speed experienced. As the low passes the pressure will become steady, the wind will change direction, remain stable or even drop. As the front continues through you will see the pressure fall again briefly and then suddenly rise. Now you will experience strong, gusty wind. Once the low moves away the pressure will become steady again and the wind will ease and stabilise
The terms you see printed on most barometers (Rain, change etc) are traditional and pretty well meaningless, so ignore them.
One hand on a dial barometer is measuring the atmospheric pressure; the other hand is the pointer, the one you set yourself to line up with the measuring hand so that you can detect movement of the measuring hand. If you line them up, go away for an hour and come back to find the measuring hand has moved significantly away from the pointer, you prepare for a blow!
I hope this helps.
The barometer measures atmospheric pressure. There are areas of high pressure and areas of low pressure moving around the globe. These areas are in constant movement. In some sailing areas, like the UK and northern Europe, these areas move more rapidly and more frequently than in other parts of the globe, such as the more tropical climes.
Generally speaking, if you are in a low pressure area or in a high pressure area the wind is likely to be stable.
However, air flows from areas of high pressure to areas of low pressure.
Air is wind.
So, it’s the interface between these areas that is important to the sailor.
When a low pressure area approaches a high pressure area, the wind flows from the high pressure area to the low pressure area. Depending upon how rapidly the low pressure area approaches the high pressure area, and on how high and low the pressure in the two areas is, the wind flowing from one to the other will be either gentle or fearsome.
The barometer will tell you how rapidly the pressure is changing and therefore how quickly the areas are approaching each other and how steep is the gradient between them. Big gradient, big wind; rapid approach, big wind soon. Big drop in barometric pressure, big wind. Rapid change, big wind soon.
If the barometer reading drops rapidly, you are in for a big blow. If it drops less dramatically, you will see a less aggressive increase in wind. Similarly, when the pressure rises rapidly you can expect a big blow.
Typically, when an area of low pressure, a ‘low’, approaches the UK you will see the barometer start to fall. Rain and wind will arrive. The speed and severity of the barometer 'fall' (movement to left of the dial) in pressure will be a direct indicator of the wind speed experienced. As the low passes the pressure will become steady, the wind will change direction, remain stable or even drop. As the front continues through you will see the pressure fall again briefly and then suddenly rise. Now you will experience strong, gusty wind. Once the low moves away the pressure will become steady again and the wind will ease and stabilise
The terms you see printed on most barometers (Rain, change etc) are traditional and pretty well meaningless, so ignore them.
One hand on a dial barometer is measuring the atmospheric pressure; the other hand is the pointer, the one you set yourself to line up with the measuring hand so that you can detect movement of the measuring hand. If you line them up, go away for an hour and come back to find the measuring hand has moved significantly away from the pointer, you prepare for a blow!
I hope this helps.
jpcarter30
Well-Known Member
Cheers Salty, that's what I need to know.
stephenh
Well-Known Member
Got this from don't know where....
the first words are from the Shipping Forecast ( as defined)
and this regards change in the last 3 hours :
steady ....... up to 0.1mb
falling / rising slowly....... 0.1 - 1.5 mb
falling / rising.........1.6 - 3.5mb = F6/7 within 12 hrs ?
falling / rising quickly........3.6 - 6.0 mb = F6/7/8 within 6hrs ?
falling / rising very rapidly......6+ mb = F7/8/9 within 3hrs ?
The above all very approximate in my experience but a good indication of how movement of the barometer relates to time and weather.
I like to log the baro. every hour , must buy one of those electronic ones - does it all for you...
the first words are from the Shipping Forecast ( as defined)
and this regards change in the last 3 hours :
steady ....... up to 0.1mb
falling / rising slowly....... 0.1 - 1.5 mb
falling / rising.........1.6 - 3.5mb = F6/7 within 12 hrs ?
falling / rising quickly........3.6 - 6.0 mb = F6/7/8 within 6hrs ?
falling / rising very rapidly......6+ mb = F7/8/9 within 3hrs ?
The above all very approximate in my experience but a good indication of how movement of the barometer relates to time and weather.
I like to log the baro. every hour , must buy one of those electronic ones - does it all for you...
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