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Old 16-02-2016, 12:06   #16
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Re: Occluded fronts

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Old 16-02-2016, 12:13   #17
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Re: Occluded fronts

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Originally Posted by seaward 42 View Post
There is nothing special about it or to much to say, simply and short...

Occluded Front is when a cold front overtakes a warm front.

or bit longer....

A developing cyclone typically has a preceding warm front (the leading edge of a warm moist air mass) and a faster moving cold front (the leading edge of a colder drier air mass wrapping around the storm). North of the warm front is a mass of cooler air that was in place before the storm even entered the region.
As the storm intensifies, the cold front rotates around the storm and catches the warm front. This forms an occluded front, which is the boundary that separates the new cold air mass (to the west) from the older cool air mass already in place north of the warm front. Symbolically, an occluded front is represented by a solid line with alternating triangles and circles pointing the direction the front is moving. On colored weather maps, an occluded front is drawn with a solid purple line.
Changes in temperature, dew point temperature, and wind direction can occur with the passage of an occluded front. Let say temperatures ahead (east of) the front were reported in the low 40's while temperatures behind (west of) the front were in the 20's and 30's. The lower dew point temperatures behind the front indicate the presence of drier air. A noticeable wind shift also occurred across the occluded front. East of the front, winds were reported from the east-southeast while behind the front, winds were from the west-southwest. .
Thanks but not clear how this answers the question.

Mike
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Old 16-02-2016, 12:19   #18
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Re: Occluded fronts

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Thing is I am not aware how to identify whether an occlusion I am viewing on a surface pressure chart is a cold or warm occlusion?.
Look at the temperature on the opposing sides of the occlusion..
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Old 16-02-2016, 12:26   #19
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Re: Occluded fronts

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Mike,
You got me interested the question of depicted the different types of occluded fronts so I checked the NOAA manual on weather analysis (http://www.wpc.ncep.noaa.gov/sfc/UAS...alVersion1.pdf), and it does not reference different symbols for the different occluded fronts, just one symbol.
Thinking about it, I would have great difficulty trying to determine whether an occluded front is warm or cold from a surface plot of weather data. I think it would require an upper air dataset in the vicinity of the front, which would not always be available. So my further guess is the types of occluded fronts are not depicted on the weather analysis charts because the types cannot be determined from surface data and it isn't all that important for the development of a forecast.
Of course that's just a guess on my part.

John
Thanks John,

If your guess happens to be correct and in the absence of anyone else coming up with anything more convincing I have to conclude there is little point in me being aware of the different occluded fronts if I am unable to determine which are Cold Occlusions and which are Warm Occlusions. I still await a response from the Met Office.

Mike
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Old 16-02-2016, 12:29   #20
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Re: Occluded fronts

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Look at the temperature on the opposing sides of the occlusion..
This would certainly do it but where can I find the temperatures?. I have considered studying the isopleths but this does not seem to be the answer.

Mike
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Old 16-02-2016, 14:20   #21
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Re: Occluded fronts

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Thanks but not clear how this answers the question.

Mike
To be honest I answered before reading your question first, so this should be the answer how to "read" occluded fronts. simply read temperature (gradient).

The temperature gradient is strongest in front of the Warm Front and the Occlusion.


The temperature gradient is strongest in the rear of the Cold Front and the Occlusion.


The thickness gradient is stronger in front of the Warm Front and the Occlusion.


The thickness gradient is stronger in the rear of the Cold Front and the Occlusion.
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Old 16-02-2016, 23:21   #22
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Re: Occluded fronts

As far as I am aware, you cannot tell the difference between a cold and warm occluded front just by looking at the symbols on the chart.

A warm occluded front is far less than common than cold occluded front.
The warm type is more likely to form when a polar maritime air mass meets a polar continental air mass, so Western Scandinavia and North West US/Canada would be typical.

As others have mentioned, look at the temps, with a warm occluded front, the air temp behind the cold front will be higher than the air temp ahead of the warm front.
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Old 17-02-2016, 00:34   #23
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Re: Occluded fronts

Have now received a reply from Met Office. They sent me 3 links to interrogate which I have done but no nearer.

I am thinking John & Nigel are correct, there is no way of determining whether an occlusion is warm or cold by observing a synoptic chart.

I find it easy to imagine that most/all occlusions (UK) are Cold Occlusions, just seems logical, but would rather rely on something more substantive than my own guess work.

Obviously this thread is all about temperature differences, ahead & behind occluded fronts. Apart from isopleths/thickness lines (which I believe are not much use in this case) the synoptic charts I use do not include any other indication of temperature.

Perhaps someone could direct me to more suitable charts for this purpose.

Thanks

Mike
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Old 17-02-2016, 01:20   #24
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Re: Occluded fronts

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Originally Posted by Mike1956 View Post
Have now received a reply from Met Office. They sent me 3 links to interrogate which I have done but no nearer.

I am thinking John & Nigel are correct, there is no way of determining whether an occlusion is warm or cold by observing a synoptic chart.

I find it easy to imagine that most/all occlusions (UK) are Cold Occlusions, just seems logical, but would rather rely on something more substantive than my own guess work.

Obviously this thread is all about temperature differences, ahead & behind occluded fronts. Apart from isopleths/thickness lines (which I believe are not much use in this case) the synoptic charts I use do not include any other indication of temperature.

Perhaps someone could direct me to more suitable charts for this purpose.

Thanks

Mike
I have never heard of 'cold occlusion' vs 'warm occlusion'. Furthermore the concept of occlusion (i.e. cold front catching up with the warm front ahead, pushing the warm sector upwards) which dates back to the twenties (Bjerknes' school) has been long superseded by more complex thinking (e.g. the conveyor belt approach)
The international 'Bible' on frontal features as determined from sat pics, upper air soundings and nowadays extensive computer modeling, is here
MANUAL OF SYNOPTIC SATELLITE METEOROLOGY
see the Occlusion chapter
(and for Ana vs Kata COLD fronts, see under Cold fronts)
but the content may be quite challenging, and not directly useful for the average sailor.

I am wondering what precisely you expect to find from a pratical point of view
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Old 17-02-2016, 01:49   #25
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Re: Occluded fronts

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I have never heard of 'cold occlusion' vs 'warm occlusion'. Furthermore the concept of occlusion (i.e. cold front catching up with the warm front ahead, pushing the warm sector upwards) which dates back to the twenties (Bjerknes' school) has been long superseded by more complex thinking (e.g. the conveyor belt approach)
The international 'Bible' on frontal features as determined from sat pics, upper air soundings and nowadays extensive computer modeling, is here
MANUAL OF SYNOPTIC SATELLITE METEOROLOGY
see the Occlusion chapter
(and for Ana vs Kata COLD fronts, see under Cold fronts)
but the content may be quite challenging, and not directly useful for the average sailor.

I am wondering what precisely you expect to find from a pratical point of view
Thanks,

This may be the post we have all been waiting for (well at least I have)!.

The Cold occlusion and Warm occlusion is described with diagrams in my Macmillan & Silk Cut Nautical Almanac 1981 page 276. The explanation is very straightforward.

Having read this I realised I had no way of knowing whether I am looking at a cold occ or warm occ on a synoptic chart and set about trying to determine if this was possible to determine. It appears it is not and furthermore as many other things have been consigned to history.

I do take your point about the information being challenging (have not looked at it yet) when I ask questions I have to accept on many occasions the answers might be way beyond my comprehension.

Thanks

Mike
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Old 17-02-2016, 07:09   #26
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Re: Occluded fronts

actually you CAN tell the difference between a cold and warm occluded front. you just need to know how to read sinoptic chart.

this is warm occluded front between Greenland, Iceland and Norway...


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Old 17-02-2016, 07:19   #27
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Re: Occluded fronts

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actually you CAN tell the difference between a cold and warm occluded front. you just need to know how to read sinoptic chart.

this is warm occluded front between Greenland, Iceland and Norway...


Well let us all into the secret then.

Mike
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Old 17-02-2016, 19:12   #28
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Re: Occluded fronts

Seems to me to be splitting hairs. The cold airmass that occludes, is essentially the same airmass - in the time it has been split by the warm airmass wedge, the two sides might be influenced enough by their geography to assume a significant (enough) temp difference, such as Nigel illustrated with Polar Maritime and Continental characteristics. If pressed I would compare the surface temp chart to determine if it's a cool or warm occlusion.
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Old 18-02-2016, 11:07   #29
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Re: Occluded fronts

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Originally Posted by sailorF54 View Post
I have never heard of 'cold occlusion' vs 'warm occlusion'. Furthermore the concept of occlusion (i.e. cold front catching up with the warm front ahead, pushing the warm sector upwards) which dates back to the twenties (Bjerknes' school) has been long superseded by more complex thinking (e.g. the conveyor belt approach)
The international 'Bible' on frontal features as determined from sat pics, upper air soundings and nowadays extensive computer modeling, is here
MANUAL OF SYNOPTIC SATELLITE METEOROLOGY
see the Occlusion chapter
(and for Ana vs Kata COLD fronts, see under Cold fronts)
but the content may be quite challenging, and not directly useful for the average sailor.

I am wondering what precisely you expect to find from a pratical point of view
I have just had chance to interrogate the link. I note there are a couple of occlusions described as

Cold conveyor belt type & Warm conveyor belt type

Mindful that this is an international forum it may just be possible in other parts of the world these occlusions have different names?

It might be the Marathon/Snickers scenario?

From a practical point of view I have no interest whatsoever, it has nothing to do with why I asked the question.

Mike
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Old 25-02-2016, 00:43   #30
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Re: Occluded fronts

Have now received a reply from UK mat office. It is not possible to determine whether an occluded front is of the warm or cold persuasion from studying a synoptic chart.

Thanks for all above replies.

Mike
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