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Posted
  • Location: Coventry,Warwickshire
  • Location: Coventry,Warwickshire

Hi

The North East has the symbol of a trough over it on the met office site, but the sky is blue and cloudless, and troughs are associated with low pressure. Can there be a cloudless sky in a low pressure area?

Regards

There are different types of trough and the one depicted on the fax chart is probably a surface trough or upper air trough as opposed to a long or short wave trough.

Long wave troughs are areas of cold air intruding into warmer air, where the two air types meet you will normally find the jetstream.

Short wave troughs tend to be features embedded within the jetstream and tend to be identified by increased vorticity and/or embedded cold dry air within a system. Sometimes there will be a kink in the jetstream, usually they are at jetstream strong pulse regions. There is more to it than that, but you can investigate further.

habbyhints short wave trough

There are also surface and upper air troughs which tend to be blocks of air at different temperatures or humidity to the surrounding air. Upper air troughs tend to be areas of cold air aloft, like in an occluded front which will steepen lapse rates and can spark of convection. Surface troughs are often areas of warmer air at the surface which can also steepen lapse rates and spark convection. You do however need to look at the complete air temperature profile from surface to the tropopause to understand whether clouds will form. Any band of warmer air over cooler air will stop air rising and prevent cloud formation. You also need to take into account humidity or water content of the air to determine whether clouds will form.

weather front s explained

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Posted
  • Location: South Norfolk, 44 m ASL.
  • Weather Preferences: Varied and not extreme.
  • Location: South Norfolk, 44 m ASL.

BF - the way you explained it above makes it sound as though there are four types: long waves, short waves, surface troughs and upper-level. Is that actually the case? Surely a long-wave feature has to be upper level because it's a product of the jet stream, whereas short-waves tend to be a transient phenomenon associated either with topographical effects or areas of cold air aloft or at the surface?

Edited by chrisbell0033944
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Posted
  • Location: Coventry,Warwickshire
  • Location: Coventry,Warwickshire

BF - the way you explained it above makes it sound as though there are four types: long waves, short waves, surface troughs and upper-level. Is that actually the case? Surely a long-wave feature has to be upper level because it's a product of the jet stream, whereas short-waves tend to be a transient phenomenon associated either with topographical effects or areas of cold air aloft or at the surface?

Long wave troughs are associated with the jetstream and mark the boundary between warmer and cooler air at upper levels.

Short waves which are not really troughs tend to be associated with jet streaks. The term short wave trough is not one I like to use,but in the UK tends to be associated with a smaller scale version of a long wave trough.

The troughs marked on fax charts tend to be none of the above, but pockets of air which have different temperature and humidity to the general air mass. These pockets can be at any level, but have slightly different characteristics depending on the level, hence why I differentiate.

What both types of trough ( 1 - air mass boundary type 2 - air pocket type), tend to have in common is marked vorticity.

Hope that Helps, maybe John or one of the others forecasters will be able to explain it better.

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Posted
  • Location: just south of Doncaster, Sth Yorks
  • Location: just south of Doncaster, Sth Yorks

You seem to be doing well enough on your own Brick

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Posted
  • Location: South Norfolk, 44 m ASL.
  • Weather Preferences: Varied and not extreme.
  • Location: South Norfolk, 44 m ASL.

Long wave troughs are associated with the jetstream and mark the boundary between warmer and cooler air at upper levels.

Short waves which are not really troughs tend to be associated with jet streaks. The term short wave trough is not one I like to use,but in the UK tends to be associated with a smaller scale version of a long wave trough.

The troughs marked on fax charts tend to be none of the above, but pockets of air which have different temperature and humidity to the general air mass. These pockets can be at any level, but have slightly different characteristics depending on the level, hence why I differentiate.

What both types of trough ( 1 - air mass boundary type 2 - air pocket type), tend to have in common is marked vorticity.

Hope that Helps, maybe John or one of the others forecasters will be able to explain it better.

Thanks BF. Of those types I am familiar and comfortable with long waves as one of the NW guides from a while back linked to an excellent site (GeologyWales) which explained ridges, troughs, low-pressure formation, jet entry and exit, etc. However, the more I hear about short wave features, the less consistency in definition there seems to be. It seems to be either:

1. A small long wave

or

2. A consequence of a jet streak.

Then you say that troughs on FAX charts tend to be either surface or upper-level troughs, i.e. pockets of air that are distinct from the air around them. Does this mean that an area of convective warm air on a summers day is a trough?

Edited by chrisbell0033944
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Posted
  • Location: South Norfolk, 44 m ASL.
  • Weather Preferences: Varied and not extreme.
  • Location: South Norfolk, 44 m ASL.

However, the more I hear about short wave features, the less consistency in definition there seems to be. It seems to be either:

1. A small long wave

or

2. A consequence of a jet streak.

OK, ignore that - total drivel - a small version of a longwave feature might or might not be caused by a jet streak - I'm learning! :(

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