chrisbell-nottheweatherman

Getting pretty dark here - radar shows showers approaching from the SE. Just got back froom swimming - it's cooler out there but still feels close and there's that feel of something about to happen...

What sort of timescale do you think? I'll be out in a moment and won't be back until gone 7.

Pretty much the same here now I'm back from the shops, plus I would add a freshening breeze. Will be going swimming at a pool about 10 miles away soon. Will report back later.

Thanks Ian - I'm still a novice so any info is welcome! Would the same apply to more generalised frontal rain - i.e. I have seen pale blue over my location with no rain falling.

First ppn of the day on my postcode radar zoom - small shower around Ipswich. Now about to go round to my local shop - will update you with what I see cloud-wise when I get back.

Oooh that looks threatening! Milky skies here.

By the sound of it, the likelyhood of the overnight plume getting here is a bit doubtful. Is that a fair judgement?

Me too mate!

Any chance of anything in my neck of the woods tonight?

Interesting about the reference to the low humidity (though it seems quite sticky here now) and the clear blue skies as opposed to the milky skies and humidity of a long continental feed. I had often noticed that high humidity often came with hazy skies, but hadn't realised the link to a continental feed. Is it my imagination, or do we get more hazy, humid summer weather now than in the 1980s (or is it just because I was a kid then)?

Clear here now, there was some cumulus building earlier. It's going to be horrid muggy night for sleeping, but getting cooler next week thank goodness. Any risk of a storm tonight for my location?

I think you deserve a rest after your sterling work teaching me this! :unsure:

Thanks again John. I see now why prediction low or high pressure formation is so tricky! Just to make certain that I understand. A surface low will start forming in a confluent ridge behind (to the west of) a trough, then it will deepen in the left exit of the dffluent trough as the streak moves along the lobes of the stream. Therefore, the low will be found just ahead of the trough, explaining how the pattern of warm front, warm sector and cold front forms, with the cold front being at the leading edge of the polar air brought south in the trough lobe. Conversely, high pressures develop in diffluent ridges and mature in confluent troughs, but (for a reason I don't think you mentioned - probably just as well for my and your sanity!) they are found behind a trough, not ahead of it. Is that a correct understanding?

Sounds good John. Just one final quick question - you explained that a convergent trough can give rise to a high pressure area - so can divergence at a ridge form a low-pressure area?

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!

Thanks indeed John - it's beginning to fall into place now!

Also, John, just a quick question about your explanatory PDF file - are you saying that most streaks lead to the formation of both high and low pressures or that the pattern you describe is only if the flow is perfectly symmetrical? If that is the case, does any asymmetry in the flow lead to the formation of either a high or low in the jet streak?

Thanks John - I understand the polar front and the formation of warm and cold fronts as I've read, re-read and inwardly digested your guides. What I am struggling with is reconciling the pattern of cool air, followed by a warm sector, then a cold front bringing in cold air behind, with the longwave pattern of a trough being a lobe of cold polar air being brought south by a southward lobe of the jet stream.

OK, to help anybody brave enough to try to get the theory of this into my thick skull, let me summarise what I think I know: 1. Long-wave features are caused by the polar jet swinging north to form ridge of warm air and south to form a trough of cold air. 2. Ridges are associated with higher heights; troughs with lower heights. This is due to fact that warm air rises. 3. At a trough, the acceleration of the jet steam around the base of the cold polar air lowers the pressure slightly along with the fact that the cold air mass is thinner and therefore exerts lower pressure than warm air. In addition, the curve of the jetstream establishes positive (anticlockwise/cyclonic) vorticity. This vorticity, together with the lowering of pressure, establishes a spinning column of air, generating a vacuum, thus causing air to flood in at lower levels to fill that vacuum. If the rate at which the air at upper levels diverges is greater than the rate at which the air flooding in at the base converges, then the pressure will drop, so the low deepens; if the rate of convergence at the base exceeds the rate of divergence at the top, the pressure rises and the low is filled. 4. As the polar front dips southwards at the trough, just ahead of that trough, a warm front is established as the vorticity of the low builds and warm air is dragged upwards from the south. This generates a warm sector, behind which is a cold front which delineates the point at which the warm air is followed by colder air from the north. 5. Due to the fact that the warm air at the warm front tends to ride up over the colder air, convection is limited, and precipitation tends to be light. Conversely, at the cold front, cold air undercuts warm air and the rising warm air generates convection - thunder and heavy precipitation associated with cumulo- and stratonimbus clouds are commonly observed. 6. Presumably, a warm front must then pass over after the depression at the point where the cold air associated with the trough gives way to the warm air of the next ridge.

Thanks for replying John. I think I may have missed a vital point in there somewhere to do with convergence being at the base of +ve vorticty and divergence being at the top, creating low pressure. Having said that, it still doesn't make much sense to me.

Hi all. In relation to the discussion on troughs, I have re-read the following link WHAT GOES ON ALOFT? In the section on diffluence and confluence, it states that both troughs and ridges in long wave systems can be either diffluent (air entering at speed and slowing as it leaves) or confluent (slow entry, quick exit). This I understand. However, it then talks about jet streaks, which sound rather like diffluence, but it describes a northwards angling of air on entry to the streak due to pressure difference and a southward angling on exit due to the Coriolis force, which, according to their diagram, means that every jet streak has both right entrance and right exit as I understand it. Yet this suggests confluence, not diffluence, and therefore -ve vorticity. To top it all, it then starts referring to left entry which (to my understanding) it has just described as not happening. As you can see, I'm confused. To me, the page seems to contradict itself. Does the air flow into a jetstreak always enter left or right, and does it always exit left or right? Or can it vary between jetstreaks? I can understand that diffluence would generate +ve vorticity and therefore cyclogenesis and low pressure formation, and that confluence would generate -ve vorticity. What I don't get is how both diffluence and confluence are defined by what the air does both in entry and exit, yet entry can be both diffluent or confluent and so can exit. Aaargh!

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?

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?

Any idea of when this will replace v4? Other than the IE issue, it's very promising indeed. Well done guys!

Hi - I'm still very much a beginner myself, but my understanding is that warming air causes convection rather than the other way around. Most of the time, heating is caused by the effect of sunlight warming the ground and therefore the air close to the ground. That warm air rises in the same way a hot air balloon does, and will keep rising until the temperature of the rising air drops to the same temperature as the surrounding air, at which point it will stop rising. In conditions where the atmosphere is cold at upper levels and relatively warm at lower levels (i.e. a steep lapse rate) the rising pocket of warm air will rise rapidly and will keep going for longer as the temperature of the air around it will be dropping rapidly. Although the temperature of the rising pocket will also drop as it ascends, if the lapse rate is steep enough, the temperature around it will be dropping at least as quickly, so the temperatures don't equal out until the rising air has risen a long way and cooled substantially. If the pocket of air is moist, its temperature may drop below its dew point, i.e. the relative humidity of the air hits 100%. It can't hold any more moisture, so that moisture starts to condense, forming clouds and precipitation. This explains why thunderstorms associated with heavy, showery downpours occurr most frequently happen either during the summer when strong sunlight heats the ground significantly, or during winter or spring when weaker ground heating is combined with very cold upper air. In both cases a steep lapse rate is formed. No doubt one of our experts will soon correct me and provide a more detailed explanation, but that's my take for what its worth.