Learning a bit of Meteorology

[snooks]

Back in the 70s you could take down the coastal weather reports from BBC long wave and make your own forecasts,

Now you have iPlayer....with the ability to pause between weather station reports. YAY! Search for shipping forecast....Then you can draw your own synoptic charts of the UK.

I did a crib sheet when I tried it, you should be able to download it as PDF from YM plus...if you can find it

 

[KellysEye]

> I often see weather-presenters describe the jet stream as causing weather events, and just wanted to make it clear to the other readers, that is not the case.

How can you say that the Jetstream doesn't cause weather events. If the Jetstream isn't over the UK but over Portugal then we don't get the lows and Portugal does get the weather events in the form of lows. We were in Portugal in 2009/10 and from December to the beginning of April the Jetstream was over us, it wasn't worth listening to the forecast we knew what was coming. That included the storm that killed people in Madeira and France and shook our boat more than has ever happened before. I would call that a weather event caused by the location of Jetstream.

 

[macd]

> I often see weather-presenters describe the jet stream as causing weather events, and just wanted to make it clear to the other readers, that is not the case.

How can you say that the Jetstream doesn't cause weather events. If the Jetstream isn't over the UK but over Portugal then we don't get the lows and Portugal does get the weather events in the form of lows. We were in Portugal in 2009/10 and from December to the beginning of April the Jetstream was over us, it wasn't worth listening to the forecast we knew what was coming. That included the storm that killed people in Madeira and France and shook our boat more than has ever happened before. I would call that a weather event caused by the location of Jetstream.

Cruiser2B is right. The jet stream no more "causes" weather events than roads cause RTA's. As he said, jet streams form at the boundary of air masses, and it is their interaction that gives rise to revolving weather systems such as Atlantic lows. Yes, a predicative chart of jet stream routing will give a strong indication of where those lows might travel, as your experience usefully tells you, but they are a consequence, not a cause.

 

[macd]

> I often see weather-presenters describe the jet stream as causing weather events, and just wanted to make it clear to the other readers, that is not the case.

How can you say that the Jetstream doesn't cause weather events. If the Jetstream isn't over the UK but over Portugal then we don't get the lows and Portugal does get the weather events in the form of lows. We were in Portugal in 2009/10 and from December to the beginning of April the Jetstream was over us, it wasn't worth listening to the forecast we knew what was coming. That included the storm that killed people in Madeira and France and shook our boat more than has ever happened before. I would call that a weather event caused by the location of Jetstream.

Cruiser2B is right. The jet stream no more "causes" weather events than roads cause RTA's. As he said, jet streams form at the boundary of air masses, and it is their interaction that gives rise to revolving weather systems such as Atlantic lows. Yes, a predicative chart of jet stream routing will give a strong indication of where those lows might travel, as your experience usefully tells you, but they are a consequence, not a cause.

 

[franksingleton]

You can always try the site below. And my book http://weather.mailasail.com/w/uploads/Franks-Weather/bookcover.pdf due to hit the streets Jan 2. See http://www.bloomsbury.com/uk/search?q=reeds+weather+handbook&Gid=1

I have written this pocket sized book with the minimum of theory and in as practical a manner as possible. The application of weather information is largely based on my own sailing experience together with my background in the hot seat at the UK Central Forecast Office – now the national Meteorological centre.

 

[nigelmercier]

... The application of weather information is largely based on my own selling experience...

From cover:

Front cover photos: Front cover photos: ...

Need a proof-reader?

No seriously, it's what I do. PM me.

 

[franksingleton]

Oops!

That is one that the designer missed. I and the editor paid more attention to the contents than the small print on the BACK cover.

I hope that you do not judge the contents on such a typo!

PSI rely on s[ell check far too much

PPS. That copy of the book covers was a draft version. I never noticed the repetition. However, I now have a pre-print in my hand ad that error has been rectified. So, the only real typo was in my post. I am constitutionally poor at seeing such errors - I have some mild macular degeneration.

 

[KellysEye]

>The jet stream no more "causes" weather events than roads cause RTA's.

Obviously it doesn't cause the lows and I've never said that, but what I did say is that it causes where the lows go by where it is. That seems to have ben misunderstood.

 

[macd]

>The jet stream no more "causes" weather events than roads cause RTA's.

Obviously it doesn't cause the lows and I've never said that, but what I did say is that it causes where the lows go by where it is. That seems to have ben misunderstood.

You're still confusing cause and effect. The dynamic boundary between polar and warmer temperate air masses causes the lows to go where they go. The same boundary determines the route of the jet stream. Hence your are wise and prudent to acquaint yourself with forecasts of jet stream location, but not precisely for the reason you suppose.

A former professional meteorologist made the post immediately after mine. Don't you think he might have corrected me had he demurred?

 

[simonjk]

Oh dear, much confusion, perhaps I can make it a little clearer.

The jet stream is only one of the results of several mechanisms within the atmosphere. Think of jet streams as aiding the exchange of cold air from the pole and warm air from the tropics.

As well a this, the dynamics and interaction promotes the development and weakening of surface features. The jet stream will 'steer' surface lows.

There is a far more complicated explanation but you all you need know is above.

Does his help?

Simon

 

[franksingleton]

The atmospheric system is highly interactive. It is driven by the sun heating the earth which heats and cools the atmosphere. In broad terms, the global system of inter-tropical convergence zone, trade winds, sub-tropical highs and mid-latitude mobile systems and all the associated upper level wind patterns results from the heat differential between pole and equator together with the spin of the earth and the distribution of land and sea.

You cannot isolate any particular feature. All interact. Our frontal lows form with associated oscillations in the jet stream but, themselves, affect the jet stream.

In the old days, when I was a young meteorologist (and that was a long time ago) forecasters had a fairly good understanding of how the atmosphere worked. Our problem was that we could not quantify our knowledge. In truth, we were not very good. It may be a little invidious to single out one particular problem but, in the old days, I do not recollect ever trying to factor latent heat into the development of lows. Remember October 1987 and the recent deep lows with their storm force winds.

Forecasting has improved enormously due to genuinely global observing from satellites and the ever more powerful computers. I know that there is much talk about jet-streams but computer models do the sums that we could not – and still cannot – ever imagine doing ourselves.

So, why not take computer models as the best advice possible? By all means look at jet steam charts and forecasts – eg http://www.netweather.tv/index.cgi?action=jetstream;sess=. You will find them interesting and educative. But use the computer models; there is no point in having a dog and barking yourself...

 

[johnalison]

Unlike many grumblers around here I am a great fan of modern computerised forecasts, which have done more than almost anything to make cruising pleasurable for us amateur sailors. On the other hand, there are sometimes local factors that come into play, and the timing of weather movements can be uncertain, so it can be helpful to us who sail around the UK to be a little weather-savvy. I don't often get caught out these days but I know that I still have much to learn.

 

[franksingleton]

...............
........
sometimes local factors that come into play, and the timing of weather movements can be uncertain, so it can be helpful to us who sail around the UK to be a little weather-savvy. I don't often get caught out these days but I know that I still have much to learn.

Local detail is and will always be a problem for computer models. Local topographic effects ca, at least in principle, be modelled well. The problem lies in small scale weather features that have short lifetimes. These can make a monkey out the best models.

There are two answers; use your own experience and judgement; if you are not sure about that, then use a consultant who adds human experience and intelligence. To save Simon embarrassment, he would be a candidate. Personally, I would not use any small scale (limited area or meso-scale) computer model forecasts claimed to be

....exceptionally accurate
....high-resolution
....precise high quality,
Incredible detail....
....down to 1km resolution.

For North Sea and Baltic sailing, I would probably use http://www.dmi.dk/en/hav/#danmark or,
http://www.klart.se/kust/ or
http://www.yr.no/kart/#lat=64.2929&lon=12.2307&zoom=6&laga=vind&proj=3575.

Around Spain I would be looking at http://www.puertos.es/content/prediccion-del-oleaje-mapas.

For the Med, I would be looking carefully at http://212.175.180.126/DENIZ/DTS/sea.php

Unfortunately, around southern and western UK, we have nothing to compare with these.

Over the years, I have come to the conclusion that the more I know about weather, the more I realise how much more there is to know. That may or may not discourage you.

 

[nigelmercier]

I hope that you do not judge the contents on such a typo!

Not at all, I've seen some horrendous errors. In one book the author transposed "effect" and "affect" throughout.

My offer of proof-reading still stands. For a book I would buy, I've done it for a signed copy as my fee.

 

[franksingleton]

Not at all, I've seen some horrendous errors. In one book the author transposed "effect" and "affect" throughout.

My offer of proof-reading still stands. For a book I would buy, I've done it for a signed copy as my fee.

Thanks, Nigel.

Re http://weather.mailasail.com/w/uploads/Franks-Weather/bookcover.pdf, to late, I am sorry to say. However, Reeds were very good to work with. I cannot guarantee that there are no errors; they like me are only human. As I said in my PPS, that typo was corrected before hard copies were produced.

They did raise questions when the text was not clear and did so from the position of non-experts. The only issue where we disagreed – and they won – was in the use of “data.” To me, like the French equivalent “données,” it is plural and I naturally say “data are …” I find “data is …” to be ugly. However, I seem to be in a minority scarcely above one!

 

[nigelmercier]

... The only issue where we disagreed – and they won – was in the use of “data.” To me, like the French equivalent “données,” it is plural and I naturally say “data are …” I find “data is …” to be ugly. However, I seem to be in a minority scarcely above one!

Data is indeed a plural noun (singular datum) but it is common for it to be treated as singular in modern writing. For example, it is the house style of the Open University to do so. It took me a while to get used to it.

 

[macd]

Data is indeed a plural noun (singular datum) but it is common for it to be treated as singular in modern writing. For example, it is the house style of the Open University to do so. It took me a while to get used to it.

Mmm. An even more common example is the almost universal use of the plural for companies, teams, etc. Garmin is a company: singular, so an "it". By the same token Manchester United is also an "it", as is the crew of a yacht.

Whatever their style-book, proof-readers do need a hand on reality (as I'm sure you have, Nigel). In a motorcycle history book I once had a phrase changed from "thus becoming the first man to average 100mph for a race", which is newsworthy, to "thus becoming the first man to average 160.09km/h for a race", which definitely isn't. I bet the headline writers of the time were really holding the presses for that one, particularly since it was in the USA. Then there was the time a well-known newspaper changed the last phrase of a test on a Triumph from "never was the name on a tank more apposite", to...well, you've guessed it. The paper was the Grauniad (inevitably) and I've been in Triumph's bad books ever since.

 

[Cruiser2B]

The jet stream will 'steer' surface lows.

There is a far more complicated explanation but you all you need know is above.

Does his help?

Simon

Sorry Simon, that doesn't help much. I get that the Jetstream can feed cold or warm air to a low, but that again is down to there being deep undulations in the frontal boundary that would route the Jetstream from well south or north upwind of the low. If your suggestion is that this "feeding" would aid the development or weakening of the low, and that in turn would play a part in shaping the path of the low, then I accept the premise that 'the Jetstream will steer surface lows.' If that's not it, then you'll have to do a better job explaining to me how a relatively narrow band at the tropopause can push around the 7-mile thick airmass below it?

 

[franksingleton]

Sorry Simon, that doesn't help much.
…………….

Hi, Cruiser2B

The whole process is both difficult to explain and understand. Simple explanations such as Simon has tried to give can always be questioned. I will attempt a more complete explanation at the risk of losing myself and you.

As I said at #31 there is interaction between all levels in the atmosphere. The jet stream is a current of stronger flow within the whole atmosphere. Look at tidal current charts and see how you can get areas of quite strong flow within a larger area of lesser flow.

Like any such flow in a fluid there will be small perturbations. Sometimes these seem to be random. Sometimes they will be induced either by topography eg air flowing over the Rockies or due to weather events, perhaps an area of intense convective activity.

Such perturbations will travel along the flow. Such small perturbations in the upper flow will result in small decreases in surface pressure. The effect will be the start of a circulation of winds at low levels. On the eastern side of such small lows the winds will blow warmer air northwards (warm advection in technical speak.) To the west low level winds will blow colder air southwards (cold advection.)

This warm and cold air advection will have the effect of increasing the distortion in the upper winds – including at jet stream levels. With the warm air advection the air moving northwards will be lifted; the usual textbook pictures of up-sliding at the warm front is a simplification of a complicated process. Similarly, as the cold air comes southwards to the west of the low, it will undercut warmer air and, again cause lifting.

The rising air at both warm and cold fronts will be moist and warm having come from southern latitudes. As condensation of water followed; later by freezing takes place, latent heat will be produced causing warming of the air.

This leads to a further decrease in surface pressure. That leads to a deepening of the surface low, a strengthening of low level winds and increases in the warm and cold air advection. During all this time the developing low is moving east being embedded in the upper winds.

The jet stream becomes increasingly distorted and the steering effect changes as a consequence. Eventually, occlusion takes place; the supply of warm moist air is cut off to the low. The jet stream becomes so distorted that it eventual disrupts. The low pressure circulation has probably extended right up through the atmosphere. The low will then slowly dies due to friction and having lost its source of energy.

As I hope will be clear, this is a complex process and I have tried to simplify the description. You may see now why the old style pre-computer forecasting was so poor. I hope that you also see how difficult it is to write computer programs that calculate all these processes with any degree of realism.

I always have to be surprised how good forecasts are these days. I hope that you understand when I get rather tetchy at complaints that it, sometimes goes badly wrong. Fairly rare these days although perfection will probably never be achieved.

This may explain why I have tried to avoid theory in my book as much as I reasonably could. Apologies for the length. Life is too short to write it in many fewer words.

 

[Cruiser2B]

I appreciate the effort Frank, and if I understand you correctly, you're saying that upper level 'pertubations' can lead to (or at least aid in) the development of a low at the surface. I fully accept that, but that doesn't change the fact that the Jetstream is where it is, because it is above the boundary between two adjacent airmasses. If I've got it wrong, perhaps you could explain how the Jetstream moves the UK's typical Polar Maritime airmass down to Portugal?