91 replies to this topic

[Adi F]

Cheers John

So were in that skew-t would you look to see if there would be instability and showers or thunder storms

[johnholmes]

Adi Fiddler, on 9 May 2005, 02:23 PM, said:

Cheers John

So were in that skew-t would you look to see if there would be instability and showers or thunder storms

<{POST_SNAPBACK}>

quick check is look on the right hand side and it gives figures for CAPE and LI;
I believe, although this part of forecasting has developed since my time, that for thunderstorms you need a CAPE of over 300, better still 1000, and 1000-2500 could give severe, even intense storm activity
for LI -1 to -4 may give a moderate thunderstorm with some hail; <-4 for severe storms and large hail and <-10 for sev to intense storms with prob of tornado/large hail.

I tend to still rely on my understanding of how much potential instability there is in an ascent; a simple guide is that the greater the area between the ELR and the SALR which goes from the cloud base the more likely severe storms are. There is a figure we used but I cannot remember it and I believe its the basis for both CAPE and LI
CAPE= Convective Available Potential Energy and LI=Lifted Index, which is itself a pretty crude measure of CAPE. Hope that helps Adi and any others. Please PM for any extra and I will try and help.


regards

John

[Adi F]

Cheers John

Its is starting to make sense now

Regards

[BrickFielder]

It seems to be coming together quite well.
The significance of wind speeds increasing with height and changes of wind direction with height still needs to be explanined. Perhaps calculating the cloud base and affects of surface heating are not quite clear as well yet. Just when I think I am getting the hang of it though I see something like this.

[Adi F]

Yeah I have used the skew –t’s to look at wind direction for a few years now as you can see if there is any chance of shear and the wind speed

[johnholmes]

hi B and A
It will all come together including talking about the winds as well - patience boys. I've only got one brain, that hurts enough as it is. More on cloud bases and tops very soon, and not just convective cloud. The winds in a little while.

regards

John

[johnholmes]

B again
Try not to look at that type of skew-T for a while. Its pretty complicated so lets stick with the relatively simple one we can all get off Net Wx for a while

regards

John

[Mark Griffin]

Good work john all just clicked in to place. Keep it up

[johnholmes]

hi
Brickfielder made this comment
' Perhaps calculating the cloud base and affects of surface heating are not quite clear as well yet. '

the cloud base is, in convective situations, a function of the surface dewpoint and the temperature. Usually, unless there is a change of airmass, the dewpoint changes by no more than 2-3C from early morning to mid afternoon, maybe 4-5C sometimes. This may seem quite a lot but is small when compared to what the temperature can change by. On a nice clear morning with the convective cloud developing as the temperature rises, a change of 10-15C is not unusual.
As an example my ASW(Automatic Weather Station) today showed a rise of temperature of 10C, whilst the dewpoint rose from 1C to less than 4C, so less than 3C change for it.
As a rough rule of thumb, the larger the difference between dewpoint and temperature, then the higher the cloud base will be.
Its just a function of how moist the air is.
Another 'rule of thumb' that I used to use was that the actual cloud base was usually about 25mb LOWER than the condensation level. So if the condensation level was, say, 925mb, then the cloud base would be around 25mb lower, that is at 900mb.
The condensation level is that, on the skew-T, where the dewpoint line and the DALR meet. Remember that the dewpoint line follows the HMR(Humidity Mixing Ratio) lines. These are the PURPLE pecked lines going, at an angle, from left to right, on the Net Wx skew-T. The temperature line is the BLUE one going left to right, in BLUE, on the skew-T. However, just to confuse the issue, the line you must follow, is the DALR(Dry Adiabatic Lapse Rate) line which is RED and goes from right to left on the skew-T.Have a look at the last diagram I put up to show this.

There is a scientific explanation of why this is so, but for this series of tutorials it is not necessary to know it.

Hope that helps B, and for others.

regards

John

Edited by johnholmes, 09 May 2005 - 19:07 .

[Stuart]

Thank for that it make it so easy for my Understand Skew-t Diagrams

[johnholmes]

cheers Stuart,
glad it helps; its not easy, as I remember only too well on my first forecast course.

regards

John

[kold weather]

Hi John,good job by the way,I now full well its a very hard thing to get your head around,but once you do its one heck off a tool to use for short range forecasting.

[Adi F]

John I live right on the coast so apart from a decreased temp would that have any effect on the skew-t’s i.e. sea breeze?

Tell me if I am jumping too far ahead again, I need to learn to be patient.

You must be pulling your hair out with all our questions

[johnholmes]

Adi Fiddler, on 9 May 2005, 09:28 PM, said:

John I live right on the coast so apart from a decreased temp would that have any effect on the skew-t’s i.e. sea breeze?

Tell me if I am jumping too far ahead again, I need to learn to be patient.

You must be pulling your hair out with all our questions

<{POST_SNAPBACK}>

hi Adi
you are right, it would decrease the surface temp, but little else, also inject some additional moisture at very low level. Its this combination that can sometimes set off 'convergence' thunderstorms a little way inland with the right wind and atmospheric conditions. Perhaps a topic to discuss a long way down the line!

regards

John

[Adi F]

johnholmes, on 9 May 2005, 09:34 PM, said:

hi Adi
Its this combination that can sometimes set off 'convergence' thunderstorms a little way inland with the right wind and atmospheric conditions. Perhaps a topic to discuss a long way down the line!

<{POST_SNAPBACK}>

Haha John I love the way you said “a long way down the line”.

I only asked because unlike those that live inland us people that live on the coast have too factor in the constant effects of day time sea breeze and night time land breeze.

I thought I was told years ago that the skew-t’s could predict whether the breeze affect would be strong or not (i.e. go a long way in land / out to sea or stay close to shore), I am probably wrong it was probably some other measurement.

John believe it or not I was in the Artillery and I actually did a basic course on meteorology, that course was based up at Larkhill and twice a day we sent a weather balloon up and we would track it (I think they still make soundings on Larkhill). And, yes we used to produce and look at these charts but I don’t remember them being called Skew-t, I thought they were called something else. The thing is I have always loved the weather but back then I was more interested in learning how to play war games, sending balloons up in the sky really was not important for me although I understood the relevance of it all. Now I only wished I concentrated and took it in.

[johnholmes]

hi Adi
Yes, when I was in Met we used what were called T-phi diagrams, pretty similar really. Whatever is used it will be a plot of temperature against height, usually. Please don't ask me to explain T-phi or any others. 99.9% on the forum would be asleep after the first two paragraphs, and I would certainly have to rack what little brains I have left to put some text together. I have various Met books but even there it is very very complicated. So lets just keep it as simple as we can.
As time goes by, assuming its wanted, we can discuss all manner of things which these aerological diagrams(!) help with. Max, Min temp, will there be fog overnight, what time will it lift at a certain place, will it snow or rain, will there be a thunderstorm will it be sunny. The list, if not endless, is pretty long. And then we can set about using the wind data also!!
Yes, that you saw at Larkhill was one of about 12 upper air stations we used to have, and two or three out on weather ships in the Atlantic, the Bay of Biscay, between Iceland and Greenland and off the far north of Norway. Now much of it is done by satellite sensors and with masses more data being sent down. Pretty mind boggling really.
Anyway I'm going to bed now.
Thanks for your tutorials on how to as well, when I get time I must try and get the various things you suggest to work for me.

regards

John

[the_white_stuff]

Good work JH, even though I find it hard to grasp I'm slowly learning it. I keep re-reading this thread and picking up new bits each time

[johnholmes]

hi
Just to try and make it really clear on the point I made in an earlier post about the differences in changes through a 24 hour cycle between the temperature and the dewpoint.
Below is the print out(sorry its not in colour - another problem I'm having!!), but it illustrates how on relatively clear nights with clear mornings just how much the temperature can change and how little the dewpoint does.
This is why meteorologists call the dewpoint a 'conservative' measure. As it only chnages markedly when there is a change of airmass.
I hope the diagram helps to iluustrate the daily changes for you, over the past three days.




regards

John

[Adi F]

Thanks John that’s an interesting diagram that dose certainly put it into perspective

[johnholmes]

hi skew-T readers
I've attempted to put all the items about ske-T diagrams in the one post. Eventually it will be made into a web page to go alongside the Net Wx skew-T forecast diagrams, and, I think, in the Net Wx Guides. That way they will always be readily available.

Look forward to any feed back, + or -.

regards

John