Determining Cloud Tops

[VORTIME]

Hi folks,

How can one determine cloud top (and less importantly types) for a 800nm trip across Europe?

VT

[IO540]

You asked this question a few days ago :)

One needs to get forecast skew-t diagrams or tephigrams, for various places enroute.

There is also the SigWx form which indicates tops exceeding 10,000ft.

Form 215, and Form 415 have indications of tops but only for huge regions. There is also a thing called GAMET (which I have never used, and it came up blank on Avbrief when I last looked) which is claimed to include some tops data.

The only place I know of that does forecast tephigrams is

http://pages.unibas.ch/geo/mcr/3d/meteo/

and look under Animated Soundings. This data is based on the U.S. GFS model, with some processing on top.

I heard there are forecast skew-Ts available from GFS directly, from

http://www.arl.noaa.gov/ready/cmet.html

but I haven't found them yet.

Looking at a skew-t or tephigram, you get an idea of the cloud cover at a given level from:

Difference between the temp and the DP:

<1C means 7-8 octas cloud
1-2C means 6-7 octas cloud
2-3C means 4-5 octas cloud
3-5C means 2-4 octas cloud
>5C means clear

For example

http://weather.uwyo.edu/cgi-bin/sounding?region=europe&TYPE=GIF%3ASKEWT&YEAR=2006&MONTH=10&FROM=0100&TO=0100&STNM=16080

shows solid IMC from ground to about 600mb and then a bit of fresh air but not much above that. Most of the IMC is however above 0C.

This one

http://weather.uwyo.edu/cgi-bin/sounding?region=europe&TYPE=GIF%3ASKEWT&YEAR=2006&MONTH=10&FROM=0100&TO=0100&STNM=03882

shows IMC up to 700mb (3000m, about 10,000ft) and then it gets better. Again, the 0C level appears not too far down for the climb.

This one

http://weather.uwyo.edu/cgi-bin/sounding?region=europe&TYPE=GIF%3ASKEWT&YEAR=2006&MONTH=10&FROM=0100&TO=0100&STNM=34731

shows a clear top at about 6000ft.

The above chart URLs are actual baloon ascents (from another U.S. site) because I don't think one can paste links to the forecasts from Unibas, but the idea is the same.

The Unibas forecasts are not to be fully relied on of course. Allow another 2000ft on top and make sure you can climb up there; preferably file the flight plan for that level. And carry oxygen; you will need it for much of Europe.

Cloud types I can't help with, but the SigWx form shows the nasty ones and you will never get above those unless you have a jet, anyway. That's why one tries to get into VMC so any avoidance can be done visually.

[VORTIME]

I did indeed but all the answers related to the legalities of VFR on top and not cloud tops :) !! IO540, thanks for your indepth information, hopefully we can get some activity on this thread as there might be tips and tricks that would be useful to european pilots,

Cheers
VT

[IO540]

From the vast number of contributions to this thread (??) so far I do wonder how IFR pilots of sub-turboprop planes do their long range flight planning.

Years ago, before the internet, and before the internet started leaking all the precious 3D weather data which the "CAA authorised" UK Met Office wants to charge for, pilots were unable to plan things this way.

They either needed to have weather understanding bordering on that of a professional forecaster (which I still think wouldn't have done them a lot of good without access to the data; I know of some forecasters and they get their stuff off the internet too), or they just took off and went, and if they picked up an inch or two of ice which they couldn't get rid of (because the rubber boots were not up to the job, or they didn't have any) they would descend, or climb, and hope.

One could say the same for long VFR flights - they require a much more aggressive planning strategy than VFR - but it's very likely that almost nobody does those in Europe.

At the same time I do know that plenty of people who hang out here either do decent flights, or they used to. I wonder what they do, or did.

On the face of it, the SigWx should do the job if one can flight plan the route for say FL150, but IME the cloud top data on that is way off the mark. I suspect this may be because it is intended for jet pilots and they don't care much for anything enroute because they can handle it as they go along.

[High Wing Drifter]

In additiont to IO540's answer, this seems to be pretty comprehensive:

http://www.peter2000.co.uk/aviation/prague/index.html

[bookworm]

I heard there are forecast skew-Ts available from GFS directly, from

http://www.arl.noaa.gov/ready/cmet.html

but I haven't found them yet.


Choose your location and then choose the model you want from the "Sounding" menu.


Looking at a skew-t or tephigram, you get an idea of the cloud cover at a given level from:

Difference between the temp and the DP:

<1C means 7-8 octas cloud
1-2C means 6-7 octas cloud
2-3C means 4-5 octas cloud
3-5C means 2-4 octas cloud
>5C means clear


For those with less experience than you IO540, it's worth pointing out that that is a very crude rule of thumb. It's certainly true that, all else equal, the closer the temp and dewpoint, the more likely you'll find cloud. But you'll overestimate stratiform cloud that way and underestimate the tops of cu.

[IO540]

Thank you for that NOAA pointer, bookworm. Brilliant. When I get a bit of time I will compare their forecasts with the Unibas forecasts. The really fun thing (for some) would be to do a historical comparison of these two sites against real ascents...

I agree the rules I posted are rough. However, the forecast itself is very rough. A single skew-t chart, with an MSLP chart, can tell one more about the atmosphere that just about anything else. It therefore follows that if a skew-t forecast was ever exact, chaos in the atmosphere would be abolished and weather forecasters would be out of a job :)

[Piltdown Man]

Flying long range in our own aircraft sounds marvellous, but in Europe flying "VFR on top" is in my opinion, fraught with danger. Firstly, whilst you can calculate the cloud tops before flight, what do you do when you find out that the data supplied was inaccurate or out of date? Then, on an extended leg, how much time are you going to spend looking for holes to decend through if it all clags up together with the airspace which they are connected to (the hole) and how much time will you spend navigating? How will you guarantee you can descend at your destination? The METAR/ATIS for your destination will not neccesarily cover the area where you transit from "VFR on top" to just plain old VFR.

May I therefore suggest that you plan to go in the weeds but if you constantly find yourself doing these trips, you are probably loaded so spend your money on an IR and do things the easy way.

[IO540]

How will you guarantee you can descend at your destination

One has to be very sure the weather will be clear at the destination, over a wide area.

Often, especially if flying south from the UK, this is not a problem. Getting back to the UK under VFR is usually harder but the IMC Rating comes in very handy there.

May I therefore suggest that you plan to go in the weeds but if you constantly find yourself doing these trips, you are probably loaded so spend your money on an IR and do things the easy way

IFR is not necessarily "the easy way". Being able to climb/descend through a hole in clouds (if VFR) in just one step away from having to climb/descend through 10,000ft of freezing IMC (if IFR).

I also don't see what being "loaded" has to do with a discussion of flight planning.

[VORTIME]

To be more specific, I'm referring to IFR in a Columbia 400. Ideally one would aim to be VMC for 99% of the flight.

VT

[IO540]

That one has a ceiling of 25k which should get you above anything that doesn't "stick up". Especially with a turbo.

However:

I have scrapped a number of flights this summer due to forecast tops around the departure/destination of around 22k. On the face of it you would be OK there but in all those cases I found that these high tops were due to the stuff being pushed up; they were not the normal stratus cloud which rarely seems to top 16k. In these cases I was looking at the 0C layer being about 10k so one would have had 12k of freezing IMC to climb through. Rightly or wrongly I would not do that without full TKS.

So it seems to me that if one is to make use of a 25k operating ceiling, one also needs all over de-ice.

Lancair love the 25k ceiling because it enables them to achieve very impressive TAS figures. But can you imagine the oxygen flow rate? Over 18k you also need a mask, not a cannula, and you can't use a demand regulator with a mask, so you are looking at a flow rate about 4x higher than it would be with a cannula+demand reg. I would be suprised if the oxygen lasted as long as the fuel :)

[pumper_bob]

No one seems to mention the option of asking for 20 degrees either side of track to pass a cumulus of larger proportions??? Also my friend was talking only yesterday of a rule of thumb for calculating the height of clouds in your path, but he is on his way to Orlando for a 2 moth break away from aviation!!! Soon as i have comms with him i will let you in on the secret. Aparently he was within 200' of the line training pilots figures and he was using a texas instrument £9.99 special!!
But seriously, why not ask for a routing round the cloud, especially if it is lighting up like a crimbo tree on free electric:p :p

[IO540]

One does ask for left/right around anything big sticking up, but to do that one needs to be in VMC in the first place.

That's where the discussion of cloud top determination comes from. Let's say you are doing an IFR flight from the UK to Poland; that's some 700-800nm and European airway MEAs, not to mention permitted routings, will put you either in IMC or above it. The option of going below cloud is rarely available.

So one needs an idea of the tops. Let's say they are forecast at FL120. You flight plan it for FL140, with an option to climb substantially higher (ATC will always let you climb "due weather" or even without any reason, in the FL100-FL180 range, IME).

Then, as you are zooming along in the sunshine, you can do avoidance of CBs etc etc as you suggest.

Airliners and well equipped other planes can do this from inside IMC too, using radar, but then airframe icing comes into it. European airway routes will likely take you into 0C or below, even in the summer, so you'd better be in VMC for most of the enroute section.

One can plan IFR flights around the bottom of the airways, say FL070, but one is very likely to just sit in cloud there, and I would personally reserve this option purely for descending below the 0C level, having been picking up too much ice. You've got to make sure there is no terrain close to the 0C level :)

The above is it in a nutshell.

[Piltdown Man]

IO540 - If you are regularly doing 800 nm trips in your own aircraft I'd suggest that you may have an income sufficient to fund an Instrument Rating. Having an IR and the appropriately equipped aircraft makes the requirement to fly "VFR on top" superflous (which is where this thread started), obviously subject to ice. The priviledges of the rating will also allow greater certainty of using your aircraft to get from A to B. And then if we compare IFR to VFR, especially in controlled airspace then I honestly believe that there is no contest! Having reasonable experience of both, IFR is by far and away the easiest method of getting from A to B.

[IO540]

PM

I do have an IR and I do fly both VFR and IFR around Europe.

I don't think you have actually read anything that has been written in this thread.

[VORTIME]

Why does everyone want to talk about VFR/IFR when the question relates to determining cloud tops ... ?

VT

[mm_flynn]

Why does everyone want to talk about VFR/IFR when the question relates to determining cloud tops ... ?
VT
Because VFR/IFR is a great subject for debate and opinion. How to find cloud tops needs concrete recommendations, knowledge, and URLs.


I use - http://pages.unibas.ch/geo/mcr/3d/meteo/ which gives a reasonable view of cloud tops over a location for a 3 day period.


http://131.54.120.150/ which in the flight hazards area gives an icing and turbulance 'forecast' which gives an indication of tops.

http://www.wunderground.com/modelmaps/maps.asp?model=GFS graphs a set of parameters over Europe which can give an indication of weather at 850, 700, 500 mb

As well the usual products from Avbrief and the local metoffice provide usefull information.


It would be realy nice though if someone had the University of Basel time phase of cloud tops done as a geographic/time phase (from departure at Departure time to Dest at ETA) colour coded to show temperature levels.

Finally there are forecast skew-Ts available from GFS directly, from
http://www.arl.noaa.gov/ready/cmet.html and somewhere on the site you can download what I understand to be the whole model run updated every 12 or 24 hours (about 70mB)

[scooter boy]

Great thread.

I totally agree with IO540 - being VFR on top is the essence of IFR flight.
Solid IMC all the way is no fun :bored: (due to inadvertent embedded CB entry/icing etc...)- even in a boeing and therefore even with a stormscope penetrating cloud at high level on an airway can focus one's concentration.

Despite my current IR and TKS equiped A/C with O2 on tap I scrapped Padova to Plymouth on Sunday because of severe convective activity forecast from central France up to the UK. Unlike my Mooney, Easyjet have WX radar, can climb a lot higher than me and can also land in winds gusting to 30mph so I took the easy option (a no brainer). I will collect my aircraft this coming weekend and bring it home (just another excuse to go flying really).

I have to say though that the tephigrams still look like a bunch of wiggly lines to me! :confused:

What we really need in europe is WX beamed by data link into the cockpit - I would subscribe in an instant.

SB

[Bluebeard777]

Could someone point me towards a source of instructions for reading these tephigrams and getting something useful from them? ("Just a lot of wiggly lines" indeed!)

[IO540]

Do a google.co.uk on the word tephigram.

The first two hits are what you want.

There is a lot of info which can be gleaned from them; a lot more than I know about. I tend to use them for cloud tops, 0C level versus cloud tops (to estimate how much ice one might have to climb/descend through) and not a lot else.

[grob103]

Soundings tutorial at:
http://www.itadvice.co.uk/weatherjack/tut-soundings/tut-snds-01.html
Might be Skew-T instead of a tephigraph - not sure what the difference is myself! :\

[IO540]

Same kind of chart, showing how temp and DP vary with height. The axes are skewed differently.