PDA

View Full Version : Question for the met / glider boffins


QDMQDMQDM
11th Jul 2006, 20:41
Why was it so phenomenally bumpy today?

Beautiful day, perfect vis. I flew Eggesford -- Booker and back at altitudes between 1200 and 2500 feet and it was incredibly bumpy with a lot of thermal activity. There was about 10 knots of westerly and a few cumulus clouds of minimal vertical extent, bottoms at about 3000feet.

Why so bumpy? I was surprised to not see a lot more in the way of cumulus, given the lift I was getting and even more surprised to watch the cumulus die in the afternoon and the bumps remain. I set off at 09.30Z and it was even very bumpy then.

Answers on a postcard?

Lucy Lastic
11th Jul 2006, 20:59
Good, wasn't it. I got better climbs in the thermals than my engine could give me.

Thermals seemed to have narrow cores and were quite violent, but some seemed to be related to local land features with local heating effects

During the day, as predicted, the effect of the advancing high killed the cumulus but we still had the blue thermals.

Great visibility, though and a fantastic flying day (assuming you could avoid the Bicester gliders)

gaxan
12th Jul 2006, 11:43
I was towing yesterday, I found one thermal that gave us a climb rate in excess of 1000fpm when the normal ROC on tow is about 450fpm.Not bad for a combination! One of the better days this year for gliding. Thursday should be good too.

QDMQDMQDM
12th Jul 2006, 11:56
That's the difference innit? Glider types like bumps. The rest of us don't.

Thanks for the replies!

TD&H
12th Jul 2006, 13:05
Sometimes the thermals meet an inversion before they get high enough (and cool down enough) to form clouds. Or even, as happens in the really hot climes, they never do cool down far enough to form clouds. A 'blue day' or 'blue thermals'.

Read some of Derek Piggot's books on gliding to find out more. Or his autobiography 'Delta Papa' for a fascinating book on flying in all types of aircraft.

Ever notice that clouds on a good thermal day with a steady wind form up in lines? Cloud streets. Also there's a relationship between overnight low temperature and daytime maximum to give height/altitude of the cloudbase. Too long since my gliding days to recall off the top of my head.

Certain days, with the correct movement of fronts, wind directions, etc. can give very good thermal conditions for 500km plus flights in the UK. Days when your speed goes from very slow, not much above stall circling in a tight thermal core, to Vne between the thermals. Maintain high speed between the thermals, then pull back and zoom climb, losing speed in the next thermal, before zooming off again.

Makes pootling along at a steady 90 snotts in a Cessna boring!!

QDMQDMQDM
12th Jul 2006, 15:44
Thanks, TD+H, most illuminating.

Anywings
13th Jul 2006, 05:35
Also there's a relationship between overnight low temperature and daytime maximum to give height/altitude of the cloudbase. Too long since my gliding days to recall off the top of my head.

Rough rule of thumb under normal conditions is 400ft of cloudbase for each degree C of difference between overnight low and daytime max; so 10C over night and 20C next day = 4,000ft. Doesn't always work in practice, but it's usually pretty close.

IO540
13th Jul 2006, 06:15
Rough rule of thumb under normal conditions is 400ft of cloudbase for each degree C of difference between overnight low and daytime max; so 10C over night and 20C next day = 4,000ft. Doesn't always work in practice, but it's usually pretty close.

I don't doubt it for a moment, but it would have the authors of much PPL training material revolving in their graves at 2575 rpm :O

I'd like to see a physical derivation of it though. The other "rule" (which is more intuitively obvious) is that if one waits for the weather to settle down into the day, then multiplying the diff between the temp and dew point by 400

http://www.itadvice.co.uk/weatherjack/tut-various/bradbury.html

should give the base of any CU.

Anywings
13th Jul 2006, 06:37
''The other "rule" (which is more intuitively obvious) is that if one waits for the weather to settle down into the day, then multiplying the diff between the temp and dew point by 400''

Quite right, but if you're planning a gliding task at 7am (or the night before!) and you want to get a rough idea of what the cloudbase will be (to help you decide the size of task) you can't afford to wait until the weather settles down into the day. I did say it was a rough guide but it works reasonably well in practice under normal conditions.

ProfChrisReed
13th Jul 2006, 21:40
These are both the same rule - as Anywings points out, you may need to guess the dewpoint, and the best guess = the overnight minimum temperature. It's surprising how often these are within a degree of each other.

Of course, sometimes a different airmass sneaks in with a different dewpoint, which lead to gloomy glider pilots who are either 2,000 ft lower than they predicted, or at work looking at a cloudbase 2,000 ft higher. That's gliding - gloom and elation in about a 90:10 ratio.

IO540
14th Jul 2006, 20:02
and the best guess = the overnight minimum temperature

Why?

For example, if the sky is overcast, the day/night temp fluctuation is much smaller.

ProfChrisReed
15th Jul 2006, 18:43
Don't know why, but empirically it seems to be right about 9 times in 10, even for overcast nights. I believe it might be linked to the fact that the amount of moisture a parcel of air can hold varies with its temperature. Thus if the temperature doesn't drop overnight because of overcast, the air nearest the ground has a higher dewpoint because it remains warmer.

The relevant starting point for a thermal is the air layer near the ground - thermals work by conduction from the warm ground to the adjacent air, so the sun has to heat that layer of air before convection starts. So, even if the air at (say) 2,000ft has a much lower dewpoint, that air will never be heated more than a minimal amount by the sun. Once convection begins, the upper (colder) air sinks to replace the rising air in the thermal, and can then be heated by the ground - this might explain why the cloudbase on a good soaring day usually rises to 500ft or more above the 400 x min/max difference, whereas on a weak day it can be below that height.

I regularly attempt to produce my own soaring forecasts, including for local competitions from my club, and use both forecast and actual atmospheric soundings to predict cloudbase. My check for whether I've done the sums right is to compare my forecast couldbase with 400 x max-min, and these almost always correlate to within about 10%.

Of course, if a front comes through at 9 am, all bets are off as we now have a different airmass.

IO540
15th Jul 2006, 20:39
Interesting. I would have expected the night air to cool through radiation to close to its DP, due to the latent heat of evaporation preventing it cooling much further. But only under a clear sky.