Drove down to Bourg St Maurice this afternoon (approximately 850m) from my apartment in Les Arcs (approximately 1850m). It was about 5 degrees in Bourg, and moderate rainfall. Typically I'd expect it to be snowing when I got back to my apartment if it was 5 degrees in the valley, but today it was still raining when I got back to my place. I usually assume a Lapse Rate of about 1 degree per 100m (which I know is very much a rule of thumb, and usually seems to be a bit in the high side to me), but today it was way off that as it was only four or five degrees colder when I'd gained about 1000m of altitude. Why was the Lapse Rate different today?
Obviously A snowHead isn't a real person
Obviously A snowHead isn't a real person
The lapse rate you quote is the 'dry adiabatic lapse rate', while the circumstances you describe were far from dry. The moist adiabatic lapse rate is about half that i.e. 0.5 degrees C per 100m.
Last edited by Obviously A snowHead isn't a real person on Tue 31-01-17 17:29; edited 1 time in total
Well, the person's real but it's just a made up name, see?
Well, the person's real but it's just a made up name, see?
The approx. 1 degree per 100m ROT is the theoretical dry adiabatic lapse rate. The real lapse rate depends on temperature/humidity and local environmental factors including the wind. When the air is saturated the lapse rate is usually much lower, as condensation created in the rising air releases heat.
@foxtrotzulu, @Wags, thanks. I knew there was a difference between wet and dry rates, but didn't realise it was that large.
What confused me is that typically if I'm in Bourg St Maurice and it's raining and about 5 degrees it will be snowing when I get up to 1800m (often snowing at a lower altitude than that). Today must have been a bit unusual.
Anyway, snowHeads is much more fun if you do.
Anyway, snowHeads is much more fun if you do.
@rob@rar, my rule of thumb figure is 2C per 300m. So 10C over 1500m in heavy snow/rain. That's about the same as @foxtrotzulu's suggestion perhaps a little more generous. There are plenty of complex formula to work it out. Even so with a 1000m climb from 5C (which should bring it to around freezing) you'd expect it to be snowing as snowfall normally kicks in a little above the freezing level. I guess it's a bit chaotic up there. When the Atlantic rips in it can be messy (hence ALARM!)
The problem is that large features, such as high mountains and deep valleys, have a massive effect on air movement/mixing so all sorts of random things can happen.
Then you can post your own questions or snow reports...
Then you can post your own questions or snow reports...
Yes air mixing and saturated air greatly reduce lapse rate.
It's a bit hard getting your head around rain and dry air and rain and saturated air.
Here in the Pennines it was 3c this morning and raining but no snow at 2,500 feet. With drier air e.g. From the north west, it would have been perhaps -4c on Raise or the tops of the North Pennines.
The lapse rate also tends to increase at higher latitudes and the further away from the sea. So for example it is greater in the cairngorms than Snowdonia. It is usually at its highest in late spring apparently.
The high lapse rate in dry air also explains why snow making works better in dry air.....I think
After all it is free
After all it is free
I've given up trying to work it out
It's only in Spring I can 100% rely on shite when I go to bed and get up for an old man's pee to see temps at -6 that I then know how to gauge the morning
As for the coming days there at times to be no logic, today +5 though as I was walking the dogs flocons were falling out of the sky.
I know that in various valleys near to us temps can be wildly different.
In fact tomorrow I'm taking 10 "geezonaires" up to the Col Izoard and am hoping for colder temps
You'll get to see more forums and be part of the best ski club on the net.
You'll get to see more forums and be part of the best ski club on the net.
The complexity of it all is easing the pain of my stupidity. Always like that when it happens.
Ski the Net with snowHeads
Ski the Net with snowHeads
Same here today, +3 in our village (1274m) and the same down in the valley at 700m, and not much colder at 2000m. Rain rain rain.
snowHeads are a friendly bunch.
snowHeads are a friendly bunch.
And to add a further complication the saturated adiabatic lapse rate (SALR) increases with altitude to eventually become almost identical to the Dry Adiabatic Lapse Rate (DALR). This is due to the decrease in temperature with height, which reduces the amount of water a parcel of air can contain and therefore a reduction in the amount of latent heat released when condensation occurs. It thus follows that there is no standard SALR, but in the levels we are interested in, ie where most of the weather, is it varies between 4 and 7 Deg C per 1000m. The DALR is generally fixed at 10 deg C per 1000m.
We could then go on to study the Environmental Lapse Rate (ELR) and instability - but it's late.
Rob - if you are interested in learning a bit more the Haynes Meteorology Manual is very good (yes that is the lot that used to tell you how to put your Austin 1100 back together again!) For mountain weather in France 'Petit Manual de Météo Montagne' (Thillet and Schueller) is also good as it goes into quite a lot of local phenomena but the French is a bit A level.
And love to help out and answer questions and of course, read each other's snow reports.
And love to help out and answer questions and of course, read each other's snow reports.
Quote:
This is due to the decrease in temperature with height, which reduces the amount of water a parcel of air can contain and therefore a reduction in the amount of latent heat released when condensation occurs. It thus follows that there is no standard SALR, but in the levels we are interested in, ie where most of the weather, is it varies between 4 and 7 Deg C per 1000m. The DALR is generally fixed at 10 deg C per 1000m.
Yes, that lot in French would be a tad beyond me!!
Fascinating stuff though - as I drive up the winding road from the valley to my apartment I cheer each half degree drop in temperature. Though of course, for weeks and weeks it's been getting warmer driving up. Or even yo-yoing, with several inversion levels, to keep us all guessing!
So if you're just off somewhere snowy come back and post a snow report of your own and we'll all love you very much
So if you're just off somewhere snowy come back and post a snow report of your own and we'll all love you very much
In my little corner of West Berkshire we live at the bottom of a valley. The temperatures at night in winter are nearly always 4c colder than the 'top' of the valley which is 40 metres higher. Something to do with cold air flowing down the hills I presume.
You know it makes sense.
You know it makes sense.
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cold air flowing down the hills I presume.
yes, that's why in the fruit-growing Evesham area the trees are on the slopes, not in the bottom of the valley.
Otherwise you'll just go on seeing the one name:
Otherwise you'll just go on seeing the one name:
Quote:
In my little corner of West Berkshire we live at the bottom of a valley. The temperatures at night in winter are nearly always 4c colder than the 'top' of the valley which is 40 metres higher. Something to do with cold air flowing down the hills I presume.
The Chilterns valleys have this reputation for being very cold in the winter and hot in the summer. It is something to do with the air getting trapped in the valley botttoms due to narrow exits.
@pam w, Like you I check the temperature as I drive up from Bourg st Maurice to Arc 1600. The height gain is about 900m, and more or less as text book the temperature drop is about 6 degrees. However, in the early hours of the morning it is quite often only a couple of degrees. Of course on many such mornings I open the curtains of my apartment and cannot actually see Bourg since the whole valley is full of mist. We have a fine view down into the Isere valley and across to Mont Blanc
Last summer whenever we were down in Bourg the temperatures were well into the 30s and we raced through what we had to do there to get up the mountain as soon as possible and get into the cooler air. We tried to plpan activities such that we never went below 1500m just because of the temperature.