I've been flying since 2000 and flying at altitude (say FL150-200) since 2005, and don't recall ever seeing ISA conditions.
Mostly it is ISA+10 and often ISA+15.
Maybe in the depths of the winter, or at ~ FL300 one does actually see ISA conditions?
I guess it's because we tend to fly a lot in the summer months, and then usually when there is stable weather (=little temperature drop with altitude).
In the winter, you won't see ISA a lot, but lots of sub-ISA. Freezing level should be at 7500 feet, but it's often much lower in winter, frequently at ground level.
Remember though that ISA is not just one temperture at a certain a altitude, but a constant decrease of temparature with altitude. Sometimes in winter, you have two or even three freezing levels, with positive temperatures in between.
ISA is merely a standard which however in practice is never encountered.
What I have found in winter flying is that while you are way below ISA on the surface (obviously), by the time you get to say FL150 the surface situation does not track up there i.e. the higher altitude temps are less affected by the season.
However one particular thing I wonder is whether the atmosphere is warmer today than say 20 years ago. I know jet pilots who say that e.g. 30 years ago they would usually be VMC at FL200 while today they often find solid IMC at FL300, and this is away from any fronts.
ISA is based on averages; how often do you encounter the average in everyday life? 2.4 kids comes to mind!
I see sub ISA all the time above and below FL200 but generally only in winter. Temperatures definitely correlate as you go higher ie temperatures in winter at altitude are lower vs ISA than in summer. What does vary is the actual lapse rate which is never at the ISA rate.
Yea, ISA is alive and kicking. I've seen many soundings which go straight up the ISA Line in our levels. And yes, the perception has a lot to do with the fact that we are flying a lot of time during the summer.
Over average, ISA still works pretty well. Particularly if you use it outside the expectation to see a straight ISA atmosphere all the way through. But let's say the pressure differences or temperature gradients within a certain range are often enough close enough to work out. e.g. if you have a temperature at FL100 and you want to know it at FL120 or 130 for which you have no charts (assuming you are looking at forecasts and have no forecast TEMPS plots) you can still use the standard calcs and will get pretty accurate results. I did some of this calculating density altitudes for mountain crossings recently, where there is no real data available often enough and that works very well indeed.
One bit to recall about the soundings is that they most of the time are hours old by the time we get them. So ISA calcs are still useful when looking at forecasts unless you have access to forecast soundings, that is balloon ascents calculated in advance using forecast models. Not many of us have access to these so ISA is the thing to use.
In a broader sense, do we have climate change? Sure we do. There has not been a single period in the existence of this planet where there has not been change in climate. Climate changes regularly and in cylclic forms, some of them short term, others long enough in order not to have been on any measure data this civilisation has. Mix the two and you get tens of thousands of scientists funded who will tell you that we are all going to perish from heat or freeze to death and who make a load of money with their theories. Scared people always have been willing to pay top dollar in order to avoid doomsday :)
So yes, this planet is in the process of another change of life cycle weather wise. Right now, we actually are moving towards weather we have seen and found normal in the 1960ties: Real winters and real summers with wet and nasty springs and falls. At least that is central Europe at the moment.
Best regards Urs
This morning at FL230 : ISA -7 The summer of 2013 is history...
Yes, I went from ISA+6 out of Paris to ISA-3 over Frankfurt yesterday.