UKC

Interesting new research about enhanced rain banding?

Seems too obvious to be new?

https://youtu.be/dskwM5Y3-P8?si=UuY2B3kfBLd5SLP6&t=666

I'm not sure I understand that freezing line.

If you compress a gas (as I think you will if wind hits an obstruction and forces it to rise), it warms up: pV=nRT.

If the air is warmer, I would expect the freezing level to rise, i.e. falling snow will melt higher up. That seems to be the opposite effect to what has been shown in the 'purple line' in the video, which shows the snow melt altitude to drop at rising hills.

This sounds a bit like the analysis of how to get least wet; by walking or running in rain and wind.

The differential fall speed between snow and rain, and the freeze/melt altitude determine how much of a 'snow column' falls as rain. If the entire snow column is moving, then I can see that the freeze/melt altitude determines how much of that snow column drops out as rain.

It's worth pointing out that all rainfall starts as snow...

What about Langmuir’s chain reaction?

air also cools with altitude..

I think I can see what you mean about the air experiencing some compression as it is blown against the side of the hill but it's the subsequent lifting that cools it.  As it rises the atmospheric pressure on it drops so it expands.  Expanding gases cool and that's why the freezing level is lower in the areas with the highest rates of lifting.

Yes, it does. But clearly there are other effects, otherwise the freeze/melt altitude would be a flat line, wouldn't it?

Actually, there's a difference between 'air cools with altitude' and 'air temperature drops as an air mass rises'. Yes, the temperature of the surrounding, static air mass falls as we climb though it, but, in this case, it is the air mass rising. A more complicated situation.

Well, that's a plausible theory. But why is it rising? What force is acting on it to cause it to rise?

Yes, if the air column rises and expands, it will cool. But it's the same air column that has moved from lower ground to higher ground, and has thus been compressed (if only because it has to squeeze into less space...), so will be warmer, relative to the preceding air column (because of exactly the same mechanism that accounts for your cooling as pressure reduces). So, even if it cools as it rises  it is starting off from being warmer.

Weather is complicated, it seems. Who'd a thunk it...?

It's a little bit more than plausible - it's the standard orographic lifting model.  And the force that's causing it to rise is the wind (in the example in the video it's left to right.)  You wouldn't get any of this without wind.  (I'm not saying it can't rain without wind but that would need some other kind of lifting.)

Rather than thinking about the whole air column, maybe think about any given litre of air.  As it moves upwards it has to expand because of the lower atmospheric pressure.

expansion cooling on the upslopes/windward and compressional heating downwind fits the graph? but then there's the latent heat of vaporisation...

A snow flake melting and evaporating as it meets warmer air, cause evaporative cooling immediately around it, so it may still reach the ground only smaller than it original was. That phrase we often use "it's trying to snow" when you get big wet flakes when air temp is often +1 or +2c. 

Air does cool with altitude, but how much depends it's humidity. That's how temps at equal heights on either side of a large hill or range won't be the exactly the same when a weather front passes over, as some humidity is lost as rain. 

Okay...

It was a rhetorical question. I was alluding to force, energy & warming.

I obviously need to go and read up on weather. Any primers will be welcome, since I did start a while back with a view to adding to my DofE instruction notes, as well as my general understanding. I remember struggling with the explanations I found.

I too was a bit confused by the altitude of the 0°C isotherm varying as the air mass is pushed upwards over mountains. I don't understand why the altitude of the freezing level of an air mass, and the altitude to which you would have to raise a parcel of that air to get it to freezing, are not identical. 

Because the properties of the air mass is not uniformly homogeneous; after all, we are familiar with variation in pressure, temperature & humidity across a geographical extent.

My query was due to not understanding how those variations occur, and the associated physics. I still don't, which is why I need more study...

I think I understand it now. The rate at which the temperature of a rapidly rising parcel of air declines with altitude ('adiabatic lapse rate') is purely determined by expansion (+ condensation). The rate at which temperature declines with altitude in non-rising air ('environmental lapse rate') is also determined by expansion (+ condensation), but also by the slower-acting processes of radiation and convection, which act to homogenise the temperature of the air mass over time. Therefore the environmental lapse rate is lower than the adiabatic lapse rate.

So as air is pushed over the Cumbrian hills, the whole air mass cools at the adiabatic lapse rate, which is greater than the environmental lapse rate, so the freezing level drops.