I don't have any answers, David, and they're good questions. But here are some observations:
The air at my altitude (about 1000m) is drier and thinner than in England. That probably means its specific heat capacity and thermal conductivity are lower so radiation plays a more significant role compared to ambient air temperature than in a more humid, sea level environment.
The thinner air also means less of the sun's radiant heat is absorbed by the atmosphere.
The south and west facing banks where I am seeing STs absorb and re-radiate more heat than horizontal surfaces would.
These three factors might mean that so long as the sun is in the sky a butterfly can maintain a sufficiently high thoracic temperature to fly. It's perhaps worth noting that at higher altitudes small tortoiseshells regularly fly over snow, where they get a double dose of radiation from the sun and reflected from the snow.
Down in the valley the situation is different. There, I find early butterflies among the terraced vineyards where hotspots readily build up. It is because of the very favourable conditions here that the vines were planted in the first place.
In England, where moist air from the surrounding sea acts as a buffer the temperatures drop less at night and rise less while the sun is in the sky.
Whatever the causes, STs are now out in numbers here, not just in ones and twos. No other species are, though. The brimstones and commas, and peacocks to a lesser extent, normally wait for a more appropriate time of year.
That's all I can say! Any more would be pure speculation!
Guy
Here's a late afternoon view taken the other day in my home village. The slope in the foreground is inclined towards the east, hence the long shadows. But the other side of that hill, in the full sun, it is gloriously warm.
