Experience alone determines this little difference.
In summer, on cloudy and stormy days, mercurial vapors rise more readily and quickly than in the temperature of autumn or winter.

From 60 degrees upwards towards the boiling point (660 deg.), the vapors of mercury rise in greater abundance and collect in larger globules on cold surfaces.
For various reasons I prefer a high temperature and short exposure.

It accelerates the process.

It renders the lights of the picture more strong and clear, while the deep shades are more intense.

It gives a finer lustre to the drapery.