dry weather is definitely much better.
the grapes are wet, water will mix with grape juice
while crushing the grapes. It will alter the quality
dew and mist are all enemies when the harvest comes.
Actually hot sun is an enemy as well as the fermentation
process will start sooner.
harvest more and more in the middle of a dry night
to prevent both water and sun to corrupt their wines.
more about Wine making
your question here