With some basic observations you can figure out that length and persistency of contrails depends pretty much on weather. I live on south side of Alps and contrail behavior is one of indicators to do my own forecast. Short, tiny contrails mean air in upper troposphere is generally dry and that is common when we have dry, cold northerly wind across the Alps, bringing sunny, dry and cooler weather. When contrails become long and in particular, when they spread into wide swaths of cirrus-like clouds and persist, that means moist air is coming, forecasting bad, (means cloudy, possible rainy) weather is getting in. It works every time. No rocket science here.