"there were no regular seasons. It could just as well pour a flood from the skies in the middle of summer as heat up a miserably cold country in the middle of winter."

That means unseasonable weather. If fields are planted with crops and accustomed to a certain amount of dryness or rain or heat, etc. and thew weather becomes 'unseasonal' or has one in 500 year events a couple of times each year, regardless of season, it becomes impossible to grow crops.