Old weather lore is a personal love of mine, especially for interpreting the natural world and the influence upon it by the Gods and spirits.
Here are some monthly thunder omens as detailed by Olaus Magnus in Historia de Gentibus Septentrionalibus:
In January thunders mean higher winds than usual and a taller growth of the earth's crops.
In February, that death will come to a great many, particularly to those who live luxuriously.
In March, that strong winds are ready to burst out, that there will be a fruitful season, and noisy legal disputes.
In April, that rain will fall, which will be good for the crops, and the fields will wear a cheerful face all the year long.
In May they signify everything adverse: a dearth of crops and every commodity frighteningly expensive.
In June they forecast a more plentiful fertility, but sickness is much to be feared.
In July too an abundant supply of corn, but ruin for peas, beans, and fruit.
In August, that men shall live together at peace, yet there will be terrifying illness.
In September, that a fruitful season is at hand, together with wars, civil discord, and carnage.
In October any thunder is reckoned to be ominous, showing that tempests are threatening on land and sea.
In November, though they happen in very infrequent years, they promise fruitfulness in the coming year.
In December thunders are a sign of general plenty and that men shall live agreeably together.
