Almanacs are a mathematical tool for calculating thithis, stars, yogams and karnams. Apart from these a one year prediction is also given for example - this year there will be more rain eg. In the south west of Vindhyas there will be a severe cyclonic storm in the karthika month and 75% of the rains will be in sea, 15% in mountains and remaining in the vilages and cities adjacent to them.
On what basis is this prediction being done? There are Vakiya panchangam, Thiru Ganitha Panchangam, Pambu panchangam etc.Again I think this may vary from region to region. Any scriptural evidence as to how they calculate and predict the natural calamities?