Ceraunoscopy--from cerauno/keraunos (Anciet Greek for 'lightning, thunderbolt') and scopy (Ancient Greek for 'to see')--is a form of augury and aeromancy in which one draws and interprets omens or divines past, present, or future through the examination or observation of thunder and lightning.
The phenomena of thunder and lightning were once believed to be a form of communication from the gods and our ancestors.
Ceraunoscopy consists of two practiced--brontoscopy is the art of divination by the sound of thunder, and ceraunomancy is the study of divination by the examination of lightning bolts.
This practice was known to be used by mainly the Ancient Roman augurs, some of the forefathers of the augury we still know and use today, but it was also used by the Celts, Greeks, Babylonians, and Egyptians, to name a few others.
The most well known method of Certaunomancy consists of dividing or mapping out the sections of the sky into east and west and divining based upon where the bolt appears.
The East related to the family and one's own kind.
The West related to 'the enemy' in ancient times.
Bolts that come from the EAST are a good omen, received as a sign of harmony and success to come. Bolts from the WEST are a considered bad luck to see and very inauspicious. Bolts from the NORTHEAST are favourable and lucky. Bolts from the NORTHWEST are an omen of trouble to come and receiving bad news. Bolts from the NORTH are the worst to receive, as they were believed to promise great loss and possibly even death.
In order to receive and interpret these omens correctly, the diviner should stand facing SOUTH to be able to observe the phenomena. I cannot honestly say I know why to stand facing south, just that that is how it was done before and how I do it now.
Thunder was believed to bring news from the gods on the matters of war, death, loss, and success and happiness.
To be heard from the LEFT, it is to be a good omen, denoting happy news to come or a victory in your current struggles. To be heard from the RIGHT, it is the opposite, a promise of misfortune and unhappy times to come.
Bearing these rules in mind, it also depends on the day of the week the thunder is heard on.
Sunday: indicative of the death of a learned man, who holds a prominent position in society--a judge, a scholar, or possibly a politician.
Monday: believed to foretell the death of a woman.
Tuesday: a sign of a good harvest, success, and good fortune.
Wednesday: was once said to foretell the death of harlots by the Ancient Romans, but other cultures regard it as a sign of bloodshed or conflict as a general rule.
Thursday: another promising harvest or success.
Friday: the Romans believed it to indicate the loss or death of someone great.
Saturday: promises widespread grief, loss, or trouble, such as an epidemic, tragedy, or economical strife.
These are, however, very, very old interpretations and are difficult to relate to our lives now. After much studying, my notes play out as such:
Sunday: death or loss of a man
Monday: death or loss of a woman
Tuesday: an auspicious occurrence promising success and good fortune.
Wednesday: conflict, stress, and cutting ties (or bloodshed, if you've a quick fist, like me).
Thursday: your luck will turn around; a sign that now is the time to take new ventures and risks.
Friday: a hefty or great matter; unhappy affairs.
Saturday: prepare to receive bad news; crisis, or great trouble.
Also, the length and volume of the thunder clap or rumble has a part to play,as well as whether the thunder was accompanied by lightning or not. The LONGER the thunder lasts, the longer the promise indicated by day and direction will last. The LOUDER the thunder is, the more you will either suffer or benefit from what is foretold. If the thunder is followed by lightning, it foretells of an abrupt change of circumstance. As for whether it is for the better or worse, that depends on both how the thunder was read,as well as how the lightning is to be read.