Does praying help prevent natural disasters?

Saint Januarius is the patron of Naples and annually a flask of his (supposedly) blood is presented during a public procession in the city. If, so the legend goes, the dry blood becomes liquid again the city will be spared of any disaster and misfortune. It must be said that the supposed premonitory miracle and its interpretation is very complicated. Factors like timing, how much and how the blood liquefies, color and density of the resulting liquid play a role.

Fig.1. The eruption of Vesuvius in 1631, Saint Januarius is shown above the vulcano. He's a silent guardian, a watchful protector.... most of the times.

The first supposed miracle happened in 1389 and in 1924 geologists Giovanni Battista Alfano (1878-1955) and Antonio Amitrano compiled a list, looking on how well the premonitory signs correlates with disasters or the lack of such. In years where the miracle failed to happen Mount Vesuvius erupted eleven times and nineteen earthquakes hit the city. In years when the miracle happened Vesuvius erupted just five times. So it seems that divine protection works half the times. 
Also whenever Vesuvius seems to become active a procession with the relics of the saint is organized to implore divine protection.These processions helped diplomat and amateur vulcanologist Sir William Douglas Hamilton (1730-1803) to reconstruct the activity of Vesuvius in recent history.

Fig.2. "The recent eruption of Vesuvius: scenes of terror and piety in the face of the eruption", by Achille Beltrame (1871-1956), cover of the newspaper "La Domenica del Corriere" April 1906.

Naples is not the only city with a holy protector. In Catania it´s Saint Agata. According to local folklore a veil of the saint has the power to stop a lava flow. During the disastrous eruption of Mount Etna in 1669 Saint Agata was invoked. For a time the lava flows could be diverted by a group of brave men, building dams of lava rocks in front of the advancing flow and digging alternative paths, also the walls of the city of Catania resisted. But finally the lava entered in a breach of the walls, claiming 15.000 victims.

Fig.3. Fresco by painter Giacinto Platania (1612-1691) of the eruption of Mount Etna in 1669, Platania witnessed the eruption himself and the painting is quite realistic, showing the lava flows stopped by the city walls.

Still in 1971 divine protection was claimed for the city of Sant'Alfio, near Catania, and it apparently worked this time. 

As why those inconsistent results over time - as they say the lord works in mysterious ways.