This is part of a continuing series of theoretical weapons. The weapons are theoretical as the United Nations has already banned the weapons (but not banned the building of research of defense from these weapons).
Possible applications of weather modifying weapons.
1) Use surface modifiers on oceans including but not limited to submerged nuclear heaters, airborne solar powered lasers, surface spreaders like oil slicks. This will help modify the temperature of the ocean in certain critical areas at critical times, influencing weather esp winds that bring rains.
Example- Modifying or Enhancing El Nino to influence rain to specific countries.
2) Use of air borne or aircraft borne lasers to start forest fires
3) Use of lasers to enhance the rate of melting of strategic glaciers.
4) Modify and interfere with the timing of an active volcano to prevent big rupture, rather to go for controlled releases.
5) Use of harmonics to influence seismic wave activity in geological reasons.
- Weather Weapons (decisionstats.com)
- What made the roman weapons powerfuler than other armys (wiki.answers.com)