A letter in The Guardian makes the following claims:
While seemingly fantastical, weather has been weaponised. At least four countries – the US, Russia, China and Israel – possess the technology and organisation to regularly alter weather and geologic events for various military and black operations
...
warfare now includes the technological ability to induce, enhance or direct cyclonic events, earthquakes, draught and flooding
Does this technology exist?
To some extent yes.
Well know technique is cloud seeding to induce rain. It was used during the Vietnam War, operation Popeye was carried out to induce heavy rains over Ho Chi Ming Trail.
"Few aviation writers and historians seem to be aware that there were three WC-130As. These three were originally trash-haulers, borrowed from TAC in late 1966 for use in Operation "Popeye", the rain-making mission in Southeast Asia, set to begin the following year. The intent of the mission was to create enough year-round rain to keep the Ho Chi Minh trails impassable with mud. Tests were conducted over Laos in 1966, and the operational missions began in March of 1967 from Udorn RTAFB, Thailand. They were flown by crews of the 54th WRS, rotated on a regular basis from Guam. In addition, 54th crews were sometimes called upon to conduct synoptic weather reconnaissance from Udorn over various areas of Southeast Asia, out to and including the Bay of Bengal.
The A-models were modified for weather reconnaissance, probably at WRAMA, with the AN/AMR-1 system. They were not configured for atmospheric sampling. Two were kept at Udorn, with the third rotating to and from Guam for maintenance, repair, and crew changes, from June 1967 through late 1970. When the third one was not enroute to/from Thailand, it was used for normal weather reconnaissance activities from Guam. In late 1970 the A's were replaced with three 1958 B-models, and the rain-making mission continued through mid-1972 with whichever B- or E-models were available from the 54th. After re-conversion to transport, the A's were transferred to Air Force Reserve units. During their brief stint as rain-makers, they flew a total of 1435 "combat" sorties, and it is reported that at least one of them received battle damage. All three A-models wore the standard Southeast Asia camouflage colors and markings, but with no unit designations of any kind. In 1973, 56-0519 was given or loaned to the South Vietnamese Air Force, and it became one of the spoils of war on April 30, 1975. The last reliable sighting was in April of 1999, which reported her corroded and derelict at Tan Son Nhut Airport, Ho Chi Minh City." (source)
Another military attempt at weather control was Project Stormfury:
Project STORMFURY was an ambitious experimental program of research on hurricane modification carried out between 1962 and 1983. The proposed modification technique involved artificial stimulation of convection outside the eyewall through seeding with silver iodide. The invigorated convection, it was argued, would compete with the original eyewall, lead to reformation of the eyewall at larger radius, and thus, through partial conservation of angular momentum, produce a decrease in the strongest winds.
However, it not weaponizing, as it's was mainly aimed at preventing/reducing naturally occurring hurricanes, rather than inducing them.
The US Project HAARP (High Frequency Active Auroral Research Program) powers many of the current crop of secret weather control stories: (haarp weather control). Haarp can produce artificial aurora by heating limited parts of the ionosphere, but it's only a minor effect produced by a megawatt scale device, and the ionosphere is far removed from surface weather. Some claim that HAARP was used to knock out the Russian's latest mars probe, however:
Even if HAARP was turned on, a full-power blast would have hit the Russian probe with no more than 1.03 milliwatts of radio energy per square centimeter — about the same as pointing a 60-watt light bulb at it from 69 feet away
That's just not enough energy for HAARP to be a plausible weather control agent. Directly back to the subject at hand: If weather's been weaponized, it's not been done through HAARP scale projects.
Related Question: Can HAARP affect weather or earthquakes?
People have attempted to weaponize lightning in some form or another at various points.
Scientist Looks to Weaponize Ball Lightning
In the mid-’60s, the U.S. military started exploring ways that the phenomenon might be weaponized. Take this 1965 Defense Technical Information Center report on Survey of Kugelblitz Theories For Electromagnetic Incendiaries, (Kugelblitz is German for ball lighting). The document summarizes and evaluates the ball lightning theories then prevalent, and recommends "a theoretical and experimental Kugelblitz program… as a means of developing the theory into a weapons application." This led to an Air Force program called Harness Cavalier, which seems to have ended without producing anything conclusive.
Wikipedia also states that an Electrolaser could be used as a weapon:
As a weapon, to make a thunderhead deliver a precise lightning strike onto a target from an aircraft; in this case, the aircraft and laser can be compared to a triggered spark gap, in that the relatively minor amount of initial input from the laser allows a large amount of energy to flow between the cloud and the ground.
Although I don't know how likely that is.
