The ionosphere which HAARP is apparently studying is very thin, nearly
a vacuum by normal standards. I don't think it is at all likely to be
a way to control the weather. Its electromagnetic properties are much
more interesting from the point of view of communications and sensing,
and I think that is enough to explain the military's interest.
The thing is, weather is a big, big phenomenon. If you want to
control it by brute force, you will need hugely massive amounts of
energy, many orders of magnitude (powers of 10) beyond what we can
manipulate now. Alternatively you can try to use fine tuning control,
taking advantage of the chaotic dynamics of the system. There have
been a lot of demonstrations recently of controlling chaotic systems,
and conceivably these ideas could apply to weather. However for these
to work you need to have a very accurate picture of your system, and
you need to be able to predict where it is going in the short term,
as well as where it would go if you tweak it.
So we need better computers, and more importantly we need a global system
for monitoring the atmosphere in detail. These both will be along
eventually, but it's going to be a while. A spinoff from this will be
very accurate weather forecasts at least over the short term. They will
probably need to be able to predict how much rain each house will get
over the next 24 hours, something like that level of accuracy. Then
they can try seeing if tiny perturbations can give control. It's going
to be a big and very exciting task.
Once this is achieved, conceivably weather warfare will become possible.
However countermeasures may be possible as well. Given that the enemy
is using microscopic tweaks to lead to big effects based on his models,
you can confound him by counter-tweaks. This presupposes a certain amount
of parity between the opponents; without that then sure, one side will
have an advantage, just like with other forms of warfare.
Hal