First, in the context of smoke "tar" is a general term that means particulates. Vapour from THC contains tars, just not nearly as much as smoke does.
From what I know tar (in the context of smoke) contains the byproducts of pyrolysis / combustion, not just fine particles in general – if that was the case, the air would be full of tar, wouldn't it?
The reason higher temperatures create thicker clouds is the increase in particulate matter in the vapour stream. This is also why higher temperatures produce harsher vapour: the particulates irritate the throat, and of course the vapour is hotter.
While that might be true, let's not forget that we actually strive to inhale certain substances, namely cannabinoids. *Those* (and in some cases the temperature, as you mention) will irritate the respiratory system. While cannabis smoke will contain about 15% cannabinoids, vapor contains about 95% (
https://www.ncbi.nlm.nih.gov/pubmed/17429350). So I think it's pretty safe to say that when we cough, it's not because of those 5% non-cannabinoids (which, by the way, are not tar). Let's also not forget that not any kind of respiratory irritation is a sign of harm; most of us do not avoid combustion because it sometimes makes us cough, but because of its long-term effects.
This has been studied. See
Availability of Hazard Identification Materials for Marijuana Smoke (R. S. Tomar, J. Beaumont, & J. C. Y. Hsieh August 2009). They conclude that the inhaled smoke from smoking cannabis contains much the same mix of tars that tobacco smoke does. They speculate that this could mean the same risks exist for both, but they do not present any evidence to prove this.
That's because *any* pyrolized plant material contains carcinogenic substances, that's not exclusive to combusted tobacco. Smoking peppermint, wood or sage will harm you in the same manner as smoking tobacco or cannabis. It makes no sense to make a (costly) study to prove something we already know for a long time. Tobacco smoke has some special characteristics, that is true, but the major harm factor is the combustion itself. We know that tobacco smoke and cannabis smoke are not equally carcinogenic, but they both are.
From my own research (which no one should take as either exhaustive or conclusive) I do not believe anyone has found such evidence yet. I believe there are two reasons for this. First, cannabis users have far less exposure to tars than tobacco smokers.
Those kind of assumptions are not helpful. You can't say any cannabis user will have less exposure to tar than a tobacco smoker – compare someone chainsmoking his bong every day with someone who will have a cigar every Sunday evening. Both will be exposed to smoke condensates.
Second, I believe that cannabis smoke might well contain something that mitigates the risk. This has not been proven but there are indications that this might be true. The inability to connect cannabis smoking to cancer despite strenuous effort is one of them.
Because in order to actually *prove* a connection you'd need long-term studies, and those are not published yet as far as I know. It's just not possible to prove a connection to a symptom that needs decades to develop within a few years. On the other hand you are right that there seem to be strong signs that canabinoids have anticarcinogenic properties, but we should separate those two effects wisely.
Now, that study is about smoking not vapourizing, but as I pointed out, cannabis vapour does contain particulate matter that we refer to as tars. The exposure to them is reduced, however, and I believe it is safe to say that the risk is much lower.
Okay, let's agree that the risk is much lower (that's why we are here, right?), but I can't agree on the term „tar“ here. Sorry. :-)