Going from a 70+ dewpoint day to a <65 degree dewpoint day is like chugging rocket fuel. I can put out way bigger numbers for way longer.

Getting to the point where I don't even try to do a workout if dewpoint is high. Just try to keep the legs spinning.

I don't think simply riding in a room at 90 degrees would help much. Heat isn't typically the issue, heat + humidity is. Your body simply can't cool off. In a race, that's when ice socks come into play...

Tucsonan who trains evenings and afternoons here:

Heat is similar to elevation in that:
1) You perform poorly because less oxygen is available for your muscles.
(In heat, blood is sent to your skin for cooling purposes and less blood is available for movement).
2) Workouts and races are moreexhausting.
3) One doesn't recover as well as one would at low elevations or low temperatures.
4) In both situations, it might be wise to add occasional high quality workouts at low elevation or low heat.
5) In both situations, ones athletic economy might improve because of learning to train with little oxygen.

Differences-
1) Ones body does not respond to heat by producing more red blood cells.
2) In heat, your body becomes depleted of different vitamins and minerals.

Conclusion:
Heat training makes you good at performing in heat. It might help a little with performing at elevation. It might improve your economy.
But heat is exhausting and it is harder to recover.
Unlike elevation, there will be no increase in red blood cells caused by living in heat.

My guess is that heat is only beneficial for athletes that have to prepare for a hot race.
Otherwise the costs probably exceed the benefits.

That said I hate being cold and there is nothing worse than living somewhere without a real summer.

the key about altitude is not only is it less air, but less pressue. high heat humidity will deliver less oxygen, but is also during high pressure

  
For a long time, the scientific consensus has been that there is a heat adaptation that occurs over 2 weeks of prolonged heat exposure. That includes:

* Increased propensity to sweat --
* Lower mineral/electrolyte loss from sweating
* higher plasma volume

There is no further acclimation after 2 weeks.

I think the consensus has been the heat acclimation improves relative performance only for hot conditions:

https://www.ncbi.nlm.nih.gov/pubmed/25943678

But there is the occasional study that suggests it might improve performance more generally:

https://www.ncbi.nlm.nih.gov/pubmed/20724560

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My latest book: "Out of the Melting Pot, Into the Fire" is on sale on Amazon and at other online and local booksellers

This is not true. When you train continually in heat your body responds by increasing plasma volume (due to continual losses). Additional plasma volume means additional red blood cells too at an absolute levels (not necessarily by percent though).

If the changes suggested in this infographic happen to me, I'd take them! (not a comparison to altitude though)


Also, here is a graph of some of the changes and the time course of the adaptations

Thanks, this was the type of info I was looking for. So generally speaking, which might be best for an athlete?
1) Train all the time in an environment of higher heat and humidity in order to maximize the heat aclimisation effect. (For conversation sake, let's say 6 months out)
2) Train in an athletes normal environment. Then 14 days out from an event, move into a heat aclimisation plan.

My next question would be this:
If an athlete completes a heat aclimisation cycle, and the race conditions are not one of heat and humidity, but cool and dry, would there be any performance gains for the athlete relative to the field that did not do any heat aclimisation?

My opinion is to try to train in a way where you maximize the benefits of the heat and minimize the deleterious aspects. So, longer lower intensity training in heat (like long rides and runs) and more of the lactate threshold and VO2 work in cool conditions (I'll do these inside with plenty of moving air) . So to answer the first part of your question I'd say your #1.

The answer to your second part is without a doubt YES. Any effort you do in the cool and dry will feel very easy when highly heat trained.

Also it must be said that newer research is coming out now pointing to the upregulated genes and resultant proteins from heat training, all of which suggest a cascade of very beneficial adaptations. Your gut also must go through adaptations to deal with the high internal temperatures for long periods. It's not always pretty adjusting to it, but the changes in the gut for the heat trained are very beneficial in endurance racing over someone that isn't. You'll digestion a hot day, they won't.

Couldn't agree with you more. When I went to race at IM Tahoe after training in North Carolina's summer, I didn't feel the altitude at all. I was pleasantly surprised.