To clarify:
Dehydrated foods require water only to reconsitute into something more pallatable. I've eaten a dehydrated meal without reconsituting it. Top Ramen is also dehydrated food. Your body will supply the moisture needed for digestion, and even if you reconstitute dehydrated food, your body will still use a significant amount of water to digest it. If you don't have a supply of water to reconstitute a dehydrated meal, then don't eat, you don't have enough water to eat anything anyways.
Freeze-dried and dehydrated food can be stored in your vehicle, regardless how hot it gets. We got to 140 this summer here (which translates to much warmer in the car)and my freeze dried/dehydrated supplies did just fine. Canned foods may get too hot to store in such an environment, because the moisture inside the can generates exessive internal pressure and will burst the seal when heated.
Most of the freeze dried/dehydrated meals I've prepared lately require 2 to 2 1/2 cups of boiling water to rehydrate. Some freeze dried food doesn't require hot water to rehydrate properly.
As a refresher, freeze dried food is fresh food that is taken to sub zero temperature and then exposed to a vacuum, which sublimates the frozen water in the food into vapor and draws it out, thus preserving the general texture of the food. Freeze dried food thus has a very low residual moisture content. Dehydrated food is fresh food that is typically dried by exposing it to airflow at ambient or slightly warmer conditions(below the temperature that would cook the food). Conventionally dehydrated food may have as much as 30% residual moisture, which means it may be susceptible to spoilage. Generally, freeze dried food is better than conventonally dehydrated food because the freshness is maintained better (I've had freeze dried peaches that I rehydrated that were just about like eating a fresh peach). Processing methods for both have improved over time, and although freeze drying is a relatively expensive process, conventional dehydration can be nearly cost free, except for a certain amount of time and effort expenditure, if done at home. I have more than a few home made recipes using dehydrated ingredients that are pretty darned tasty. BTW, dried beans are a dehydrated food, so is rice and noodles. Most home dehydration is for meats (jerky), vegetables, and fruits.
Some dehydration methods use salt in lieu of airflow. Norwegians still use salt aboard their boats to preserve fish rather than ice.
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The ultimate result of shielding men from the effects of folly is to fill the world with fools.
-- Herbert Spencer, English Philosopher (1820-1903)