Preference goes to non-miraculous explanations of how the fire was maintained.
I feel like this question would be better suited to physics but giving a scientific explanation here.
What is fire?
Fire is an exothermic reaction between wood and oxygen refered to as "burning". Burning requires energy in order break the chemical bonds, but produces more energy then it consumes (hence exothermic), in what is called a "runaway reaction"
How does water put out fires?
it can do this one of 2 ways:
- The water goes thru a phase change taking a lot of energy from the fire which can lower the woods temperature bellow the point that wood burns. This stops the reaction
- Burning wood requires both wood and oxygen (Which limits the speed that wood burns). Water can prevent oxygen from coming into contact with wood, this prevents the reaction from occurring (this is how pouring sand, salt, and fire extinguishers smother a fire to "put it out").
What does this mean?
If the amount of water that you pour on a fire is small enough it will not put out the fire. The fire will be strong enough to evaporate the water without losing enough heat to stop burning.
Generally speaking, rain isn't a bucket of water poured at once from the heavens onto the ground... It is a bucket of water that is slowly dripped onto the ground over a long period of time.
So why is rain bad for fire then?
There is a 3rd way to kill a fire. Which is to starve it of fuel (wood, or other flammable material. When it rains, wood gets wet which makes it harder to burn. If it has been raining long enough, its possible that the wood will require more energy to dry out then it will release... (Or it will require more energy then your fire is capable of providing... Either way, wet wood makes it hard to start a fire/maintain a fire.
Special preparations needed for rain?
Have a supply of dry firewood. This doesn't come from historical sources, but from years camping out in the woods... Including rainy days/nights. If you have a large enough supply of dry wood, you ca keep a fire going in almost any rainstorm (Obviously not thru a hurricane, monsoon, or other major weather event).
The scientific method (Testing this for yourself)
Go camping (or somewhere you can build a fire outside). Build a small fire. Try slowly pouring water onto the fire... You will notices that the fire doesn't go out, you will get a lot of steam, but the fire will continue to burn. This is because the fire is evaporating the water as it lands onto the fire... Now take a bucket and dump the entire contents on the fire at once. You will notice that the fire went out. This is because the amount of water that you poured onto the fire out paced the fires capability to evaporate it and so it cooled the fire below the critical point of the exothermic reaction, halting the reaction.
Not only are fires strong enough to evaporate water from a light rain (and thus not burn out). But fires also heat up the air around the fire, this creates an updraft, which is a bunch of hot air rising. The updraft will deflect water droplets as they fall, preventing most of the water droplets from falling onto the fire in the first place. This is especially true if the rain is light, as opposed to heavy