The setting: On a recent backpacking adventure, it began to rain very hard just as we were setting up camp. Luckily, I had already started a small fire. It may not have been possible to start a fire to begin with if I had waited too long after the rain started. Many items got wet including our backpacks, the clothing we were wearing and even the inside of our tent. Oh yea, so did the dog but she didn't mind!
After our tent was set up and all our gear was properly protected inside the tube tent that I bring specifically for gear, the next task was simply to wait out the rain. But I wanted to keep the fire going, otherwise it would not be possible to start it up again with the amount of rainfall we were getting.
So what can someone do to keep a small fire burning strongly for hours during an intense rainfall?
There are a few strategies at play:
First off, you obviously don't want the fire to completely go out, so we need to protect it somehow from getting rained on.
Secondly, you need to keep enough fuel and kindling dry as everything will continue to become wet and may be useless to burn even if you have a fire going already.
The first step is to isolate your fire in a small pit, preferably surrounded by stones and rocks. Your surrounding pit constructed of rocks should be between 6 inches and 12 inches high.
While your fire is strong, you want to burn as much as possible to achieve a very high heat and develop long burning, hot embers and coals that will stay very hot even when not in flames.
As you burn you should begin collecting branches and sticks to cover the fire but not so that they are close enough to catch flame. The purpose is to protect the fire/embers underneath from the rain and to have these branches and sticks exposed to a high level of heat to stay dry.
Ideally, the next step would be to complete this "fireplace roof" with wide strips of bark that you can layer to even further protect the fire/embers and branches from rain.
Then finally, cover that top layer with leaves or grass or anything that is absorbent and easily combustible (that you would burn, nothing valuable like your sleeping bag!).
This creates the final layer of protection for the fire and provides a means of keeping some relatively dry tinder in case you can not produce flames again from the hot embers underneath.
That's it! I would highly recommend continually checking on the inner fire and intermittently add some thicker branches or small logs inside.
*UPDATE!* (12/14/13) - Andy Putrello of Solo Scientific brings up a good point: Be careful about heating up shale or layered rocks as they can be potentially explosive due to air pockets within the rock that can rapidly expand when heated. Keep them away from the fire!
THINGS TO REMEMBER: It is extremely important that you actually keep the fire small, but the embers hot! In fact, you don't even need flames if you have enough embers that are very hot and will continue to burn for hours in a dry condition. Secondly, it is also important that you do not allow the overlaying roof of the fire to catch flame. This will cause the roof to burn/collapse and expose the inner fire to water. Make sure your roof is far enough from the ground to feel enough heat from the fire to help it stay dry but not too close to catch flame. Try to close as much gap over the fire as possible. The more open spots or leaks in the roof, then the more water will get into the inner fire and cause it to cool or go out entirely.