Falling barometric pressure, a good indication that rain is coming, reveals itself to the observant camper in a number of ways, like the following. They are often forerunners of approaching storms. More gas bubbles rise in there water when there is a lot of decaying vegetation. In swampy areas, you can even smell the gases. Smoke particles collect water and become visible, making the campfire smokier.
Birds get quiet before a storm.
Some have observed that the birds seem tosense the duration of a rain. If it is raining early in the morning but likely to clear up later in the morning, the birds waith until it is drier. But if they are out and about in the wet, most likely the rain has set in for the day. Another useful piece of information comes with the chirping of crickets. County the number of chirps in one minute's time, add 40, and you will get the approximate temperature.