Wool coats have a variety of benefits over other natural fiber or synthetic coats. You can wear one in the rain without worry, so long as you give it the proper care once you're out of the rain.
Absorbency
Wool is somewhat water-repellent; small amounts of liquid as in a light rain may simply roll off wool coats. When wool does absorb liquids, it can take in up to 35 percent of its weight.
Insulation
Wool insulates against wind, cold and heat. The absorbent qualities of wool do not diminish its insulating power, even when the coat is wet.
Smell
A soaked wool coat can take a long time to dry and may have a strong, unpleasant smell. However, some wool blends minimize the wet-wool smell.
Prevention/Solution
To properly dry a wool coat and keep the damp smell from lingering, hang it in a ventilated, warm and dry location until it dries. You can use a clothes dryer set on air-dry or the lowest heat setting, but monitor the coat closely to keep it from shrinking.
Comparison
Other natural textiles, such as cotton, do not insulate you from the cold in the rain. Synthetic coats can cause you to sweat more in warm rain and may develop mildew smells.