The cold snap seems to have broken, and the weather system moving in from off the Pacific is bringing warmer temperatures and dumping rain, melting the recent snow and soaking most of the Puget Sound area.
Rainy, warmer weather also help set up the environmental conditions for horses to rapidly develop rain rot, also called rain scald. Preventing rain rot isn't particularly complicated; you simply have to keep your equine buddy clean and dry. Unfortunately, that can be easier said than done, in our climate.
To be blunt, rain rot can be a total pain to get cleared up, once your horse comes down with a galloping case of it. It's not life-threatening, but it's awfully unattractive, and can become stubbornly entrenched. If this is the first winter/spring you've spent in a rainy climate with your horse, you might not have seen this condition before, and might you're uncertain as to how to recognize whether your equine companion has a case of rain rot.
If you surf around the internet looking for information, you'll find several articles that mention rain scald as something that occurs to "horses in poor body condition"—and that's more than a little misleading. When you live in a climate like ours, perfectly normal, healthy horses can develop rain rot.
You'll likely first notice little bumps on your horse's skin, or a sort of rippled, odd texture to the skin and hair, often along his back or hips, or even on his neck. In more advanced cases, the hair looks matted and crusted. If you gently brush at it, the hair and scabs lift away, leaving patches of bare, irritated, sometimes serum-oozing skin, beneath. It really is pretty distinctive, and once you've seen it, you won't mistake it for ringworm or any of the other skin conditions horses can develop.
Some horses seem more prone to the condition than others. Rain scald is most often caused by a fungus, Dermatophilus congolensis. The fungus requires an oxygen-poor environment, so you're going to have to gently clean away the thick hair and crusty, scurfy scabs to let the air reach the affected areas, before the infection will clear up. The easiest and most gentle way to do this is with daily bathing, until the hair starts growing back on those bald patches. You should use an anti-microbial shampoo, like chlorhexidine or dilute betadine scrub.
It's also important to remember that the condition is transmissible; you should thoroughly clean and disinfect any brushes, curry-combs, blankets, and tack. If you're having trouble getting the condition to heal, you might need to give your vet a call—extreme cases may require a course of antibiotics, or your horse might have another skin condition, complicating treatment.
Rain rot isn't particularly uncomfortable, and it's certainly not a reason for stall rest. While you're waiting for your horse's skin to clear up enough that you can ride, again, don't hesitate to make use of the extra time for grooming, groundwork, and bonding.

