The simple answer is: it depends entirely on the specific cushions. While many modern outdoor cushions are designed to withstand occasional rain, they are not meant to be permanently wet. Most are made from water-resistant or quick-drying fabrics, which means they can survive a sudden downpour. However, leaving them constantly exposed to rain will significantly shorten their lifespan.
Water-resistant fabrics shed light rain, but prolonged exposure will eventually allow moisture to seep in. Once the inner foam filling gets wet, it becomes very difficult to dry completely. This trapped moisture can lead to mold, mildew, and unpleasant odors. Furthermore, constant wetness can break down the foam, making it lumpy and less comfortable, and cause the fabric to fade or degrade.
For the longevity of your cushions, the best practice is to bring them indoors or store them in a waterproof container during heavy or prolonged rain, and especially during winter months. If they do get wet, allow them to air dry completely in the sun before placing them back on the furniture. Always check the manufacturer's care label for specific instructions, as materials like Sunbrella® are highly weather-resistant, while others require more diligent care. Ultimately, bringing cushions in during bad weather is the safest way to protect your investment.