There’s nothing wrong with dry air, even if it has a reputation for being harsh on the skin and mucous membranes. On the other hand, there are few proven benefits of humidified air. It doesn’t soothe the airways, and it won’t necessarily help breathing during a cold or an asthma attack. (One exception is croup, a condition that humidified air helps considerably.) On the other hand, when air is too humid it provides a perfect culture medium for molds, bacteria, and allergens. For those reasons, I don’t encourage humidifiers. If you insist on having one, they come in different sizes and prices, depending on factors such as noise level and their alleged ability to limit mold production. Whether the moist air is cold or warm makes no difference, since it ends up taking on the room’s own temperature.