Evaporation is the process by which water changes from liquid to gas, known as water vapor. We call water vapor humidity. Keeping an open bucket of water inside a house may cool the air slightly, and add a little humidity, as the water slowly evaporates. Humidity and temperature work together to determine the dryness of the atmosphere. A simple evaporative cooler, sometimes called a swamp cooler, may be more effective than a bucket alone. An evaporative cooler adds a fan; moving air across the evaporating water cools the atmosphere.
Bucket and Towel
To add humidity to a hotel room, some guests fill the room's ice bucket with water, then soak a hand towel in the water. Place half of the soaked towel in the bucket, leaving the rest of the towel hanging over the side of the bucket. Set the bucket next to the room's fan vent in the room with the free end of the towel flapping the the moving air over the vent.
Add Humidity with Plants
House plants can increase a room's humidity slightly when water evaporates from the soil and the plants transpire as water evaporates from their leaves. Think of the plants as living receptacles of water, in lieu of a bucket. Group the plants together. To further increase moisture, fill a shallow tray with pebbles and water. Set the plants in the tray on the pebbles. The water in the tray will evaporate slowly.
An evaporative cooler capitalizes on hot, dry air outside the house. The air is drawn into the cooler and passes through wet pads. The air cools and an interior fan circulates the cooled air and moisture. (Similar to air from the fan being blown over the damp towel.) Evaporative coolers works best in hot, windy, dry climates. They can be rigged from buckets and fans or purchased.
Alternatives to Air Conditioning
Air conditioning units use refrigerants, which may harm the environment if released into the atmosphere, and tend to dry the air. With evaporative cooling--whether letting a soaked towel waft over a fan or using a commercial evaporative cooler, relies on the natural process of air cooled by water. Evaporation keeps moisture in the air and can reduce irritations of skin, throat or eyes. An evaporative cooler requires you to keep open the doors and windows, circulating fresh air.