A ceiling fan has been standard equipment for hot weather since the 1800s. By circulating air downward in a room, they make rooms feel about four degrees cooler on summer days. However, the great majority of ceiling fans have a switch that reverses rotation of the fan, making them useful in winter, too. Instead of circulating air downwards, clockwise rotation in winter draws air up from below, blending it into warm air that naturally collects at the ceiling. The warmed air is driven out to the walls and then pushed back down into the room. This distributes heat more evenly around the room and reduces heat loss through the ceiling. A ceiling fan uses about as much electricity as a 100-watt light bulb but can cut energy consumption in a room 10 percent by allowing the furnace to run less often or at a lower level of operation. Fans can also be placed on the ceiling above stairwells to prevent heat migration from lower floors to upper floors in winter.