Keeping cool in the summer is much easier in modern times than in ancient ones. Ancient cultures that lived in the desert, as well as modern ones that live traditionally, such as the Bedouin, relied on sweating, since their dwellings were not climate controlled. Places like Phoenix, Arizona were inhabited by the ancient culture of the Hohokam, which built an extensive system of canals to bring water from the Salt River to their crops. This tribe eventually either moved away or died out. For modern Phoenix residents, the prospect of facing a summer without air conditioning makes the disappearance of the Hohokam seem quite understandable, despite archaeologists having no official explanation.

In fact, the city of Phoenix was quite instrumental in the development of climate control systems. It is one of the hottest metropolitan areas in the entire United States, with more than 100 days per year over 100 degrees Fahrenheit and temperatures often reaching over 110 in July. Without climate control, it’s unlikely that the city would have become the sixth largest in the nation. In the early 20th century swamp coolers were perfected in Phoenix. Although swamp coolers do not handle humid heat well, they are quite effective in dry heat. Later, those who perfected swamp coolers moved to air conditioning units, which work effectively in any humidity (and actually reduce the humidity of the air they cool).

A swamp cooler works on the principle of evaporation. In order to evaporate, water has to take in heat from the surrounding air. A swamp cooler is usually a box with pads inside three of the sides and the top. The fourth wall is dominated by a large fan, and the floor of the box is a sump for water. A pump carries water from the sump to the top of the pads, where it trickles back down and soaks the pads. The fan blows air out, and air is pulled back into the box through the soaked pads. The expelled air is usually quite cool, especially in dry weather.

An air conditioning unit works on a completely different principle. Instead of evaporating water, it takes advantage of the principle of Boyle’s gas laws. When a gas is pressurized, it gets warmer, and if a vacuum is drawn on it, it will cool. Air conditioning uses Freon, which has a boiling point near room temperature. By pressurizing the Freon, and allowing it to cool, and then decreasing the pressure, the system moves heat from inside the room to outside. Phoenix air conditioning has developed and is now a major fixture in virtually all inhabited buildings in the city. For air conditioning, Phoenix was an early pioneer and continues to carry the torch.