Every winter, the Water Plant receives a number of calls from residents who have noticed a change in the appearance of the water running from their faucets. Their descriptions range from "cloudy" to "soapy" to "old dishwater gray." If you notice this, you need not be alarmed. What you are observing is harmless; it is just air.
This phenomenon is caused by the temperature of Lake Michigan water, which is generally between thirty-three and forty degrees during the winter months, and the fact that air dissolves more easily in cold water. After water is treated by the City, it is pumped into distribution mains under pressure (about sixty pounds per square inch). While on its way to your home, the water begins to warm as it flows underground. This warming trend continues as the water enters the pipes in your home. Water entering a water heater warms up and all along, it remains under pressure, This pressure forces the air to remain in solution even though its concentration is several times in excess of the water's current capacity.
As soon as a household faucet is opened, the pressure is relieved and dissolved air begins to release. If you observe a glass of the "cloudy" water, you will soon notice that it begins to clear at the bottom. As the bubbles rise to the top of the glass and pop, the band of cloudiness slowly begins to disappear.
These tiny bubbles are visible upon close inspection. They scatter light, causing the cloudy or gray appearance, just as the clouds of water vapor in the sky would. Once the cloudiness dissipates, the water is perfectly clear. No residue is visible at the bottom of the glass, offering further proof that this is simply air!