Why does tap water sometimes come out cloudy white — then clear on its own?

That white, milky glass of tap water is not chlorine, minerals, or anything leaching from the pipes. It is air — millions of bubbles so small they hang in the water like fog. Water arrives at your faucet cold and under pressure, and both conditions let it carry extra air in solution — a taught term meaning the air is fully dissolved into the water itself, invisible. The instant the water leaves the tap, the pressure is gone. The USGS puts it plainly: the water is no longer under pressure, so the air comes out of solution as bubbles, much like a freshly opened soft drink. The bubbles rise, reach the surface, and pop. Within a minute or two the glass is clear again.

Air under pressure

A municipal system pushes water to your house at real force — a typical home plumbing line runs around 40 to 60 psi. Pressure is one of the two dials that control how much gas a liquid can hold: squeeze harder, and more air stays dissolved. Your glass, sitting open on the counter, is at roughly zero. Cross that gap in a fraction of a second and the water suddenly holds far more air than it can keep. The surplus has nowhere to go but out — as a burst of microscopic bubbles, everywhere at once. That is the cloud.

Why winter makes it worse

Temperature is the second dial, and it turns the same direction. Cold water holds more dissolved gas than warm water — near-freezing water can carry roughly twice the dissolved oxygen of warm water (about 14.6 milligrams per liter at 0°C, versus around 7 in the mid-30s °C). So in winter, the water reaching your house is loaded to the brim. Then it warms in your pipes and glass, which lowers what it can hold just as the pressure release does. Two dials, both dropping at once: that is why the milky-glass effect shows up mostly in cold months and often after the utility has worked on the lines, which can stir extra air into the mains.

The bottom-up test

Here is the field check, straight off the drafting table: fill a glass and set it down. If the haze is air, the glass clears from the bottom up — bubbles rise, so the clear zone grows upward as they escape at the surface. Total clearance time: a minute or two, no residue. If instead the water clears from the top down, or particles settle on the bottom, that is not air — that is sediment, and it is worth a call to your water utility. Air clouds are cosmetic. The plumbing is performing exactly as built.