Why tap water sometimes looks milky white, and why it always clears up

Fill a glass at the kitchen tap on a cold morning and it can come out looking like skimmed milk. Give it a minute, undisturbed on the counter, and it turns back into ordinary, transparent water. No filtering, no stirring, nothing added.

For anyone who has ever wondered whether their pipes are failing or their water supply has gone bad, the answer is almost always far less alarming than it looks.

The cloudiness is air, not contamination. Tap water carries dissolved gases, mostly oxygen and nitrogen, the same way a sealed bottle of soda carries dissolved carbon dioxide. Under the pressure inside a water pipe, these gases stay dissolved and invisible.

The moment water leaves the tap, it drops from high pressure to ordinary room pressure, and the gas that was held in solution can no longer stay dissolved at the same concentration. It comes out of solution as countless microscopic bubbles, scattered through the entire glass rather than rising as one. Each bubble is tiny enough to bend and scatter light in every direction, and when billions of them are suspended together, the eye reads that scattered light as solid white rather than as individual specks.

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This effect has a proper name, gas supersaturation, and it shows up most often when the water is cold. Cold water can hold more dissolved gas than warm water, so when winter mains water or water fresh from a chilled pipe is suddenly depressurised at the tap, it has more gas to release.

That is why the milky-glass effect tends to be more common in colder months or with water drawn straight after a cold spell, and rarer with water that has been sitting and warming in a tank.

Plumbing can amplify the effect without causing it. A faucet aerator, the small mesh screen fitted to the end of many modern taps, is designed to mix air into the stream deliberately, for a softer flow and lower splash. That built-in aeration adds extra bubbles on top of whatever gas the water releases naturally, which is part of why aerated taps produce cloudier-looking water than an old-fashioned spout. Long pipe runs, sudden pressure surges after maintenance work, and recently installed plumbing can all have the same amplifying effect, since each introduces a bit more turbulence at the point where the water meets the air.

What makes the cloudiness self-correcting is simple physics. Once the bubbles form, they are buoyant, and they rise. A glass left standing clears from the bottom upward, because the lowest layer of water has the shortest distance for its bubbles to travel to the surface and escape.

Within ten to thirty seconds, depending on the size of the glass and the temperature of the water, the bubbles have escaped into the air above the water's surface and the glass returns to full transparency. If the cloudiness were caused by something dissolved or suspended in the water itself, such as sediment, minerals or bacterial growth, it would not behave this way. True contamination tends to settle slowly, leave a residue, alter taste or smell, or simply fail to disappear. Air bubbles do none of that; they vanish entirely and leave no trace, which is the simplest test available at home.

It is worth distinguishing this from other reasons tap water can look off. A genuine yellow or brown tinge usually points to rust or sediment in old pipework. A persistent cloudiness that does not clear after several minutes of standing is more likely linked to mineral content or, rarely, to a treatment or supply issue worth reporting to the local water authority. But the fast-clearing white haze that disappears top to bottom within half a minute is, in nearly every documented case, nothing more than dissolved air finding its way back into the atmosphere. It is a property of the water leaving the pressure of the pipe, not a property of what is in the water at all.