When water is cold, such as during the winter, there is more air in it. When the cold water enters your home, the water warms up and air is released from the water. The air in the water will give it a milky or cloudy appearance. When you open the faucet to fill a glass, the air is released as bubbles similar to what you see when shaking a soda. As the water sits in the glass, it will clear from the bottom of the glass to the top. The air is not harmful. The more air in the water, the longer it will take for the water to clear and more cloudy or milky it looks.