What device is described as a fabric tube that indicates the direction of the wind?

The wind sock is specifically designed to be a fabric tube that visually indicates the direction and relative strength of the wind. It is commonly used at airports and other outdoor locations to provide a quick and easily understandable visual representation of wind direction. The opening of the sock points into the wind, while the wider end flutters in the wind's direction. This design allows pilots and others to gauge not just where the wind is coming from, but also its intensity, based on how much the sock inflates and moves.

In contrast, a wind vane, while also related to wind, is more rigid and typically indicates the wind's direction without conveying the wind's strength. An anemometer measures wind speed, providing quantitative data rather than visual directional information. A weather balloon is used primarily for collecting atmospheric data at various altitudes and does not serve to visually indicate wind direction. Thus, the wind sock is the most appropriate answer as it directly fulfills the function described in the question.