The efficiency of power lines is affected by wind conditions. When cold weather generates demand for more electricity, power lines heat up and then stretch and sag. In these circumstances wind can have two effects, it can help to cool the line, but may also cause the line to sway and touch high trees resulting in a short circuit discharge to ground, which can sometimes trigger a major power outage. Gill wind anemometers are often used to monitor power line condition and are normally placed on the pylon. Due to the height, the vertical component of the wind speed is also important.