Final answer:
Wind shear is reported on a METAR when a significant change in wind direction and speed over a short distance occurs, and it is an important factor in aviation safety and severe weather prediction.
Step-by-step explanation:
Wind shear is reported on a METAR when there is a change in wind direction and speed over a short distance, such as during conditions that may lead to clear air turbulence or thunderstorms. It is not exclusively associated with any single weather event like thunderstorms or snowstorms, but rather with the rapid change in wind patterns that could affect aviation safety. Understanding wind shear is important for predicting severe weather phenomena, including the potential development of tornadoes.