Which scenario does not require a special observation?

In the context of aviation weather reporting, a special observation is necessary when there are significant changes in weather conditions that could impact flight safety. While the start of a new thunderstorm during the presence of an ongoing thunderstorm might seem like it would require special reporting due to potential hazards, the criteria for a special observation typically focuses on immediate and significant changes that affect weather reporting, such as those that can be measured and observed distinctly.

In this case, the start of a new thunderstorm might simply be part of the ongoing thunderstorm activity and does not represent a distinct "change" in conditions but rather an addition to them. Thus, it does not require a separate special observation.

Conversely, significant changes in wind direction, visible drops in temperature, and alterations in humidity levels usually indicate critical shifts in weather that could directly affect aviation operations, necessitating special observations to inform pilots and air traffic controllers adequately.