Which weather phenomenon can indicate the presence of low-level windshear?

Virga is the phenomenon that can indicate the presence of low-level windshear because it is often associated with the evaporation of precipitation that falls from clouds but does not reach the ground due to dry air. This situation typically occurs when there are strong vertical wind gradients and turbulent airflow, which are characteristic of windshear conditions. The presence of virga suggests that there may be significant changes in wind direction and speed at lower altitudes, which can impact flight operations, especially during takeoff and landing.

In contrast, stratus fractus consists of low, irregular, and broken clouds that generally form in stable air conditions and do not necessarily indicate turbulence or windshear. Altocumulus castellanus, while it may show some instability, primarily indicates potential vertical development and does not specifically point to low-level windshear. Patches of fog indicate low visibility and typically occur in stable conditions, providing no clear indication of significant wind changes at low altitudes. Therefore, virga serves as a more direct indicator of the potential for low-level windshear due to its association with evaporative processes and turbulent airflow.