When is low level windshear likely to be greatest?

Low-level windshear typically occurs when there are significant differences in wind speed or direction in the lower levels of the atmosphere, particularly in the vicinity of boundaries such as temperature inversions. The top of a marked surface-based inversion is a location where a significant change in wind patterns often exists.

At the top of the inversion, wind speeds may increase sharply, especially if there is a contrast in air temperatures that results in stable layers below. The stability provided by the inversion can restrict mixing, so any disturbances lower in the atmosphere, such as those caused by surface winds, may lead to enhanced wind shifts or changes just above the inversion layer. This creates conditions ripe for windshear, particularly during takeoff and landing phases of flight when aircraft are operating at lower altitudes.

Factors such as surface friction can influence wind regimes, but the presence of a temperature inversion creates a strong boundary where low-level windshear is particularly pronounced due to the increase in wind speeds just above the inversions. Therefore, this context provides a clear understanding of why low-level windshear is most likely to be greatest at the top of a marked surface-based inversion.