Refer to figure.
Windshear could be defined as an abrupt and localized change of wind velocity, that is strength, direction or both. It can occur both vertically and horizontally.

There can be several causes of low-level wind shear:

• The passage of the front in which the isobars change direction;
• A marked temperature inversion;
• Topography and large buildings;
• Surface friction; and
• Cumulonimbus clouds and their associated weather of heavy rain and hail, which can give rise to the phenomenon known as Microburst.

A microburst is generally accepted to be about 4-5 kilometers across and lasts only a few minutes (around 5 minutes). Although gusts and wind shear can be encountered up to 20 miles from the thunderstorm, the downdrafts generated can be very strong, up to 6000 ft/min, and can often exceed the rate of climb of many aircraft.

The sequence of events typical for a helicopter during a take-off within a microburst is as follows.

• First, the helicopter will meet a sudden increase in headwind, which will cause an increase in airspeed and, by extension, in lift. Consequently, the helicopter will immediately climb above the desired take-off path.
• At the same time the helicopter enters a reducing headwind and increasing downdraft. A loss of energy leads to reduced airspeed, less lift, and an increasing rate of descent.
• As the aircraft continues, the outflow from the microburst is now coming from behind. The increasing tailwind reduces the airspeed even more, accompanied by an increase in the descent rate.