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 only last for 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 an approach within a microburst is as follows.

• First, the helicopter will meet a sudden increase of headwind, which will cause an increase in airspeed and a decrease in rate of descent, which takes the helicopter above the desired approach 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 even greater rate of descent.
• As the aircraft continues, the outflow from the microburst is now coming from behind and the increasing tailwind reduces the airspeed even more, accompanied by an increase in the descent rate.

The pilot’s problems are added to by the effect of vertical windshear and strong downdrafts which can cause further variations in speed and the rate of descent.

More specifically, in our case, as you approach the microburst, you will experience a 20-knots headwind. While flying through the microburst, the wind will change, and the headwind will decrease. When you fly directly under the microburst, you will experience strong downdrafts, and the horizontal wind speed will be 0 knots. Beyond the center of the microburst, the tailwind will increase (20 knots tailwind). Therefore, the wind will change from a 20-knots headwind to a 20-knot tailwind, resulting in a 40-knots windshear.

Generally, keep in mind that it is best to avoid the hazard rather than having to recover from it.

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Note:

• If, at any stage of flight, moderate or severe turbulence, wind shear, and by extension, microburst are encountered, Air Traffic Control must be informed with details of the location and severity of the phenomena. This will allow them to issue warnings to other aircraft.