When having a windshear alert plus minus 10 knots by the tower, does it mean vertical or horizontal ones?

Question

When having a windshear alert plus minus 10 knots by the tower, does it mean vertical or horizontal ones?

Answer 1

Windshear reported in knots will be horizontal.

any serious vertical windshear will be reported as a microburst or thunderstorm activity in the vicinity.  this also requires doppler radar which isnt available at most airports.  

Its alot easier for the tower to observe windspeed fluctuations at various points around the airfield.  there are usually several anemometers scattered about for just this purpose.

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Answer 2

Horizontal, since airspeed in an airplane is measured horizontally, and that is what they are referring to: instantaneous airspeed fluctuations of plus or minus 10 knots. Vertical shear is reported differently.