Vertical wind shear
Topic: Air Masses, Fronts, Cyclones and Thunderstorms · NCERT: Class 11, Ch 9 "Atmospheric Circulation and Weather Systems"
Meaning
Vertical wind shear is the change in wind speed or direction as you go higher in the air. A tropical cyclone (a strong storm over a warm sea) needs its warm core to build straight up, layer above layer. When shear is weak, the storm stays upright and grows stronger. When shear is strong, winds at different heights blow the top of the storm away from its base, and the storm falls apart. That is why weak vertical shear is one of the conditions a cyclone needs to form.
Example
During the south-west monsoon (June–September), shear over the Bay of Bengal and the Arabian Sea is strong, so cyclones are rare. They form mostly before the monsoon (April–May) and after it (October–December), when shear is weaker.
Don't confuse with
- Coriolis force: this is the turning effect of the earth's rotation, which makes a storm spin. Shear is a difference in winds between heights, and it can stop a storm from organising at all.