Hydrodynamical helicity

Hydrodynamical helicity is a measure of how "twisty" or "corkscrew-like" a flow of water or air is. Imagine you have a garden hose with water running through it. When the water comes out of the end of the hose, it can twist and turn around before it hits the ground. That twist is called hydrodynamical helicity. Helicity helps us measure the energy of the water or air as it moves, and can tell us how strong the currents are in a river or wind.