Helicity denotes the conjunct intensity of rotation and the movement along the rotation, and it is conservative in isentropic fluid. Helicity is the basic physical pattern for the maintenance of atmospheric activities. Boundary layer fluids, turbulences, severe storms and tropical cyclones are highly helical in their structures. Severe storms often occur in highly helical environments. The stability of fluid is closely associated with helicity. High helicity prevents transient energy from leaping to other energy classes, thus has great importance for the maintenance of supercell. Z-helicity is a good indicator of heavy rainfall. The relative helicity of a storm has great importance in the evolution of the storm. The occurrence of a meso-cyclone is highly depended on the value of helicity. The relative helicity of a storm is also a good indicator of hail occurrence. It is difficult for the application of relative storm helicity in predicting storms for the uncertainty of storm velocity. There exists inherent relationship between atmospheric temperature and helicity, and the surface relative helicity can be used to measure the temperature advection caused by geostrophic flow.