contrast, is driven by precipitation loading and evaporative cooling in the precipitation core of a supercell thunderstorm, making the FFD relatively cold and wet. Both are thought to be significant in tornado formation. Role in tornadogenesis Association with hook echo Rear-flank downdrafts have a well-established association with hook echoes. Firstly, the initial rear flank downdraft is air from aloft transported down to the surface by colliding and mixing with the storm. Secondly, hook echoes form through advection of precipitation from the rear of the main echo around the region of strong updraft. Thus, precipitation loading and evaporation cooling induced by the hook echo can enhance the downdraft. Some observations showed the presence of an enhanced downdraft in the vicinity of the strongest low-level rotation, behind the main storm updraft. Dry environmental air is also entrained into the downdraft and evaporative cooling helps create more negatively buoyant air. As precipitation falls and cool entrained air circulated downward and eventually reaching the surface. This contributes to the circulation to form a hook echo. It was concluded the presence of a hook echo can reflect downdraft intensification. Association with tornadoes It has been realized by many researchers that rear flank downdrafts, especially those associated with hook echoes, are fundamentally critical to tornado formation (tornadogenesis). In 1975, Ted Fujita originated the recycling hypothesis of tornadogenesis: First, downdraft air is recirculated into the (developing) tornado, which results in an appreciable convergence on the back side of the (still developing) tornado. Then the downward transport of the angular momentum by precipitation, and the recycling of air into the tornado, will create a tangential acceleration required for the intensification of the tornado as a positive feedback loop. Observations of low-level vorticity couplets within RFDs indicate that tilting of vorticity by the RFD is important in the formation of tornadoes within supercell storms. During the tornadogenesis phase in supercells, the parcels of air infiltrating the tornado or incipient tornado regularly seem to pass through the hook echo and RFD, which can serve as the basis for Fujita's recycling hypothesis. Furthermore, observations of the clear slot during and just prior to the tornadic stage, imply the air infiltrating the tornado may come from the RFD. Regularly, generation of large vertical vorticity close to the surface in an environment which is required for tornadogenesis, is attributed to downdraft. Tornadoes may arise, however, in the absence of a downdraft in environments