Antarctic aphelion astronomical twilight cape celestial sphere coalescence drainage wind erosion flood instrument flight rules (ifr) national weather association (nwa) snow crust snow level specific humidity stratus fractus tornado alley transpiration water zulu time. Altocumulus aurora divergence eclipse fresh water funnel cloud gully washer isobar lifted index (li) rotation season showalter stability index small craft advisory squall standing cloud storm winds straight-line winds tropical depression veering.