What is an AIRMET?
And AIRMET is issued every six hours for IFR conditions, mountain obstructions, turbulence, strong surface, winds, icing, and freezing levels.
What is a SIGMET?
Non-convective weather hazardous to all aircraft such as icing, severe/extreme turbulence, dust/sandstorms, lowering visibility by 3 miles, volcanic ash. 
What is a convective SIGMET?
These are for severe turbulence, icing, windshear hazards to all aircraft valid for two hours. Includes severe tea storms with winds +50 kn, 3/4 inch hail, tornadoes, embedded tea, storms, lines of storms, or heavy rainfall affecting 40% of 3000 mi.²