they are
1.atmospheric icing conds. when temp is below +5*C on ground (OAT) or below +7*C in flight (TAT) and any moisture observed ,visibilitiy below 1 mile or any precipitations etc...
2.ground icing conds. when OAT is below +5*C and on taxiways or runways it exists patches of water ,slush , snow etc.
now,before performing the de-icing / anti-icing, you have to take in account the real temp.(necessary for type of fluid and ratio) and HOT (hold over time), wich depends of temp and the weather(freezing fog, rain, snow ,sleet) and HOT starts at the same time with the beginning of de-icing operations.