Various quotes from Airbus. Either from FCOM or Airbus cold weather ops.
An aircraft ready for flight must not have ice, snow, slush or frost adhering to its critical
flight surfaces (wings, vertical and horizontal stabilizers and rudder)
When removing ice, snow or slush from aircraft surfaces, care must be taken to prevent it
entering and accumulating in auxiliary intakes or control surface hinge areas, i.e.
remove snow from wings and stabilizer surfaces forward towards the leading edge and
remove from ailerons and elevators back towards the trailing edge.
No aircraft should be dispatched for departure under icing conditions or after a de-icing /
anti-icing operation unless the aircraft has received a final check by a responsible
authorized person. The inspection must visually cover all critical parts of the aircraft and be performed from points offering sufficient visibility on these parts (e.g. from the de-icer itself or another elevated piece of equipment). It may be necessary to gain direct access to physically
check (e.g. by touch) to ensure that there is no clear ice on suspect areas.
Aircraft contamination endangers takeoff safety and must be avoided. The aircraft
must be cleaned.
To ensure that takeoff is performed with a clean aircraft, an external inspection has to
be carried-out, bearing in mind that such phenomenon as clear-ice cannot be visually
detected. Strict procedures and checks apply. In addition, responsibilities in accepting
the aircraft status are clearly defined.
If the aircraft is not clean prior to takeoff it has to be de-iced.
Why de-ice/anti-ice on ground? The aircraft performance is certified based upon an
uncontaminated or clean structure. If the clean aircraft concept were not applied, ice,
snow or frost accumulations would disturb the airflow, affect lift and drag, increase weight
and result in deterioration.
It must always be remembered that below a snow / slush / anti-icing fluid layer
there can be clear ice.
fuel added to the aircraft during the current ground stop, adding
(relatively) warm fuel can melt dry snow with the possibility of re-freezing.
My favourite airbus quote:
Ice and snow due to ground precipitation, or overnight stay, should be totally
cleared before takeoff, regardless of the thickness. Otherwise aircraft is not
certified for flying.
Ensure that all the contaminants are removed from aircraft surfaces.
So make of all this what you will but it seems fairly clear to me. All of the above quotes are taken directly from airbus publications.