Question

When water convert from solid directly water vapour what is the process called?

Answer 1

when any substance change directly from solid to vapor state, that process is called sublimation but water doesn't change directly from ice to water vapor.
example- napthalene balls and camphors change directly into vapor on heating.

Answer 2

Its called sublimation. In this process substances from solid state is directly converted to gaseous state without turning to liquid and visa versa.

e.g. camphor