Ice on a car windscreen will disappear as you drive along, even without the heater on. Why?

The phenomenon you are observing is called sublimation, which is
the change from a solid state to a gas state without passing through a liquid
state.

Let's say the temperature of the ice is 0 degrees C. This
means the average temperature of all the molecules in the ice is at that temperature.  However,
the actual temperature of any individual molecule can be greater or less than the average. That
means that some of the molecules are moving faster, and others slower, than the
average.

Occasionally, some of the water molecules leave the surface
of the ice on the windshield and are blown away by the moving wind. Since there is no way to
supply more water molecules to the ice, it gradually gets smaller and smaller until it
disappears.

Of course, a sunny day will make the process go faster
as more molecules absorb energy from the sun and are able to escape their icy
prison.

This same thing happens to comets as they get closer to the
sun. See the link for more information.