Chalk directly on the asphalt will only last until the first rain and will start to fade fairly quickly as the wind blows layers of chalk away. Tempera paint will last much longer and will weather fairly well through light rain.
WHAT HAPPENS IF IT RAINS?
The artwork is covered and protected with plastic the best we can, and in most cases will survive some rain. If there is any damage, then it's time to rebuild and repair. It sounds painful, but it's part of the process.
CAN YOU MAKE A CHALK PIECE PERMANENT?
Not really. Chalk on the street is very ephemeral and will not survive the elements, especially if it get's wet. As such, sealers and varnishes will only damage the chalk and not actually preserve it.
WHAT IS ANAMORPHIC ART?
Anamorphic is just a fancy way of saying 3D when it comes to street art. Anamorphic pieces are stretched and skewed so as to trick the eye (or camera) into seeing the image as 3D.