By default, calm winds are defined as less than 1 knot, or less than 1 m/s depending on the units you are using.  It is possible to change this definition by editing the wind classes:

1. Click the wind classes button.

2. Edit the low end of the first wind class.

All values less than the lower end of the first class will be considered calm.

Note: When setting your calm threshold it is a good idea to keep in mind the level of precision for wind speeds in your met file.