In case you don't want to read the link (pages 28 &29):
The last step in the grooming operation allows the moisture that was created during the processing step to refreeze. This binds the individual granules of tightly packed snow firmly together, creating a hard surface that will withstand passing traffic much better.
The length of time needed for a trail to set up correctly can vary from two to six or even more than ten hours, depending upon the temperature and moisture content of the snow. Trail set up can be similar to freezing a tray of ice cubes – after a short time there may be a crust but the cube isn’t entirely solid and it generally takes a few hours for it to become fully firm. A snowmobile trail is no different. Therefore, it is vital that the trail remain as undisturbed as possible during this set up period for firmer, better quality trails that will stand up longer to snowmobiling traffic.
Ideally a snowmobile trail would be closed during set up time, but that isn’t practical. Consequently, the best time to groom is generally at night when traffic levels are typically lower and air temperatures are generally colder. For the best set up, it is strongly recommended that grooming occur at night after snowmobile traffic subsides. This also provides for the safer operation of both groomers and snowmobiles since it is easier to see oncoming lights and beacons.
Most importantly, night grooming provides for more effective grooming since there is typically more time for the trail pavement to freeze solid before traffic resumes, maximizing the effectiveness of the area’s grooming dollars.It is recommended that daytime grooming be done in areas only if there is little or no daytime snowmobile use on the trail being grooming. Other exceptions would include special circumstances such as when daylight would aid operator visibility for initial early season trail set up and establishment or for trail reestablishment of the trail after big storms, extremely heavy snowfalls, and/or significant wind events.
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