It's all set in the server_cfg.ini file.
There are four different settings related to the track conditions:
[DYNAMIC_TRACK]
SESSION_START= Level of grip that the track will begin at for the first session (% of 100%. 100% = "Optimum". 94% = Green track)
RANDOMNESS= % of possible deviation from the SESSION_START value.
LAP_GAIN= Number of laps required to gain 1% of track grip. (Total of everyone's laps, not singular. 10 people, 3 laps each = 30 laps)
SESSION_TRANSFER= The percentage of the spread from beginning of the session to the end of the session that is carried over to the next session. So, for example, if you set the SESSION_TRANSFER value to = 50, and start at 90% grip, and by the end of the session you reach 98%, then the spread is 8%. So, in the next session you'll start with 50% of that spread, so the track will begin at 94%.