Surface Conditions Vs Precipitation
In track driving it becomes important to distinguish between precipitation, which is for example the rain that is coming down right now, from surface conditions, which is how wet the pavement is.
The distinction is most important in the transition periods, when the rain is starting or just after it stopped. When the rain is starting, the track is still dry, and hence dry lines are largely possible, especially if there is a customary line that most cars take through particular corners. This condition of having effectively dry pavement in rain can last for several laps depending on how severe the rain is. On a hot day when water evaporates quickly, the effect would be more pronounced and would last longer.
Once the pavement is saturated with water, the track goes into rain mode regardless of the severity of precipitation. What this means, for example, is that an all day drizzle typically results in a wet track whereas isolated showers may never quite get the track to its wet state.
This also explains why a light drizzle is not much different from moderate rain as far as traction is concerned - in both cases the pavement is soaked with water and does not offer any dry grip. Heavy rain causes puddles and rivers to form which once again changes track conditions.
When rain ends the track continues to be wet for a while. In cool and overcast conditions it may in fact remain wet for quite some time. In these conditions the track should be treated as a wet one, even if it has not rained in a while.
When the track is dring, it dries first on the customary line that most cars take. This has to do with both the cars displacing water as they drive through and the tires heating up the pavement - a typical dry tire temperature is 150-200 degrees F whereas ambient track temperature may be 40-80 degrees F depending on season.