A sipe is about a 3/4 deep hole in the tread block that sucks up water as it compresses against the asphalt using the weight of the car, and kicks out the water as it spins. They can also be lateral veins in the tread block.

The circular ones are usually 3-5mm in diameter.

And they're fairly visible on winter tires.



'99 GSX GT35R
'03 CBR 600RR