### Instruction:
What is a head rowing race?

### Input:
Head races are time trial / processional races that take place from autumn (fall) to early spring (depending on local conditions). Boats begin with a rolling start at intervals of 10 – 20 seconds, and are timed over a set distance. Head courses usually vary in length from 2,000 metres (1.24 mi) to 12,000 metres (7.46 mi), though there are longer races such as the Boston Rowing Marathon and shorter such as Pairs Head.

### Response:
A head rowing race is a time trial race where rowing boats race on a rolling start with short time gaps between start times. Courses vary in length from 2,000 meters to 12,000 meters.