Hydrostatic rating on a tent is the highest column of water a material can withstand for up to one minute before the water starts to penetrate (assuming good seams, no damage etc)
The ratings translate as follows (have seen these quoted in a couple of places, this is from campingcrazy.com)
- 1000mm or less is considered shower resistant and will soak up rain and get damp after a heavy shower
- 1500mm is considered the realistic minimum requirement for summer (and mainly dry) camping
- 2000mm is the recommended minimum if you intend camping in your tent year round
- 3000mm is mainly used on family tents so they can withstand a couple of weeks of rain!
For floors, the thing to watch is that pressure on it can lower the effective hydrostatic head, so if you are camping in very wet areas, a 10000mm rating can be very useful - often this is quoted for the groundsheet, and up to 15cm up the sides of a wet weather tent. If you can't get a high rated tent, just go old school and use a groundsheet under your tent.