Hydroplaning doesn’t happen often. But it can if your
tires do not have much tread or if the pressure in one or
more is low. It can happen if a lot of water is standing on
the road. If you can see reflections from trees, telephone
poles or other vehicles, and raindrops “dimple” the
water’s surface, there could be hydroplaning.
Hydroplaning usually happens at higher speeds. 
There just isn’t a hard and fast rule about hydroplaning.
The best advice is to slow down when it is raining.
Driving Through Deep Standing Water
If you drive too quickly through deep puddles or
standing water, water can come in through your
engine’s air intake and badly damage your
engine. Never drive through water that is slightly
lower than the underbody of your vehicle. If you
can’t avoid deep puddles or standing water, drive
through them very slowly.
Driving Through Flowing Water
Flowing or rushing water creates strong forces. 
If you try to drive through flowing water, as you
might at a low water crossing, your vehicle can be
carried away. As little as six inches of flowing
water can carry away a smaller vehicle. If this
happens, you and the other vehicle occupants
could drown. Don’t ignore police warning signs,
and otherwise be very cautious about trying to
drive through flowing water.
Some Other Rainy Weather Tips
D Besides slowing down, allow some extra following
distance. And be especially careful when you pass
another vehicle. Allow yourself more clear room
ahead, and be prepared to have your view restricted
by road spray.
D Have good tires with proper tread depth. See “Tires”
in the Index.