Hydrogen Bond

however any covalency will increase the network stability relative to purely electrostatic effects. The hydrogen bond in water dimers is sufficiently strong to result in the dimers persisting within the gas state at significant concentrations [1977] (for example, ~0.1% H2O at 25 °C and 85% humidity) to contribute significantly to the absorption of sunlight and atmospheric reaction kinetics [266].
80% of the molecules of liquid water have one (cooperatively strengthened) strong hydrogen bonded O-H group and one non-, or only weakly, bonded O-H group at any instant (sub-femtosecond averaged and such as may occur in pentagonally hydrogen bonded clusters), the remaining 20% of the molecules being made up of four-hydrogen-bonded tetrahedrally coordinated clusters [613].