High-resolution spectroscopy is allowing scientists to detect the presence of water dimers—the simplest cluster of water molecules (inset)—in a sample of water vapor. The pink line is the measured spectrum and the white line is the calculated dimer spectrum. (The monomer contribution—indicated by the yellow trace—has been subtracted.) The vertical bars show the positions of the dimer rotational lines. The spectrum was measured in water vapor in equilibrium at 23 °C and 0.022 atm.