Line

Under clear weather, the atmospheric absorption/emission at (sub)-millimeter wavelengths is dominated by the rotational and fine structure line of molecules in their ground electronic and low vibrational states. Three molecules plays a major role

is the most abundant polar molecule present in the atmosphere. Its electric dipole moment is 1.88 Debye. ATM 2009 also takes into account the water isotopomers.

Being an homonuclear molecule, has a zero-valued electric dipole moment. However, it has a triplet electronic ground state, with two electrons paired with parallel spins, implying a weak magnetic dipole moment. The weakness of this dipole moment is compensated by the large abundance, implying line intensities similar to ones.

is an asymmetric top molecule, such as . Despite the small ozone abundance, its lines have significant peak opacities, especially in the submillimeter domain. This is dues to linewidths much narrower than water. This can be explained by two facts: 1) the ozone dipole moment is 0.53 Debye, about 3 times less than water and 2) ozone is mostly concentrated between 11 and 40 km of altitude, implying linewidths 20 times narrower than sea level lindwidths.