The results of numerical calculations of the intensity and spectra of optical emissions from blue jets produced by runaway air breakdown in the atmosphere are presented. It is found that a positive runaway streamer develops in the altitude range 20--34 km following an intracloud discharge that possesses a continuing current of ~1.7 kA. The ionization front of the runaway streamer propagates upward with a velocity ~90 km/s and produces optical emissions with a maximum intensity ~400 kR and a duration ~153 ms. The comparison between theory and observation yields good agreement for such important blue jet characteristics as maximum intensity of optical emissions, color, front velocity, duration, maximum radius and vertical dimensions and supports the viability of runaway air breakdown as a driving mechanism for this particular type of high altitude discharge. ¿ 1998 American Geophysical Union