The use of hydrogen as an energy vector calls for an in-depth analysis of the associated risks and the development of mitigation strategies. In the case of liquid hydrogen storage, there are virtually no norms for uses concerning the general public and there is little background on safety measures for an industrial use –with the notable exception of rocket propulsion, which is a very specific domain. The present works addresses the accidental release of liquid hydrogen, resulting in a pool that would create a flammable cloud because of its fast evaporation. This cloud is characterized by strong temperature and composition gradients that are likely to strongly affect the propagation of a flame kernel, in the event of an accidental ignition. The present work is dedicated to the prediction of the initial acceleration of the flame front in a canonical case: a tube with an axial stratification typical of a liquid-hydrogen spill. Numerical simulations and analytical developments are performed and compared in order to unravel the driving mechanisms and identify the risks of various scenarios. It is shown that ignitions in lean regions may be particularly hazardous because of flame-front instabilities as well as composition gradients favoring acceleration.