The paper analyses numerically the scenarios of ignition kernels formation in the shock tube in the intermediate temperature range. Three modes of mild ignition are distinguished among which are: ignition related to the shear heating in the developed boundary layer, ignition due to the shear heating in the re-circulation zone behind the reflected shock and ignition in the central region of the tube due to the axial compression. All three modes define short ignition delays at low temperatures compared to the ideal shock-tube values. Moreover, it is shown that the gas-dynamic mechanisms responsible for additional local heating of the test mixture provide close rates of heating. As a result, all the data on ignition delays fall in a certain range defined by the mixture composition and pressure, independent on the particular mode of mild ignition. (C) 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.