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The commonly used time scales in entrainment-mixing studies are examined to seek the most appropriate one, based on aircraft observations of cumulus clouds from the RACORO campaign and numerical simulations with the Explicit Mixing Parcel Model. The time scales include the following: (evap), the time for droplet complete evaporation; (phase), the time for saturation ratio deficit (S) to reach 1/e of its initial value; (satu), the time for S to reach -0.5%; and (react), the time for complete droplet evaporation or S to reach -0.5%. It is found that the proper time scale to use depends on the specific objectives of entrainment-mixing studies. First, if the focus is on the variations of liquid water content (LWC) and S, then (react) for saturation, (satu) and (phase) are almost equivalently appropriate, because they all represent the rate of dry air reaching saturation or of LWC decrease. Second, if one focuses on the variations of droplet size and number concentration, (react) for complete evaporation and (evap) are proper because they characterize how fast droplets evaporate and whether number concentration decreases. Moreover, (react) for complete evaporation and (evap) are always positively correlated with homogeneous mixing degree (); thus, the two time scales, especially (evap), are recommended for developing parameterizations. However, and the other time scales can be negatively, positively, or not correlated, depending on the dominant factors of the entrained air (i.e., relative humidity or aerosols). Third, all time scales are proportional to each other under certain microphysical and thermodynamic conditions.