The probability of occurrence of ice crystal number densities in young cirrus clouds is examined based on airborne measurements. The observations have been carried out at midlatitudes in both hemispheres at equivalent latitudes (52-55°N/S) during the same season (local autumn in 2000). The in situ measurements considered in the present study include temperatures, vertical velocities, and total ice crystal concentrations, the latter determined with high precision and accuracy using a counterflow virtual impactor. Most young cirrus clouds typically contain high number densities (1-10 cm^-3) of small (diameter &lt;20 <font face="Symbol">m</font>m) ice crystals. This mode dominates the probability distributions and is shown to be caused by rapid cooling rates associated with updraft speeds in the range 10-100 cm s^-1. A second mode containing larger crystals extends from ~1 cm^-3 to low concentrations close to the detection threshold (~3 x 10^-4 cm^-3) and could be associated with lower updraft speeds. Results of a statistical analysis provide compelling evidence that the dynamical variability of vertical air motions on the mesoscale is the key factor determining the observed probability distributions of pristine ice crystal concentrations in cirrus. Other factors considered are changes of temperature as well as size, number, and ice nucleation thresholds of the freezing aerosol particles. The variability in vertical velocities is caused by atmospheric gravity waves leading to small-scale temperature fluctuations. Inasmuch as gravity waves are widespread, mesoscale variability in vertical velocities can be viewed as a universal feature of young cirrus clouds. Large-scale models that do not account for this subgrid-scale variability yield erroneous predictions of the variability of basic cirrus cloud properties. Climate change may bring about changes in the global distribution of updraft speeds, mean air temperatures, and aerosol properties. As shown in this work, these changes could significantly modify the probability distribution of cirrus ice crystal concentrations. This study emphasizes the key role of vertical velocities and mesoscale variability in vertical velocities in controlling cirrus properties. The results suggest that, in any effort to ascribe cause to trends of cirrus cloud properties, a careful evaluation of dynamical changes in cloud formation should be done before conclusions regarding the role of other anthropogenic factors, such as changes in aerosol composition, are made.