The passage of two cold front systems over the United Kingdom are compared and contrasted, using the results of a detailed aircraft and ground-based study. The measurements are interpreted by means of three-dimensional, reverse-domain-filling trajectories using both global models and limited-area mesoscale models. This method provides a three-dimensional picture of the interleaving air-masses in the frontal zone as defined by their Lagrangian histories. The two systems studied differ in that the first is associated with an intense surface low in January and the second is associated with a relatively weak surface low in April. In the intense surface low case the trajectory study suggests that a dry intrusion with stratospheric characteristics penetrated deep into the troposphere along the upper level front. Measurements indeed revealed an unsaturated layer with anomalously high ozone. This layer was intersected at four levels in the troposphere (at 8.5, 7.1, 5.2 and 3.7 km), and the lower the intersection, the lower the measured anomalous ozone and the higher the water vapor content. It is argued that this is best explained by the dry-intrusion layer becoming mixed with background air by three-dimensional turbulence, also encountered by the aircraft, along the upper level front. Evidence for this mixing is apparent on tracer-tracer scatterplots. In the weak surface low case, by contrast, the dry intrusion has a more complex structure, with up to three separate layers of enhanced ozone and low humidity. Strong evidence for mixing was apparent only in the lowest layer. The weaker system may therefore be much more efficient at transporting upper tropospheric/stratospheric ozone to the lower troposphere. The transport of boundary layer air to the upper troposphere in the warm conveyor belt (WCB), however, was found to be around 8 times stronger in the intense system. Sonde measurements suggested that the WCB was ventilated by convection from the surface front in some regions to about 5--6 km, while it was stably stratified in other regions, suggesting layerwise long-range transport.