30-day back trajectories were calculated every minute of flight time within the TCRs. a–d, Pressures from the time of minimum pressure of the trajectory to the flight track are plotted for all back trajectories from ATom 1 Pacific (a) and Atlantic (b) and ATom 2 Pacific (c) and Atlantic (d) observations. Colours distinguish separate trajectories. The general slope of increasing pressure with time indicates a general descending motion of the air. e, Histogram showing instantaneous descent rates (one point every 3 h) for all trajectories within the indicated pressure bins. The skew at all altitudes towards positive descent rates is evidence of an overall descending motion of the air. The mean descent rate is higher at higher altitudes, and almost 0 at the lowest altitudes, which is to be expected as this is often within the marine boundary layer where the air cannot descend further. f, Average fraction of time that trajectories spent in cloud between the time of minimum pressure and the flight track. In-cloud time is taken to be times when relative humidities are 90% or more (an overestimate). It is binned by the pressure on the flight track (not the pressure of the trajectory itself, as in e). For measurements made at pressures of less than 850 hPa, the air spent less than 5% of its time in cloud on average. For air at pressures of more than 850 hPa, this time increased to around 14%. This shows that most of the particles descend with the air instead of being removed by clouds. g, h, Histograms of the latitudes of trajectories between the minimum pressure and flight track for ATom 1 (g) and ATom 2 (h), coloured by pressure of the point on the flight track. Except for at the lowest altitudes, air parcels entering the flight track mostly remain within the tropics (with histograms peaking around the equator). Peaks shift towards the summer hemisphere with the season, in the same manner as the TCRsSource Data.