Clouds & Particles More 1. What happens in clouds? Solution Worksheet 3 1. Convection currents in the pool of melted wax in a candle appear because the wax is hottest closest to the wick. The wax rises to the surface at the wick, travels across the surface to the edge of the candle, cooling down as it goes and then sinks to the bottom of the pool of wax at the edge. This circulation forms "convection cells". 2. A lift typically ascends at about 2 m s-1. Gliders and aircraft have to be very careful of cumulonimbus clouds because of their strong vertical air currents. For this reason, pilots are given particular warnings from the local airport weather service when there are cumulonimbus clouds around.