Over the weekend, reports were made of an impressive cloud formation over the ACT known as mammatus clouds. These clouds resemble moisture laden bulges which look like they are sagging beneath the cloud base of surrounding clouds. Firstly, we need to know how the mechanics on how a cloud usually forms. A parcel of air contains moisture in the form of gas which is invisible also known as water vapour. If this air parcel rises for any reason, whether it is via convection or moving over elevated ground, it will become cooler, less dense due to a decrease in air pressure, and as a result the air will condense into very small droplets forming cloud. In a typical thunderstorm cloud with good structure, there will be a defined cloud base (known as the lifted condensation level). This is the point at which the air is cool enough for the moisture to form cloud. Imagine a parcel of air keeps on rising due to convection. When it is very high in the atmosphere, these droplets will grow larger and potentially even freeze into crystals. Once the parcel of air reaches the highest part of the cloud, the upward momentum weakens and the parcel of air containing large droplets and ice crystals begins to sink. This sinking often occurs just ahead of the centre thunderstorm where the downward movement occurs in the convection cycle. Most other droplets would evaporate at the cloud base as the air gets warmer, however these large droplets and ice crystals take more energy (warmth) to evaporate. As a result, they sink lower than the cloud base and form nodules known as mammatus clouds (derived from the term mammary due to their feminine features). These cloud formations are most likely throughout the summer/early autumn months as low pressure troughs sweep over southeastern Australia producing thunderstorms. Contrary to popular belief, mammatus clouds do not deliver severe weather, but instead it is the strong convective thunderstorm following which produces the most extreme conditions. On the odd occasion, these cloud formations will develop without severe thunderstorms. The theory remains the same however, while afternoon sunlight can make these structures look even more dramatic.