Widespread ascent frequently occurs when one air mass slides up and over another. This may occur with the slow convergence of air from a region of high pressure to a region of low pressure.
Widespread ascent also occurs in frontal zones and within troughs of low pressure. The Figure below is an example of moist airlifted at a frontal zone and cooled to saturation. The cold air acts like a wedge. If the saturated air is buoyant, Cumulus clouds form. If buoyancy can be maintained through a considerable depth of the troposphere then Cumulonimbus clouds may form. Cumulonimbus clouds are responsible for thunderstorm activity.