The life cycle of depressions
Figure 13: The four stages of a mid-latitude cyclone after Nieman and Shapiro, 1993; Nieman et al., 1993; Shapiro and Keyser, 1990).
Once a frontal wave has been initiated, and depending on other characteristics of the atmosphere (e.g. such as the jetstream), pressure may start to fall at the apex of the wave, where the “triangle” of warm air protrudes pole-wards into the cold air. If air starts to be removed from above the weather system (sometimes by the jetstream, or also by air flowing out from the top of the warm front) or equally if lots of warm, moist and light air becomes incorporated into the centre of the depression, air pressure will then continue to fall at the surface. This will result in an increased pressure gradient and strong winds, as well as cloud and precipitation due to the fronts themselves.
There are four recognised stages in the evolution of the low pressure system; these are depicted in Figure 13 (after Nieman and Shapiro, 1993; Nieman et al., 1993; Shapiro and Keyser, 1990). These four stages correspond to the:
- Initial frontal wave.
- A developing cyclone.
- Mature cyclone and the beginning of occlusion of the frontal system.
- Cyclone mostly occluded and dissipation begins.