Cloud Head

Böttger et al. (1975) suggest that rapid cyclogenesis culminating in storms of hurricane intensity occur 3–5 times per cold season over the eastern Atlantic. Their statistical investigation of years 1968–1973 revealed that all cyclone events producing winds of hurricane force in this period were preceded by features they called cloud heads, and that no heads occurred without being followed by major surface pressure falls.

The cloud head is a cold-topped cloud structure extending polewards of, and separated from, the main frontal band but elongated along a similar orientation, with an anticyclonically curved poleward edge. Fig. 1 shows a cloud head (H) which preceded the great storm of 1987. The low pressure area was associated with a wave along the polar front, but the rapidly deepening phase had yet to occur and the extreme winds were nearly 24 hours away. One might be tempted to associate the cloud head with a frontal feature, such as an occlusion, but in fact, as is usually found with this type of image, the development along the front has proceeded no further than the wave stage. The cloud head is indicative of rapid ascent, leading to the creation of low-level vorticity through vortex stretching and release of latent heat.