Some principles of weather – Bernard Foot

This session looked at common weather phenomena and how they are caused. It looked at how the rising of air at the hot equator and the descent of air at the cold poles resulted in the establishment of North-South circulations of air and East-West bands of low and high pressures around the earth.

These bands result in prevailing winds, which are deflected (to their right in the northern hemisphere) by the Coriolis force. Factors which result in local wind variations were discussed, and in particular frontal depressions which are a major factor for the UK. The characteristics of warm fronts (layered clouds ahead of the front with constant rain just before the front), and cold fronts (heaped clouds behind the front with showers just behind the front) brought about by these depressions were described, and the “Red sky at night…” saying was explained.

The presentation went on to explain the formation of clouds and precipitation: clouds form when air is forced to rise resulting in its cooling through expansion to below its dew point. Rain and hail results from the accretion of water droplets in cloud into rain drops, which can freeze when encountering suitable nuclei (such as dust) or existing hail stones. Snowflakes result from the accretion of ice crystals. There was discussion of the Cumulonimbus, the Genghis Kahn of the cloud world, and the extreme weather that is associated with this cloud type – hurricanes and tornados. The presentation also looked at the formation of radiation fog (i.e. ground-level cloud, typically in early autumn), and some basic concepts were explained – such as the relationship between high pressure and fine weather, and what weather charts can tell us.