H C Neil

Glossary

This glossary is not a complete glossary of meteorological terms. It is intended to explain certain terms that I use in my forecasts and will be added to as I need to use new terms.

Col: This is the boundary area between high and low pressure systems. When under a col, apart from meaning that a small change in the positions of the high and low can bring about quite large changes in our actual weather, being under this boundary itself means quiet conditions with no extremes; light winds, no rain, slight cloud.

Convective rain There are three types of rain, frontal rain caused by the passage of a front (warm, cold or occluded), orographic rain (see below) and convective rain. Convective rain is caused by rising warm air (convection) and is related to the temperature at a particular spot. Therefore, depending on wind, there is not much movement sideways; the warm air goes up at one location and the rain comes down on the same spot. This can, therefore, be highly localised resulting in some areas receiving very heavy rain while areas close by remain dry. It should be noted that thunderstorms are created by convection.

Jet stream: From time to time I may mention the jet stream. Here is a brief explanation as to how the jet stream affects the weather (with many thanks to my friend Will Hand, who works for the Met Office, and who has helped me with this explanation. His weather web site is here.) It is the jet stream (a very strong current of air at high altitude) that causes the high and low pressures to form, high pressure to the south and low to the north of it. Therefore, when the jet stream is north of the UK we will get the fine weather associated with high pressure and when it is to the south, we will get low pressure and more stormy weather.

Orographic rain: rain caused by warm moist air being forced upwards, and therefore cooling to cause precipitation, by the Black Mountain rather than by the more normal interaction between warm and cold air). This usually brings very fine but densely packed droplets in the rain.

Sting jet: This is the result of an interaction between air masses which results in the jet stream being diverted to ground/sea level where it continues on a narrow path. When the interaction takes place, in our case normally over the Atlantic, a cloud known as a baraclinic leaf forms. It is called this as it is shaped like a leaf and sometimes even appears to have veins like a leaf. The tip of the leaf turns anti-clockwise until roughly facing east (towards us) and then points in the direction that the sting jet will travel. The path of the sting jet is typically only a few miles wide. A sting jet is not a very common occurrence and, due to the narrow path, it is very rare at any one location. One passed over Gwynfe just four or five years ago.

Spanish Plume: This refers to a major thunderstorm event that starts when a plume of hot air from Spain suddenly moves north into France. The thunderstorms move north across France but sometimes stop when they reach the English Channel and turn sharp right into Holland. When the thunderstorms cross the channel into England, they can, and often do, change course when they reach the land. For us here in Wales, this means that we may have a day or two's notice as the thunderstorms attack France but we may only have a few hours' notice should they turn left towards us as they reach England.