Weatherwatch with Irv Lee

Not Quiet on the Western Fronts

The high altitude winds, 'the jetstreams', which propelled all the 'around the world' balloon attempts, are also the driving forces behind the movement of weather systems

The British weather is famous the world over. Fronts, weather systems and changeable conditions are just a fact of life to the regular UK pilot. Before exploring the detail of a low pressure system with its warm and cold front, it's worth asking 'Why us?'.

To the west of the UK is the Atlantic Ocean. To the south west, there's nothing much but water before the warm islands of the Caribbean. To the north west, across cold seas are Greenland and the Canadian Arctic. The air in both these areas takes up the characteristics of the surface below it. To the south west, the air is warm and able to absorb a lot of water vapour from the ocean. To the north west, the air is cooler, and consequently has less moisture in it.

These two characteristics of temperature and humidity define different 'air masses'. The more southerly warm tropical air mass meets the northerly cooler arctic air mass somewhere out to the west of the UK. Unfortunately, instead of agreeably mixing where they meet, air masses with different characteristics tend to do the opposite.

Along the frontier - or 'front' - initially aligned east-west, where the northerly and southerly air masses touch, warm air at the boundary is cooled, forcing the release of its moisture as cloud and rain.

Given any relative movement between the two masses, the warm air, being less dense, will move upwards and over the cold air. Expanding in the lower pressures above, this rising air cools, and, unable to hold as much water, releases further cloud along the front. What can cause this relative movement at the boundary? The jetstreams - the high velocity winds which propelled British balloonist Brian Jones around the world - are a high altitude feature of the frontier between a cold and warm air mass.

The high jetstreams blow eastwards, dragging the warmer air first upwards then along with the stream. The rising warm air displaced from lower levels has another two effects: a low pressure area results as the warm air escapes upwards - the classic Atlantic low. The cold air mass undercuts the escaping air in a pincer movement, bending the air mass boundary from a straight line into the well known inverted 'V' on the weather chart.

Driven by the powerful jet-stream winds above, the whole developing low pressure system moves eastwards, still with its cloud and rain on the frontiers (fronts) which still separate the original air masses. Due to the starting latitude, the frontal system almost inevitably meets part, if not all, of the UK, resulting in famous British changeable climate as cloud, rain, warmer and cooler air passes though in a matter of hours.

Occasionally, a mass descent of dry air over nearby continental Europe forms a 'blocking high pressure'. This stable system is able to deflect the high jetstream (and therefore the path of Atlantic low systems), keeping Britain in a warm southerly airflow for a while, free from the usual attack from the west.

Finally the high pressure over the continent fades, and places the UK back in the path of the rain bearing frontal systems from the Atlantic. In winter, with less warmth, the colder air mass is larger and the Atlantic boundary between the air masses is further south compared to summertime.

This gives appropriate seasonal differences between Scotland and England as the main path of the systems will be correspondingly displaced in latitude.

This article first appeared in FLYER magazine's December 2000 edition

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