During the autumn, winter and early spring the upper jet stream (a core of fast running wind of up to 200 mph) in the upper atmosphere often influences are weather.

Beneath the meandering jet stream low pressure systems develop following that core of strong winds, which enables the meteorologist to predict with accuracy the track of these storms.

In summer the usual trend is for the jet stream to be positioned to the north between north-west Scotland and Iceland and weaker, allowing the Azores high to build northward bring the British Isles spells of fine warm summer weather. The summer of 2007 and 2008 was influenced by low pressure bringing us poor conditions, due to the fact that the jet stream was positioned in a winter pattern bringing us autumnal type weather with flooding from heavy rains. The greater the contrast of temperature along the polar front determines how active a system will be, if the temperature range is large a fully blown deep depression will form bringing us strong winds and rain.

These low pressure systems are made up of two weather fronts, the warm front brings a change from cooler or colder air to milder humid air, the cold front brings a narrower zone of heavy rain and squally winds through with a return to cooler or colder conditions. The wedge of air between the warm and cold front is called the warm sector. Sometimes along the cold front especially in winter a spell of torrential rain with hail, thunder and at times snow occurs. These Atlantic storms often move from west to east but on occasions can head up from the south or from the east.

Next Week: Fog

The answer to last weeks question: South-east.

Glossary:

Front: The boundary between two masses of air of different temperatures and humidities. As they move, fronts are named by the relative temperature of the air behind them. A front followed by warm air is called a warm front, and one followed by cool air is a cold front.

Weather for coming week Monday 13th October-Friday 17th October.

Pressure falling through the period Monday-Wednesday then a recovery in pressure during Thursday and Friday.

Monday brings cloud, some sunshine and just the odd shower, most places dry.

Tuesday cloud, light showers, rain through eventide and night.

Wednesday rain then sunshine and blustery showers, windy.

Thursday and Friday dry bright spells with sunshine, cooler gusty westerly wind flow.

Maximum temperature 14-16c 57-61f.

Minimum temperature 6-9c 43-48f.

Last Week's Observations (Monday-Sunday)

Highest day temperature............21.2c on Sunday 12th October.

Lowest night temperature............2.6c on Wednesday 8th October.

Wettest day................................3.1 mm on Tuesday 7th October.