During settled anticyclonic spells during autumn, winter and early spring fog will form, especially out within the rural setting. In some lowlands, these conditions can linger all day keeping the mercury in the thermometer low. In winter if the temperature falls below freezing level (0c 32f) freezing fog will result, coating trees, plant life and surfaces in rime. The term fog is used when visibility is less than 1,100 yd.

When relative humidity reaches 100 percent, the air becomes saturated, water vapour begins to condense into droplets. At ground level, mist and fog forms. Every droplet condenses on to a tiny particle of dust, salt, soot, or sulphate. Water vapour will normally condense only on to a surface, the particles. These are called condensation nuclei.

Over land there are usually 5 or 6 million cloud condensation nuclei in every litre of air, and over the oceans there are about 1 million. The larger the particle, the bigger the droplet. Fog can cause many problems on our road and motorway network, accidents often occur, flights delayed.

Next Week....Moisture in the air.

Glossary: Glory: A circle of bright light, often coloured, that is seen around the shadow of a person's head when cast on a bank of fog. It is caused by light passing through very small droplets of water.

Weather for week Monday 20th October-Friday 24th October.

Low pressure on Monday and also Thursday Night into Friday. Higher pressure between.

Rain on Monday and Thursday Night into Friday with drier, brighter weather and sunshine between. Winds between south-west/west and north-west gusty at times. Lighter winds Friday.

Maximum temperature 13-15c 55-59f.

Minimum temperature 2-6c 36-43f with frost in places on some nights.

Last Week's Observations (Monday-Sunday)

Highest day temperature...........18.0c on Monday 13th October.

Lowest night temperature...........2.2c on Thursday 16th October.

Wettest day...............................4.8 mm on Wednesday 15th October.