1. Fog, mist and temperature inversions TimeTemp 18.00 In the early evening, the hill tops cool more rapidly than the valleys (being high and exposed). The cool air is dense and flows down to the valley bottom.

2. Fog, mist and temperature inversions TimeTemp 20.00 These cold “katabatic” winds can collect in depressions or behind obstacles - eg railway or motorway embankments - creating pools of cold air. These may become “frost hollows” where frosts occur frequently.

3. Fog, mist and temperature inversions TimeTemp 24.00 Cold Air By night, cold air pools are collecting in the valley, continually replenished with fresh cold air from the hill tops.

4. Fog, mist and temperature inversions TimeTemp 04.00 Cold Air As the air continues to cool it gets closer to its “dew point” temperature at which excess water vapour begins to condense out as tiny droplets.

5. Fog, mist and temperature inversions TimeTemp 06.00 Low lying mist / fog Just before sunrise the temperatures reach their lowest point. The air in the valley has reached dew point and a blanket of mist (or fog if it is denser) covers the lower ground.. Dew point

6. Fog, mist and temperature inversions TimeTemp 06.00 Low lying mist / fog Many valleys contain water sources (rivers etc) which add to the moisture content of the air, encouraging fog. Settlements are also located in valleys so urban pollutants often form “condensation nucleii” on which water vapour can condense.

7. Fog, mist and temperature inversions TimeTemp 08.00 Low lying mist / fog Over the middle of the valley (above the cold air) the temperatures are higher than at ground level. This is different to the normal situation where temperatures drop as you go upwards. This is called a temperature inversion and can trap pollutants 5 C 8 C

8. Fog, mist and temperature inversions Low lying mist / fog The warmer layer of air acts as a lid which stops the rising pollutants from chimneys etc escaping. 5 C 8 C Time 09.00

9. Fog, mist and temperature inversions Low lying mist / fog 5 C 10 C Time 10.00 The fog in the valley stops the valley warming but the hill tops are rapidly warming. This can keep the temperature inversion lasting for hours until the sun is strong enough to evaporate the fog and allow the valley floor to warm up. Pollution levels can become very high.

10. Fog, mist and temperature inversions Once the fog clears the valley floor heats up more rapidly than the higher hillsides. The normal temperature arrangement returns with cooler air at height. Now the pollutants can finally disperse, finding themselves more buoyant as they rise into the colder, denser upper air. 15 C 12 C Time 13.00

11. Fog, mist and temperature inversions This is the last slide.