1. Condensation

2. Atmospheric moisture has its most direct influence on land only when it is in its condensed form. Condensation is the direct cause of precipitation. It is the reverse of evaporation. Water vapour is changed from the vapour state and becomes droplets of water.

3. Condensation - conditions The necessary condition: cooling of air to below its dew point until it is saturated [individual / combined changes in air volume, pressure, temperature / R.H.] The sufficient condition: presence of condensation nuclei [hygroscopic particles – wettable substances]

4. Condensation - conditions The necessary condition: cooling of air to below its dew point until it is saturated - radiation cooling - advective cooling - orographic and frontal uplifting & cooling - convective or adiabatic cooling [refer to note p.17 ]

5. Condensation - conditions The sufficient condition: presence of condensation nuclei - Condensation does not take place easily in clear or pure air which can be cooled below its dew point without condensation occurring. - So, the air is said to be supersaturated. - It continues to hold water in vapour form after dew point has been passed. - Its R.H. is over 100%.

6. Condensation - conditions The sufficient condition: presence of condensation nuclei [hygroscopic particles – wettable substances] They attract water-vapour molecules when the moisture content is near saturation point. e.g. dust, sea salt from evaporated spray, sulphur acid from combustion, volcanism, in numerous supply esp. in urban areas

7. Condensation - forms a. Dew and Frost Dew consists of relatively large water droplets which collects or deposits on cold or cool ground surfaces at night under clam conditions when the lower layer of the atmosphere is saturated due to cooling to dew point at night. Frost consists of ice crystals deposited on a cold surface. The formation of frost is the same of dew. The different is that the dew point of frost is below 0 o C, and water vapour transform to ice crystals directly through sublimation process. It is formed at night under calm conditions when the temperature is below freezing point.

8. Condensation - forms b. Mist and Fog Mist consists of very fine, condensed water droplets. These are held in suspension in the air because of their smaller size. A mist often forms at very low level of ground level. The droplets form a thin, fine veil which hinders the visibility. The visibility is obscure, but still exceeds 1000 m.

9. Condensation - forms b. Mist and Fog Fog contain small condensed water droplets which are larger than those in a mist. Thus, it forms a thick veil which causes poor visibility below 1000 m. Fog is usually formed by advection of warm, moist air and by intense radiation at night, namely advection fog and radiation fog respectively.

10. Condensation - forms b.i. Advection Fog when warm, moist air passes over a cooler or cold land or sea surface horizontally; where cold and warm ocean currents meet each other; when warm moist air from the ocean merges with cool dry air from the land.

11. Condensation - forms b.i. Advection Fog The lower layer of warm air is cooled below dew point by contact with cooler air or surface, and condensation results in the formation of advection fog. Condensation is assisted by the presence of hygroscopic particles which act as nuclei for condensation. Advection fog is common in spring in Hong Kong. It often disperses during the day when the sun appears and temperature rises. Why?

12. Condensation - forms b.ii. Radiation Fog A radiation fog is formed by condensation when moist air comes into contact with the cool ground and becomes cooled due to the radiation of heat from the ground. It occurs in cold weather when the sky is clear (which permits maximum radiation) and there is calm, stable condition. Hygroscopic particles in the air act as nuclei for the moisture to condense on. Radiation fog is common in winter and in industrial regions.

13. Condensation - forms c. Clouds Clouds consist of extremely tiny droplets of water (0.02 to 0.06 mm in diameter), or minute crystals of ice. Generally, the formation of cloud is the same of the formation of fog.

14. Condensation - forms c. Clouds In order for cloud droplets to form, it is necessary that microscopic dust particles serve as condensation nuclei. There should be a fall of temperature down to dew-point. - Where the air temperature is well below freezing, clouds may form of tiny ice crystals. However, water in such minute quantities can remain liquid far below normal freezing temperatures, the liquid is said to be supercooled. Water droplets may exist at temperatures down to -12 o C to -30 o C; ice crystals below -30 o C.

15. Condensation - forms c. Clouds They can be classified on the basis of two characteristics: - form: stratiform (blanket like, covering vast areas) by altitude: 1. High clouds (6000-12000m, with ice crystals) e.g. Cirrus / Cirrostratus / Cirrocumulus 2. Middle clouds (2000-6000m) e.g. Altostratus / Altocumulus 3. Low clouds (ground level-2000m) e.g. Stratus or Nimbostratus / Stratocumulus

16. Condensation - forms c. Clouds - form: stratiform (height > / = horizontal dimensions) 1. Cumulus 2. Cumulonimbus (extending from a height of 300-600m at the base up to 9000-12000m)

17. Condensation - forms c. Clouds