1. Fog Progression Presented by: Meshari Al-Hasan Supervised by: Prof. M. Fahim

2. Agenda  Introduction  How it will form ?  Types of Fogs.  Mass & Heat Transfer in Fog.  Summary

3. Introduction  Fog is defined as an obscurity in the surface layers of the atmosphere which is caused by a suspension of water droplets.  Fog is reducing the visibility to less than 1 km, If the visibility exceeds 1 km but less than 2 km then the obscurity is called mist.

4. Introduction  Fog is a visibility hazard, contributes to accidents related to all kind of transportations.  Fog which contain smoke particles is called smog, and considered as air pollutant, causing eye, skin irritation, shorten the breathing, pain in inhaling... etc.

5. How it will form ?  Fog usually formed when the difference between temperature and dew point is 5 °F or less.  take place when air temperature falls and the dew point is reached and condensation of suspended water vapour occurs.  It’s formed due to the condensation of tiny liquid droplets in air, usually occurred when relative humidity is 100%, or when condensation nuclei (water coalescer) is suspended in air, such as dust, smoke or salt.

6. Main Types of Fogs  There are three main types of fogs: 1. Advection fog 2. Radiation fog 3. Up-Slope fog  There are other types of fog such as steam fog, Ice fog, precipitation fog.... etc.

7. 1. Advection Fog  Advection, or wind fog, Forms when quite warm, moist and stable air is blown across a cooler surface (land or water).  Usually saw at sea or near tropical areas.

8. 1. Advection Fog

10. 2. Radiation Fog  This type of fog is a thin layer that forms because land cools down overnight. When the dew point is reached and condensation occurs, the fog moves slowly upwards.  Radiation fog is common at lakes, harbours and in river valleys.

11. 2. Radiation Fog

12. 2. Example of Radiation Fog

13. 3. Up-Slope Fog  forms when winds blow air up a slope.  This flow upwards causes the warm air to cool as it rises, which causes the moisture in it to condense and form fog.

14. 3. Up-Slope Fog

16. Mass & Heat Transfer in Fog  Fog formation process involved a simultaneous heat and mass transfer.  The mass transfer is due to convection, while heat transfer is due to both conduction and convection.  Concentration and temperature are varies with Time and direction.

17. Mass & Heat Transfer in Fog  Scientifically, Fog is classified based on the way of formation: 1. By cooling (warm air cooled from ground below). 2. By warming ( cool air warmed from above).

18. Mass & Heat Transfer in Fog  To calculate amount of fog vs. time & position, these assumptions are made: 1. System contain abundant nuclei, so all any supersaturation produces fog formation. 2. System is always dilute. 3. Has a single average heat capacity.

19. Mass & Heat Transfer in Fog  The equations used for material and energy balance are:  [In absence of Fog] Mass balance: Energy balance:

20. Mass & Heat Transfer in Fog [In presence of Fog] Mass balance: - r 2 Energy balance:, r 2 = Where r 2 is the rate of fog formation, and is the molar heat of evaporization. Derivation of mass balance equation during fog presence is shown in the next slide.

21. Mass & Heat Transfer in Fog  General Mass balance equation Acc. = Mass – Mass out + produced + r 2 A Δz dividing by A Δz and let Δz go to zero, equation become: - r 2

22. Mass & Heat Transfer in Fog  The boundary conditions between foggy region and no fog region, z sat, are governed by these equations:  Calculated temperature and concentration profiles for are shown in the next slide.

23. Mass & Heat Transfer in Fog  Temperature and Concentration profiles for different kind of fogs are shown below: Cooling Fog Warming Fog

24. Mass & Heat Transfer in Fog  Example1: Air at 50° C is saturated and placed over water at 20° C. How deep will the layer of fog be after 10 min ?  Solution From figure (cooling fog), = 0.5

25. Mass & Heat Transfer in Fog D= 0.23, (From table 5.1-1) (Air-Water mixture) Z = 0.5 = 0.5 = 12 cm

26. Mass & Heat Transfer in Fog  Relative Humidity (RH) of Air contributes to the fog formation.  RH is related proportionally to the dew point temperature, at the same temperature.  This is can be illustrated from the psychrometric chart, as shown in example 2.

27. Psychrometric Chart

28. Mass & Heat Transfer in Fog Example 2: At 30 o C dry bulb temperature, find the dew point temperature at 20, 40, 60% RH. At the given temperature, from the psychrometric chart:  When RH = 20%, dew point = 5 o C.  When RH = 40%, dew point = 15 o C.  When RH = 60%, dew point = 21 o C.

29. Summary  Fog is visibility hazard, reduces the visibility to less than 1 Km.  When mixed with smoke, it will form (Smog) which is a serious air pollutant.  Classified mainly to three types: advection, radiation and up-slope fog.

30. Summary  Formed due to the condensation of tiny liquid particles in air, when air is cooled till reaching Dew point temperature.  The process of fog formation involved a simultaneous heat and mass transfer.  Relative Humidity of air is related proportionally to the dew point temperature

31. The End Thanks