Fog Progression

Presented by: Meshari Al-Hasan

Supervised by: Prof. M. Fahim

Agenda

Introduction

How it will form ?

Types of Fogs.

Mass & Heat Transfer in Fog.

Summary

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.

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.

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.

Main Types of Fogs

There are three main types of fogs:

1.Advection fog2.Radiation fog3.Up-Slope fog

There are other types of fog such as steam fog, Ice fog, precipitation fog .... etc.

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.

1. Advection Fog

1. Advection Fog

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.

2. Radiation Fog

2. Example of Radiation Fog

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.

3. Up-Slope Fog

3. Up-Slope Fog

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.

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).

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.

Mass & Heat Transfer in Fog

The equations used for material and energy balance are:

[In absence of Fog]

Mass balance:

Energy balance: 2dz

T2dk

dt

dT pC

~ c

2dz

1c2dD

dz1dc

Mass & Heat Transfer in Fog

[In presence of Fog]

Mass balance: - r2

Energy balance: , r2 =

Where r2 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.

2

2

dz1cd

D dz

1dc

2

2

rvapH~

Δ2dz

Tdk

dt

dTpC

~c

vapH~

Δ

dt2dc

Mass & Heat Transfer in Fog

General Mass balance equation

Acc. = Mass – Mass out + produced

+ r2 A Δz

dividing by AΔz and let Δz go to zero, equation become:

- r2

) z@z j @z (jA )c z(A dt

d111

2

2

dz1cd

D dz

1dc

Mass & Heat Transfer in Fog

The boundary conditions between foggy region and no fog region, zsat, are governed by these equations:

Calculated temperature and concentration profiles for are shown in the next slide.

dtsatdz

)pC

~c

vapΗ~

Δ(2c )sat@z region, (Fog

dz1dT

D- )sat@z region, Fog (no dz

dTD

dt

dzsat)sat@z2(c )sat@z region, (Fog dz

1dcD- )sat@z region, Fog (no

dz1dc

D

Mass & Heat Transfer in Fog

Temperature and Concentration profiles for different kind of fogs are shown below:

Cooling Fog Warming Fog

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), = 0.5Dt

z

4

Mass & Heat Transfer in Fog

D= 0.23 , (From table 5.1-1) (Air-Water mixture)

Z = 0.5

= 0.5

= 12 cm

s

cm2

Dt4

sec)600sec)(/23.0(4 2cm

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.

Psychrometric Chart

Mass & Heat Transfer in Fog

Example 2: At 30 oC 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 oC. When RH = 40%, dew point = 15 oC. When RH = 60%, dew point = 21 oC.

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.

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

The End

Thanks