Fog and Dew in Psychrometry

Fog and dew formation both occur above the saturation region. Fog can be considered a type of low-lying cloud and is heavily influenced by nearby bodies of water, topography, and wind conditions.

When hot humid air is mixed with cold air the result may be fog. When very cold and dry air mixes with warm air at high relative humidity, the resulting mixture condition may lie in the two-phase region above the saturation region, as a result, there will be condensation of water vapor and some amount of water will leave the system as liquid water. A two-phase region is a region above saturation region where the saturated moist air and water are in equilibrium.

The second important point is that in the case of fog, the convective radiative heat transfer from the ground level constantly pushes in a random manner the water molecules upward preventing the particles from coalescing into water. The convective heat transfer raises the vapor pressure of moisture by increasing the kinetic energy that prevents gaseous water vapor from combining into liquid water.

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Let us start from the basics. Fog and dew formation both are non-equilibrium processes. A fog is visible because the fog is a dispersion of tiny water particles in the air with a different refractive index than air.

Question: How do moisture particles remain suspended in fog?

Explanation

Dispersion of moisture vapor in water – fog: Stokes law

The tiny droplets of water indeed tend to fall under the effect of gravity. However, as with any other particle falling in the atmosphere, they accelerate downwards until they reach a terminal velocity. For a spherical particle of mass?m and radius?R, falling in a fluid of viscosity?η, the terminal velocity, neglecting buoyancy, is given by, Stokes law, Vt = m g/ 6∏ η R. Terminal velocity is defined as the highest velocity attained by an object falling through a fluid.

For an aerosol droplet of radius?≈10^?6m, Vt,?this velocity is very small, typically?10^?3m/s. At the same time, the droplets are being constantly pushed upwards by convection currents, since the hot air rises. This explains why fog remains dispersed in the air. A very important point yet difficult to conceive is fundamentally, that the ground level has much smaller specific heat than upper layers. Therefore, the ground level is always hotter than the upper layers. And, therefore, the radiative heat transfer takes place by convection from the ground to the upper layers. The same thing happens with fog, fog is constantly being pushed in a random manner preventing the particles from coalescing into water. The convective heat transfer raises the vapor pressure of moisture by increasing the kinetic energy that prevents gaseous water vapor from combining into liquid water.

Why fog is visible and dew not?

I could not get the refractive index data of moisture vs air, but I got the density (mass/volume) of water vapor is 0.804g/liter, which is significantly less than that of dry air at 1.27g/liter at STP. Therefore, the air is significantly denser than water vapor under identical conditions. Therefore, the speed of light is different in these two mediums and hence the fog appears cloudy.

Fundamentally how fog is different from dew?

Fundamentally, both are non-equilibrium processes.? An equilibrium process in psychrometry ends at saturation. At saturation rate of condensation = rate of evaporation.

Let us go further inside

Fog and dew formation both occur above the saturation region. Both convert to two phases water particles and air. In fog, the tiny water particles remain dispersed because of higher kinetic energy and convective radiative heat transfer in air while in the case of dew, water particles coalesce into bigger particles due to cooling and loss of kinetic energy into bigger drops.

Phase rule

In thermodynamics, the phase rule is a general principle governing "PVT" systems, whose thermodynamic states are completely described by the variables pressure (P), volume (V), and temperature (T), in thermodynamic equilibrium. If F is the number of degrees of freedom, C is the number of components and P is the number of phases, then

F = C – P + 2

Let us look at this case

At saturation [Both fog and dew formation occur above saturation] ?

C = 2 [components], P=1

F [ degrees of freedom] = C – P +2 = 3

There are three intensive variables which can independently fix saturation [1] pressure [2] temperature and [3] specific volume

At saturation, the temperature is constant because both air and water are in thermal equilibrium. The air which consists of the highest moisture amount is called saturated air.

The?saturation vapor pressure?is the pressure at which? Water vapor is? In thermodynamic equilibrium with the condensed state At pressures higher than vapor pressure,? water would condense, while at lower pressures it would evaporate.

At saturation air and moisture have a fixed density ratio. Air is said to have reached saturation when the wet air attains a certain density. The humidity ratio is a pointer.

Dew point

When wet air is cooled below the saturation point at constant pressure, the dew point occurs. Water separates.

At dew point C [component] = 2 P [Phase] = 2

F [degrees of freedom] F = C -P + 2

F = 2

Pressure and temperature, these two intensive variables that can independently fix the dew point.

?Pressure

The higher the pressure, the less moisture the air is able to hold, resulting in a higher dew point temperature.

Temperature

The dew point is the cold air temperature at which moisture condenses out from the wet air as air cannot hold the moisture. It is an exothermic process. Gaseous moisture converts to liquid water by a compression process which brings moisture molecules closer by generating the latent heat of condensation. The temperature is indirectly linked to the dew point through relative humidity.

How and why does fog form?

Fog is a visible mass consisting of cloud water droplets suspended in the air. In the atmosphere, fog can be considered a type of low-lying cloud and is heavily influenced by nearby bodies of water, topography, and wind conditions.

When hot humid air is mixed with cold air the result may be fog. When very cold and dry air mixes with warm air at high relative humidity, the resulting mixture condition may lie in the two-phase region above the saturation region, as a result, there will be condensation of water vapor and some amount of water will leave the system as liquid water.

Due to this, the humidity ratio of the resulting mixture (point 3) will be less than that at point 4 [on the enthalpy line]. Corresponding to this will be an increase in the temperature of air due to the release of latent heat of condensation.

Generally, fog happens in winter when the cold air near the earth mixes with the humid and warm air, which develops towards the evening or after the rains.

The amount of condensate water can be found by following the constant enthalpy line from B to the saturation line. The condensate water is the difference between the specific humidity in point B and in the point where the enthalpy line crosses the saturation line. The mixed temperature is where the enthalpy line crosses the saturation line. As long as the mixed air is above the saturation line no fog will be formed in the mix.