- Dew forms when water vapor condenses (vapor $\rightarrow$ liquid) to form water droplets on an exterior surface. The exterior surface temperature ($T_{surface}$) must be at least as cool, or cooler, than the parcel's dew point $T_d$ and above freezing. That is, $32 \lt T_{surface} \le T_d$.
- Frost forms when water vapor deposits (vapor $\rightarrow$ solid) to form ice crystals on an exterior surface. The surface temperature must be at least as cool, or cooler, than the parcel's dew point $T_d$ and below freezing. That is, $T_{surface} \le 32 \le T_d$. The parcel's temperature may be above freezing and frost may still form because the exterior surface cools below freezing due to heat loss by long wave radiation.
- Self Nucleation refers to the process in which a water droplet (a cloud droplet) forms independently. No other materials or processes are involved; just water vapor mass and the process of condensation.
-
Suppose a small embryonic water droplet forms in the air parcel. The change in energy $\Delta E$ associated with the small droplet is given by
$\Delta E = \sigma 4\pi R^2 - \frac{4}{3}\pi R^3 n k T \ln \frac{e}{e_s}$
where the first term represents the energy required to create the droplet of radius $R$ (it is proportional to the droplets surface area), and the second is the Gibbs free energy of the system due to condensation, and is proportional to the volume of the droplet. - For vapor pressure $e$ less than $e_s$, the log term ($\ln \frac{e}{e_s}$) is negative, so the change in energy in forming the droplet is positive.
- A system (the parcel, in this case) in equilibrium is one that, generally, reduces its energy ($\Delta E \lt 0$) in order to achieve equilibrium
- Consequently, a subsaturated environment is not conducive droplet formation by way of self-nucleation. In fact, the environment must be supersaturated ($e$ > $e_s$) for $\Delta E$ to possibly be negative, and this is true only for relatively large $R$ (the droplet radius).
- Sizes and Parcel Concentration
-
Condensation Effects
- Certain condensation nuclei are hygroscopic, they tend to absorb water vapor from the atmosphere.
- Some condensation nuclei dissolve in water to form and aqueous solution and create droplets that can survive even in unsaturated environments. The solution results in a situation where the actual unsaturated environment of the air parcel becomes one that is, effectively, supersaturated to the forming droplet.
- Research by Duseck (2006) found that condensation nuclei size was more important for condensation efficiency than the chemical makeup of the nuclei.
-
Sources
- natural : dust, volcano exhaust, forest fire, ocean salt, sulfate particles emitted by phytoplankton
- anthropogenic : factory smoke, car exhaust, slash and burn forest clearing -
Concentrations in Terms of Location
- greater over land (102-103 per cm3) than over water (10-102 per cm3)
- greater nearer the Earth's surface
- greater nearer industrial and population centers (2-3 orders of magnitude greater - P Wang)
- cool the parcel to the dew point
- add moisture to the parcel
- mix parcels with different moisture and temperature values
-
radiation fog (video)
- thin layer of moist air
- inversion
- calm atmosphere, clear skies
- surface cools by radiation transfer
- conduction cools the parcels near the Earth's surface -
advection fog (video)
- warm, moist air parcels are blown over a cooler surface -
steam fog (video)
- cool air parcel over warmer water
- evaporation from water surface adds moisture to parcels, saturation vapor pressure $e_s$ is attained
- warmer, saturated parcels rise -
frontal fog
- ahead of a warm front (video)
$\cdot$ cool surface layer of air becomes saturated by evaporating precipitation
$\cdot$ snow sublimation cools air parcels to $e_s$
- behind cold front (video)
$\cdot$ cold air becomes saturated by evaporating surface moisture -
upslope fog (video)
- unsaturated warm, moist parcels rise and cool until $T = T_d$