Haze, mist and fog

We stated that not all clouds precipitate. Indeed, from only a very small proportion of clouds does precipitation actually reach the ground surface below. The basic problem is that cloud water droplets or ice particles are frequently too small to fall from the cloud base or to survive on the way to the ground because they evaporate. Whereas a cloud droplet is on average 8 pm in diameter, a rain drop is between 500 and 5000 pm (0.5-5 mm) this means that a small rain drop is as large in volume as 240 000 cloud drops. Assuming 240 cloud droplets cm (cf Table 2.25), there is only one rain drop in 1 L of air. Several microphysical processes occur in clouds depending on temperature, vertical resolution, dynamic and other parameters that result in growth of a particle (Fig. 2.39) and different precipitation forms (Table 2.26). 

The chemical composition of precipitation is a result of three different processes  

For a description of precipitation chemical climatology the most important cloud 

In the last two sections we considered hydrometeors, drops and ice particles in clouds, fog, mist and precipitation. This section deals with the formation of interfacial water, either from water vapor (dew and frost) or from hydrometeors (rime and interception). These forms of atmospheric water need the contact with a surface soils and vegetation but also artificial surfaces. Some meteorologists classify these phenomena as belonging to precipitation - we will not (see Chapter 4.4 for more details). Precipitation is physically a sedimentation process due to gravitational force see Fig. 2.40 for different deposition processes. 

Condensation of water vapor occurs when the dew point is undershot onto cooler surfaces resulting in a subsequent diffusion process in the direction of the surface (that is dry deposition, similar to surface removal of other molecules than H2O, but these are ad- or absorbed onto the surface - only water can condense into liquid in the earth s climate system). When the temperature is above 0°C, liquid drops (not film) are formed on the surface - most likely on condensation or crystallization centers - and we call it dew. If r 0 °C, water vapor is transferred into ice on the surface frost) in different sizes and forms, mostly depending on temperature and wind on trees, filigreed ice needles and structures are formed hoar frost). 

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Heintzenberg, I, M. Wendisch, B. Yuskiewicz, D. Orsini, A. Wiedensoehler, F. Stratmann, G. Frank, B. G. Martinsson, D. Schell, S. Fuzzi and G. Orsi (1998) Characteristics of haze, mist and fog. Beitrage zur Physik der Atmosphdre 71, 21-31... 

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