Condensation nuclei

Remarkably, condensation occurs with utmost difficulty in clean air; moisture needs a suitable surface upon which it can condense. If clean air is cooled below its dew-point it becomes supersaturated (i.e. relative humidity exceeding 100 per cent). To maintain a pure water drop of radius 10-7 cm (0.001 mm) requires a relative humidity of 320 per cent, and for one of radius 10-5 cm (0.1 mm) only 101 per cent.

Usually, condensation occurs on a foreign surface; this can be a land or plant surface in the case of dew or frost, while in the free air condensation begins on hygroscopic nuclei. These are microscopic particles - aerosols - the surfaces of which (like the weather enthusiast's seaweed!) have the property of wettability. Aerosols include dust, smoke, salts and chemical compounds. Sea-salts, which are particularly hygroscopic, enter the atmosphere by the bursting of air bubbles in foam. They are a major component of the aerosol load near the ocean surface but tend to be removed rapidly due to their size. Other contributions are from fine soil particles and various natural, industrial and domestic combustion products raised by the wind. A further source is the conversion of atmospheric trace gas to particles through photochemical reactions, particularly over urban areas. Nuclei range in size from 0.001 |m radius, which are ineffective because of the high supersaturation required for their activation, to giants of over 10 |m, which do not remain airborne for very long (see pp. 12-13). On average, oceanic air contains 1 million condensation nuclei per litre (i.e. dm3), and land air holds some 5 or 6 million. In the marine troposphere there are fine particles, mainly ammonium sulphate. A photochemical origin associated with anthropogenic activities accounts for about half of these in the northern hemisphere. Dimethyl sulphide (DMS), associated with algal decomposition, also undergoes oxidation to sulphate. Over the tropical continents, aerosols are produced by forest vegetation and surface litter, and through biomass burning; particulate organic carbon predominates. In mid-latitudes, remote from anthropogenic sources, coarse particles are mostly of crustal origin (calcium, iron, potassium and aluminium) whereas crustal, organic and sulphate particles are represented almost equally in the fine aerosol load.

Hygroscopic aerosols are soluble. This is very important since the saturation vapour pressure is less over a solution droplet (for example, sodium chloride or sulphuric acid) than over a pure water drop of the same size and temperature (Figure 5.8). Indeed, condensation begins on hygroscopic particles before the air is saturated; in the case of sodium chloride nuclei at 78 per cent relative humidity. Figure 5.8 illustrates Kohler curves showing droplet radii for three sets of solution droplets of sodium chloride (a common sea-salt) in relation to their equilibrium relative humidity. Droplets in an environment where values are below/above the appropriate curve will evaporate/grow. Each curve has a maximum beyond which the droplet can grow in air with less supersaturation.

Once formed, the growth of water droplets is far from simple. In the early stages the solution effect is predominant and small drops grow more quickly than large ones, but as the size of a droplet increases, its growth rate by condensation decreases (Figure 5.9). Radial growth rate slows down as the drop size increases, because there is a greater surface area to cover with every increment of radius. However, the condensation rate is limited by the speed with which the released latent heat can be lost from the drop by conduction to the air; this heat reduces the vapour gradient. In addition, competition between droplets for the available moisture acts to reduce the degree of supersaturation.

Supersaturation in clouds rarely exceeds 1 per cent and, because the saturation vapour pressure is greater over a curved droplet surface than over a plane water surface, minute droplets (<0.1 |m radius) are readily evaporated (see Figure 5.8). Initially, the nucleus size is important; for supersaturation of 0.05 per cent, a droplet of 1 |m radius with a salt nucleus of mass 10-13 g reaches 10 |m in 30 minutes, whereas one with a salt nucleus

Kohler Curve

DROPLET RADIUS (cm)

Figure 5.8 Kohler curves showing the variation of equilibrium relative humidity or supersaturation (per cent) with droplet radius for pure water and NaCl solution droplets. The numbers show the mass of sodium chloride (a similar family of curves is obtained for sulphate solutions). The pure water droplet line illustrates the curvature effect.

DROPLET RADIUS (cm)

Figure 5.8 Kohler curves showing the variation of equilibrium relative humidity or supersaturation (per cent) with droplet radius for pure water and NaCl solution droplets. The numbers show the mass of sodium chloride (a similar family of curves is obtained for sulphate solutions). The pure water droplet line illustrates the curvature effect.

of 10-14 g would take 45 minutes. Later, when the dissolved salt has ceased to have significant effect, the radial growth rate slows due to decreasing supersaturation.

Figure 5.9 illustrates the very slow growth of water droplets by condensation - in this case, at 0.2 per cent supersaturation from an initial radius of 10 |m. As there is an immense size difference between cloud droplets (<1 to 50 |m radius) and raindrops (>1 mm diameter), it is apparent that the gradual process of condensation cannot explain the rates of formation of raindrops that are often observed. For example, in most clouds precipitation develops within an hour. The alternative coalescence mechanism illustrated in Figure 5.9 is described below (p. 102). It must be remembered too that falling raindrops undergo evaporation in the unsaturated air below the cloud base. A droplet of 0.1mm radius evaporates after falling only 150 m at a temperature of 5°C and 90 per cent relative humidity, but a drop of 1 mm radius would fall 42 km before evaporating. On average, clouds contain only 4 per cent of the total water in the atmosphere at any one time but they are a crucial element in the hydrological cycle.

2 Cloud types

The wide variety of cloud forms necessitates a classification for purposes of weather reporting. The internationally adopted system is based upon (1) the general shape, structure and vertical extent of the clouds, and (2) their altitude.

Cloud Droplet Growth

Figure 5.9 Droplet growth by condensation and coalescence.

Source: Jonas (1994). Reprinted from Weather, by permission of the Royal Meteorological Society. Crown copyright

Figure 5.9 Droplet growth by condensation and coalescence.

Source: Jonas (1994). Reprinted from Weather, by permission of the Royal Meteorological Society. Crown copyright

Table 5.1 Cloud base height (in 000s m).

Tropics

Middle latitudes

High latitudes

High cloud

Above 6

Above 5

Above 3

Medium cloud

2-7.5

2.7

2-4

Low cloud

Below 2

Below 2

Below 2

These primary characteristics are used to define the ten basic groups (or genera) as shown in Figure 5.10. High cirriform cloud is composed of ice crystals, giving a fibrous appearance (Plate 17). Stratiform clouds are in layers, while cumuliform clouds have a heaped appearance and usually show progressive vertical development. Other prefixes are alto- for middle-level (medium) clouds and nimbo- for thick, low clouds which appear dark grey and from which continuous rain is falling.

The height of the cloud base may show a considerable range for any of these types and varies with latitude. The approximate limits in thousands of metres for different latitudes are shown in Table 5.1.

Following taxonomic practice, the classification subdivides the major groups into species and varieties with Latin names according to their appearance. The International Cloud Atlas (WMO 1956) provides illustrations.

Clouds can also be grouped in their mode of origin. A genetic grouping can be made based on the mechanism of vertical motion that produces condensation. Four categories are:

1 gradual uplift of air over a wide area in association with a low-pressure system;

2 thermal convection (on the local cumulus scale);

3 uplift by mechanical turbulence (forced convection);

4 ascent over an orographic barrier.

Group 1 includes a wide range of cloud types and is discussed more fully in Chapter 9D.2. With cumuliform clouds (group 2), upward convection currents (thermals) form plumes of warm air that, as they rise, expand and are carried downwind. Towers in cumulus and other clouds (Plates 4 and 6) are caused not by thermals of surface origin, but by ones set up within the cloud as a result of the release of latent heat by condensation.

Thermals gradually lose their impetus as mixing of cooler, drier air from the surroundings dilutes the more buoyant warm air. Cumulus towers also tend to evaporate as updrafts diminish, leaving a shallow oval-shaped 'shelf cloud (stratocumulus cumulogenitus), which may amalgamate with others to produce a high overcast. Group 3 includes fog, stratus or stratocumulus and is important whenever air near the surface is cooled to dew-point by conduction or night-time radiation and the air is stirred by irregularities of the ground. The final group (4) includes stratiform or cumulus clouds produced by forced uplift of air over mountains. Hill fog is simply stratiform cloud enveloping high ground. A special and important category is the wave (lenticular) cloud (Plate 7), which develops when air flows over hills, setting up a wave motion in the air current downwind of the ridge (see Chapter 6C.3). Clouds form in the crest of these waves if the air reaches its condensation level.

Operational weather satellites provide information on cloudiness over the oceans, and on cloud patterns in relation to weather systems. They supply direct-readout imagery and information not obtainable by ground observations. Special classifications of cloud elements and patterns have been devised in order to analyse satellite imagery. A common pattern seen on satellite photographs is cellular, or honeycomb-like, with a typical diameter of 30 km. This develops from the movement of cold air over a warmer sea surface. An open cellular pattern, where cumulus clouds are along the cell sides, forms where there is a large air-sea temperature difference, whereas closed polygonal cells occur if this difference is small. In both cases there is subsidence above the cloud layer. Open (closed) cells are more common over warm (cool) ocean currents to the east (west) of the continents. The honeycomb pattern has been attributed to mesoscale convective mixing, but the cells have a width-depth ratio of about 30:1, whereas laboratory thermal convection cells have a corresponding ratio of only 3:1. Thus the true explanation may be more complicated. Less common is a radiating cellular pattern (Plate 8). Another common pattern over oceans and uniform terrain is provided by linear cumulus cloud 'streets'. Helical motion in these two-dimensional cloud cells develops with surface heating, particularly when outbreaks of polar air move over warm seas (see Plate 9) and there is a capping inversion.

^ o Cirrocumulus-

Cirrostratus

Family A High clouds

Cirrostratus

Family A High clouds

Hygroscopic Nuclei

(Ground)

*a> x o^Family B - Middle clouds iCr> % Altocumulus—\--~

Family^- Low clouds

/ ^ Stratocumulus

Family D - clouds with vertical development

J ^ Cumulonimbus"-*

Cumulus Cumulus fair weather

Figure 5.10 The ten basic cloud groups classified according to height and form. Source: Modified after Strahler (1965).

(Ground)

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Responses

  • paul
    Can wind erosion of soil be major source of condensation nuclei?
    6 years ago
  • yvonne
    Can condensation nuclei b created over ocean?
    22 days ago

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