Changes of Cloud Form

Figure 8.3 shows how a change in surface conditions across a coastline can induce certain clouds, depending on whether the wind is onshore or offshore, and whether the land is warmer (in summer) or colder (in winter) than the sea. Convective clouds may form if the lower troposphere is unstable, e.g. when humid air from a warm ocean flows over even warmer land in summer (upper left of Figure 8.3) or when offshore cold air blows over a warmer sea (upper right), especially as the sea increases the dewpoint (Section 7.4). If the sea is cooler than the offshore wind (lower left), a shallow stably stratified cloud may form offshore, i.e. stratus or fog. In winter, relatively warm, moist onshore winds (lower right) become more stable over the coast, and form advective fog (Section 8.4) or light rainfall on rising ground. However, Figure 8.3 is a simplification; most clouds and rain form well above the planetary boundary layer and the coastline itself induces winds (Chapter 14). For instance, a sea breeze in summer may advect offshore fog (lower left) onto the land.

Clouds may change in character after they are formed, for several reasons. Here are three:

1 Destabilisation of the atmosphere by uplift

Offshore Advection Fog
Figure 8.3 Cloud development near the coast in various circumstances.

may cause cumulus to grow out of stratus.

2 Stabilisation causes afternoon cumulus clouds to flatten out as stratocumulus. This occurs when solar heating of the surface is reduced at sunset, or as a consequence of cloud shadow reducing surface temperatures.

3 A thin layer of stratus often changes to a ribbed or dappled form (Figure 8.5) because of cellular stirring due to internal instability, caused by cooling of the top through the loss of longwave radiation to space, and warming of the layer's base by radiation from the ground.

Clouds disappear when the droplets evaporate, because of either the entrainment of dry air from the environment, or warming due to subsidence, or the absorption of longwave radiation from the ground (especially with shallow stratus on summer mornings). Small fair-weather cumulus (Section 8.6) often dissipates by entrainment after only a few minutes, whilst large cumulus may last several hours. Cirrus cloud is usually long-lived because it consists of ice, which sublimes only with difficulty at the temperatures of around -40°C.

8.4 FOG

Cloud at ground level is fog, if visibility is less than one kilometre; otherwise it is mist. The better visibility in mist is due to the larger size of the droplets, i.e. about 100 pm: the larger the mean drop size, the better the visibility (Note 8.G). 'Thick' fogs have a visibility below 200 m and 'dense' fogs less than 40 m. We will consider seven kinds.

1 Hill mist or upslope fog arises from orographic lifting of stable air when the Lifting Condensation Level is lower than the mountain top.

2 Rain is usually cooled by evaporation, so the immediately adjacent air becomes cool as well as moist. Mixing of this air with that around may lead to supersaturation (Note 8.A), in which case separate patches of cloud called scud or stratus fractus form below the cloud which is yielding rain. The scud may hug a hillside or even level ground, in which case it is known as rain fog.

3 Radiation fog or ground fog is formed at night if the surface air is moist and the ground is cooled by longwave radiation to a clear sky

Table 8.2 Classes of clouds

Cloud type



Typical height

(km) Vertical motion involved

Atmospheric stability in formation

High cloud:

1 Cirrus Ci

2 Cirrocumulus Cc

3 Cirrostratus Cs

Medium-level cloud:

4 Altostratus As

5 Altocumulus Ac

Low-level cloud:

6 Stratocumulus Sc

7 Stratus St

Cumuliform cloud:

8 Cumulus Cu

9 Cumulonimbus Cb

Separate white filaments, in streaks or bands

Dappled layer like beach sand ripples

Fused veil of cirrus, forming halo round Sun or Moon

Grey, uniform fibrous sheet

Dappled, flattened globules in billows

Soft, grey layer of flakes or globules in groups, lines or waves*

Featureless elevated fog

Flat base, cabbage-shaped top piled high

Huge, heavy, dense, with fibrous top, often spread into anvil shape, producing rain

6-10 Widespread, prolonged and regular ascent at around 70 mm/s

As above As above As above As above

3-6 As above

As above As above

Below 3 Widespread irregular stirring with below 10 cm/s vertically

1-2 Widespread lifting of cool damp surface air

0.6-6 Thermal convection with large bubbles rising at 1-5 m/s

To the Internal convection of 3-30

tropopause m/s upwards

Strong wind shear

Elevated unstable layer

Turbulence within stable air


Unstable, spreading out to Sc in evening due to stability

Deep instability

* Due to spreading out of the tops of earlier cumulus clouds

Different Types Cloud Nomenclature
Figure 8.4 A composite illustration of various kinds of cloud.

Table 8.3 Typical values of the water contents of clouds in the southern hemisphere

Table 8.3 Typical values of the water contents of clouds in the southern hemisphere

The equivalent water depth is the depth of rain if all the liquid water in the cloud were to fall; it is the product of the cloud thickness (km) and the average water content (g/m3)

(Section 7.6). It is more likely when the ground has been wetted by earlier rain, and at high latitudes where long winter nights allow prolonged cooling. Also, the ponding of cold air and moisture in hollows promotes ground fog there. It is often less than 10 m thick, but may become 250 m thick in valleys. Figure 8.6 shows that fog in Canberra is most likely around 7 a.m., and in winter, when temperatures are lowest.

cooling by radiation to space

T T lapse rate f 1

warming by convection terrestrial radiation

Figure 8.5 Formation of a ribbed type of cloud as a result of convection within a stratus cloud, induced by radiation cooling of the top.

4 Advection fog or sea fog occurs where relatively warm, moist and stable air moves over water whose temperature is slightly below the dewpoint of the air (lower left in Figure 8.3, and Note 8.H), e.g. off the coast of northern Chile, and over Antarctic sea-ice, especially near a 'lead' (a line of clear water between ice floes) or the open sea in winter. Advection fogs tend to be more extensive and deeper than radiation fogs. The depth depends on the wind strength; less air is stirred down to be cooled if the speed is much below 7 m/s, whilst a stronger wind dissipates the fog by stirring it with warmer air above (Note 8.A).

5 Steam fog or sea smoke forms when cold air flows over a relatively warm wet surface (Section 4.3). Evaporation from the latter into the unstable lower atmosphere leads to convection of the moisture upwards, and then the vapour condenses to drifting filaments of wispy mist, a few metres high (Note 8.I). You can see it after a shower onto a hot roadway and it occurs at sea around Antarctica, where the air may be much cooler than the water. The likelihood of steam fog is increased by thermal pollution of waterways, and by radiation cooling on cloudless nights chilling the ground and hence the air around a shallow lake (Table 8.4).

Note the paradox that fog may be formed either by cold air over warm water (steam fog) or by warm air over cold water (advection fog).

6 Ice fog occurs when temperatures drop below -20°C or so. For instance, a person breathing out in Antarctica may become surrounded by a personal cloud of ice fog,

Ice Fog Formation

3pm 6pm 9pm midnight 3am 6am 9am noon 3am

Figure 8.6 The chance of fog at Canberra at various times of the day and year.s

3pm 6pm 9pm midnight 3am 6am 9am noon 3am

Figure 8.6 The chance of fog at Canberra at various times of the day and year.s

Table 8.4 Dependence of the frequency of wintertime fogs at dawn, on the wind, cloudiness and the difference between the temperatures of the air and a river in Sydney


Number of occasions

Wind speed (km/h)


Temperature difference (K)

With fog Without fog

9 5

Less than 5 About 5

Almost cloudless Over half overcast

7-10 1.5-4

and idling airplane engines there can quickly cause the plane to be enveloped in ice fog. Ice fog may deposit on the upwind side of solid surfaces as hoarfrost, a layer of ice crystals in the form of needles, scales, plates, etc.

7 Smoke fog or smog is characteristic of polluted cities in damp climates. It occurs when a low-level inversion traps surface air pollution, along with considerable water vapour. The droplets are so tiny that the fog may appear thick even though its water content is small (Note 8.G). The fog may be highly acidic.

The annual number of days with fog varies around the globe (Figure 8.7). In general, there are fewer in the southern hemisphere than the northern, though over eighty each year at spots on the south-west coasts of Africa and South America, where sea-surface temperatures are low (Chapter 11). Fogs occur on more than forty days each year, between

Average Number Days Fog Per Year
Figure 8.7 The average number of days each year that fog reduces visibility below a kilometre, some time during the

20-30°S along the coasts of Chile and western South Africa, but not on the west of Australia or New Zealand.

Fogs are uncommon in Australian cities. Canberra is the foggiest capital city, with around forty-six annually, followed by Brisbane (22), Melbourne (20), Sydney (17) and Perth (8). There has been a striking reduction in the number of fogs near Sydney, from thirty-one annually during 1931-5, to only five in the period 1976-80.

The dispersal of fog is either by stirring by the wind, or the arrival of drier air or the Sun heating the ground. It can endure if shielded from the Sun by cloud.

Having thus considered cloud near the ground, we proceed to discuss the kinds of stratiform and cumuliform clouds.

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  • pamphila took
    Which types of clouds tend to be thin and layered?
    8 years ago
  • joonas
    Why do clouds tend to form around 3pm and 6am?
    8 years ago
  • rezene
    Why are cumuliform clouds more likely with an onshore wind than an offshore wind]?
    8 years ago

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