Figure 8.3 shows how a adjust in surface ar conditions across a shore can induce particular clouds, depending on whether the wind is onshore or offshore, and whether the soil is warmer (in summer) or chillier (in winter) 보다 the sea. Convective clouds may form if the reduced troposphere is unstable, e.g. As soon as humid waiting from a warmth ocean flows over even warmer land in summer (upper left of figure 8.3) or once offshore cold wait blows end a warmer sea (upper right), specifically as the sea rises the dewpoint (Section 7.4). If the sea is cooler 보다 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 kind advective fog (Section 8.4) or light rainfall on rising ground. However, figure 8.3 is a simplification; many clouds and also rain form well over the planetary border layer and also the shore itself induces winds (Chapter 14). Because that instance, a sea breeze in summer may advect offshore fog (lower left) onto the land.

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Clouds may adjust in personality after they space formed, for numerous reasons. Below are three:

1 Destabilisation of the setting by uplift

Figure 8.3 Cloud development near the coast in miscellaneous circumstances.

may cause cumulus to grow out of stratus.

2 Stabilisation reasons afternoon cumulus clouds come flatten out together stratocumulus. This occurs once solar heating of the surface ar is reduced at sunset, or together a consequence of cloud shadow reducing surface ar temperatures.

3 A thin layer that stratus often alters to a ribbed or dappled kind (Figure 8.5) due to the fact that of moving stirring due to internal instability, resulted in by cooling of the peak through the loss of longwave radiation to space, and also warming of the layer"s base by radiation native the ground.

Clouds disappear once the droplets evaporate, due to the fact that of either the entrainment of dry air from the environment, or warming because of subsidence, or the absorption of longwave radiation native the soil (especially with shallow stratus on summer mornings). Small fair-weather cumulus (Section 8.6) regularly dissipates by entrainment after only a couple of minutes, whilst huge cumulus may last numerous hours. Cirrus cloud is commonly long-lived due to the fact that it is composed of ice, i m sorry sublimes only with difficulty at the temperatures of approximately -40°C.

8.4 FOG

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

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

2 Rain is normally cooled through evaporation, so the immediately surrounding air becomes cool and moist. Mixing of this air with that approximately may result in supersaturation (Note 8.A), in which situation separate job of cloud called scud or stratus fractus kind below the cloud i m sorry is yielding rain. The scud may hug a hillside or also level ground, in which case it is well-known as rain fog.

3 Radiation fog or floor fog is created at night if the surface air is moist and also the soil is cooled by longwave radiation come a clean sky

Table 8.2 classes of clouds

Cloud type



Typical height

(km) Vertical activity 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 prefer beach sand ripples

Fused veil of cirrus, forming halo round sunlight or Moon

Grey, uniform fibrous sheet

Dappled, flattened globules in billows

Soft, grey class of flakes or globules in groups, present or waves*

Featureless elevated fog

Flat base, cabbage-shaped height piled high

Huge, heavy, dense, v fibrous top, regularly spread right into anvil shape, producing rain

6-10 Widespread, an extensive and regular ascent at approximately 70 mm/s

As above As above As over As above

3-6 together above

As over As above

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

1-2 widespread lifting the cool damp surface air

0.6-6 thermal convection with huge bubbles increasing at 1-5 m/s

To the interior convection the 3-30

tropopause m/s upwards

Strong wind shear

Elevated turbulent layer

Turbulence within steady air


Unstable, dispersing out come Sc in evening due to stability

Deep instability

* because of spreading the end of the tops of previously cumulus clouds

Figure 8.4 A composite illustration of assorted kinds the cloud.

Table 8.3 typical values that the water contents of clouds in the southern hemisphere

Table 8.3 common values of the water materials of clouds in the southern hemisphere

The tantamount water depth is the depth the rain if all the fluid water in the cloud were to fall; that is the product the the cloud thickness (km) and also the typical water content (g/m3)

(Section 7.6). That is an ext likely when the ground has been wetted by previously rain, and also at high latitudes where lengthy winter nights allow lengthy cooling. Also, the ponding of cold air and moisture in hollows promotes ground fog there. That is frequently less than 10 m thick, but may become 250 m thick in valleys. Number 8.6 mirrors that fog in Canberra is most likely about 7 a.m., and also in winter, once temperatures space lowest.

cooling by radiation to space

T T lapse price f 1

warming through convection terrestrial radiation

Figure 8.5 formation of a ribbed type of cloud as a an outcome of convection in ~ a stratus cloud, induced through radiation cooling of the top.

4 Advection fog or sea fog occurs where reasonably warm, moist and also stable wait moves end water who temperature is slightly listed below the dewpoint of the wait (lower left in number 8.3, and also Note 8.H), e.g. Turn off the coastline of north Chile, and also over Antarctic sea-ice, especially near a "lead" (a line of clean water in between ice floes) or the open up sea in winter. Advection fogs have tendency to be an ext extensive and also deeper than radiation fogs. The depth relies on the wind strength; less air is stirred down to be cooled if the rate is much listed below 7 m/s, whilst a more powerful wind dissipates the fog by stirring it through warmer air over (Note 8.A).

5 vapor fog or sea smoke develops when cold wait flows over a relatively warm wet surface ar (Section 4.3). Evaporation from the latter into the unstable lower setting leads to convection that the humidity upwards, and also then the vapour condenses come drifting filaments the wispy mist, a few metres high (Note 8.I). You can see that after a shower onto a warm roadway and also it occurs at sea about Antarctica, wherein the air might be much cooler 보다 the water. The likelihood of vapor fog is boosted by thermal air pollution of waterways, and by radiation cooling top top cloudless nights chilling the ground and also hence the air roughly a shallow lake (Table 8.4).

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

6 ice cream fog occurs when temperatures drop listed below -20°C or so. Because that instance, a human being breathing the end in Antarctica may end up being surrounded by a an individual cloud of ice fog,


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

Figure 8.6 The opportunity of fog in ~ Canberra at assorted times of the day and also year.s

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

Figure 8.6 The chance of fog in ~ Canberra at miscellaneous times the the day and also year.s

Table 8.4 dependency of the frequency that wintertime fogs at dawn, on the wind, cloudiness and also the difference in between the temperature of the air and a flow in Sydney


Number that occasions

Wind speed (km/h)


Temperature distinction (K)

With fog there is no fog

9 5

Less 보다 5 around 5

Almost cloudless Over fifty percent overcast

7-10 1.5-4

and idling plane engines there deserve to quickly reason the plane to it is in enveloped in ice cream fog. Ice cream fog might deposit top top the upwind next of hard surfaces as hoarfrost, a layer of ice crystals in the form of needles, scales, plates, etc.

7 acting fog or smog is characteristic of polluted cities in wet climates. That occurs when a low-level turning back traps surface air pollution, along with considerable water vapour. The droplets are so tiny the the fog may appear thick also though that is water contents is tiny (Note 8.G). The fog might be extremely acidic.

The annual variety of days through fog varies about the globe (Figure 8.7). In general, there room fewer in the southern hemisphere than the northern, though over eighty every year at clues on the south-west shores of Africa and South America, where sea-surface temperatures space low (Chapter 11). Fogs occur on much more than forty days every year, between

Figure 8.7 The average variety of days annually that fog to reduce visibility below a kilometre, some time during the

20-30°S follow me the coastlines of Chile and also western southern Africa, yet not top top the west that Australia or new Zealand.

Fogs space uncommon in Australian cities. Canberra is the foggiest capital city, with roughly forty-six annually, complied with by Brisbane (22), Melbourne (20), Sydney (17) and also Perth (8). There has been a striking palliation in the number of fogs near Sydney, indigenous thirty-one annually throughout 1931-5, to only 5 in the period 1976-80.

The dispersal the fog is either by stirring by the wind, or the come of drier waiting or the sunlight heating the ground. It deserve to endure if shielded from the sun by cloud.

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Having thus thought about cloud close to the ground, we continue to talk about the type of stratiform and cumuliform clouds.