Figure 8.3 shows exactly how a readjust in surchallenge problems throughout a coastline have the right to induce certain clouds, depending upon whether the wind is onshore or offshore, and also whether the land is warmer (in summer) or colder (in winter) than the sea. Convective clouds may develop if the reduced tropospbelow is unstable, e.g. when humid air from a warm ocean flows over also warmer land also in summer (upper left of Figure 8.3) or once offshore cold air blows over a warmer sea (upper right), particularly as the sea increases the dewsuggest (Section 7.4). If the sea is cooler than the offshore wind (reduced left), a shpermit stably stratified cloud might create offshore, i.e. stratus or fog. In winter, relatively warmth, moist onshore winds (lower right) become more steady over the coastline, and form advective fog (Section 8.4) or light rainloss on climbing ground. However before, Figure 8.3 is a simplification; most clouds and rain create well above the planetary boundary layer and also the shore itself induces winds (Chapter 14). For circumstances, a sea breeze in summer might advect offshore fog (reduced left) onto the land also.

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Clouds may readjust in character after they are developed, for numerous reasons. Here are three:

1 Destabilisation of the environment by uplift

Figure 8.3 Cloud advancement close to the coastline in assorted scenarios.

might cause cumulus to prosper out of stratus.

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

3 A thin layer of stratus regularly changes to a ribbed or dappled develop (Figure 8.5) bereason of cellular stirring as a result of internal instcapability, led to by cooling of the top through the loss of longwave radiation to room, and also warming of the layer"s base by radiation from the ground.

Clouds disappear when the droplets evapoprice, 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 through shenable stratus on summer mornings). Small fair-weather cumulus (Section 8.6) regularly dissipates by entrainment after only a couple of minutes, whilst large cumulus may last a number of hours. Cirrus cloud is normally long-lived because it is composed of ice, which sublimes only via obstacle 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 much better visibility in mist is due to the bigger size of the dropallows, i.e. around 100 pm: the bigger the expect drop dimension, the better the visibility (Keep in mind 8.G). "Thick" fogs have a visibility below 200 m and also "dense" fogs much less than 40 m. We will take into consideration seven kinds.

1 Hill mist or upslope fog arises from orographic lifting of steady air as soon as the Lifting Condensation Level is reduced than the mountain optimal.

2 Rain is normally cooled by evaporation, so the instantly nearby air becomes cool and moist. Mixing of this air with that approximately may bring about supersaturation (Note 8.A), in which instance sepaprice patches of cloud called scud or stratus fractus form listed below the cloud which is yielding rain. The scud might hug a hillside or also level ground, in which instance it is well-known as rain fog.

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

Table 8.2 Classes of clouds

Cloud type



Common height

(km) Vertical activity involved

Atmospheric stcapacity 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

Sepaprice white filaments, in streaks or bands

Dappled layer choose beach sand also ripples

Fsupplied 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 teams, lines or waves*

Featureless elevated fog

Flat base, cabbage-shaped height piled high

Huge, hefty, thick, via fibrous peak, often spcheck out right into anvil shape, developing rain

6-10 Widespread, extended and regular ascent at roughly 70 mm/s

As above As over As over As above

3-6 As above

As over As above

Below 3 Widespcheck out irconstant stirring through listed below 10 cm/s vertically

1-2 Widespcheck out lifting of cool damp surconfront air

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

To the Internal convection of 3-30

tropopausage m/s upwards

Strong wind shear

Elevated unsecure layer

Turbulence within steady air


Unstable, spreading out to Sc in evening because of stability

Deep instability

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

Figure 8.4 A compowebsite illustration of various kinds of cloud.

Table 8.3 Typical worths of the water contents of clouds in the southerly hemisphere

Table 8.3 Usual worths of the water contents of clouds in the southern hemisphere

The indistinguishable 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 as soon as the ground has been wetted by previously rain, and also at high latitudes where lengthy winter nights allow expanded cooling. Also, the ponding of cold air and also moisture in hollows promotes ground fog there. It is frequently less than 10 m thick, but might come to be 250 m thick in valleys. Figure 8.6 mirrors that fog in Canberra is a lot of most likely approximately 7 a.m., and in winter, as soon as 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 form of cloud as an outcome of convection within a stratus cloud, induced by radiation cooling of the top.

4 Advection fog or sea fog occurs wright here fairly warm, moist and steady air moves over water whose temperature is slightly below the dewsuggest of the air (reduced left in Figure 8.3, and Note 8.H), e.g. off the coastline of north Chile, and also over Antarctic sea-ice, particularly close to a "lead" (a line of clear water in between ice floes) or the open sea in winter. Advection fogs tend to be more extensive and also deeper than radiation fogs. The depth counts on the wind strength; much less air is stirred dvery own to be cooled if the rate is much listed below 7 m/s, whilst a more powerful wind dissipates the fog by stirring it via warmer air above (Keep in mind 8.A).

5 Steam fog or sea smoke creates as soon as cold air flows over a fairly heat wet surconfront (Section 4.3). Evaporation from the latter into the unsteady reduced environment leads to convection of the moisture upwards, and also then the vapour condenses to drifting filaments of wispy mist, a couple of metres high (Keep in mind 8.I). You have the right to watch it after a shower onto a hot roadway and it occurs at sea about Antarctica, wbelow the air may be a lot cooler than the water. The likelihood of steam fog is enhanced by thermal contamination of watermeans, and by radiation cooling on cloudmuch less nights chilling the ground and therefore the air around a shpermit lake (Table 8.4).

Note the paradox that fog might be formed either by cold air over warmth water (steam fog) or by heat air over cold water (advection fog).

6 Ice fog occurs as soon as temperatures drop below -20°C or so. For instance, a person breapoint out in Antarctica may end up being surrounded by a personal cloud of ice fog,


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 possibility 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 also the distinction between the temperatures of the air and also 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 fifty percent overcast

7-10 1.5-4

and also idling aircraft engines there can easily reason the airplane to be enveloped in ice fog. Ice fog might deposit on the upwind side of solid surfaces as hoarfrost, a layer of ice crystals in the develop of needles, scales, plates, etc.

7 Smoke fog or smog is characteristic of polluted cities in damp climates. It occurs when a low-level invariation traps surface air pollution, in addition to considerable water vapour. The droplets are so tiny that the fog may show up thick also though its water content is little (Keep in mind 8.G). The fog may be very acidic.

The annual number of days with fog varies about the world (Figure 8.7). In general, tright here are fewer in the southerly hemispbelow than the northern, though over eighty every year at spots on the south-west coastlines of Africa and South America, where sea-surface temperatures are low (Chapter 11). Fogs occur on even more than forty days every year, between

Figure 8.7 The average variety of days annually that fog reduces visibility below a kilometre, some time throughout the

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

Fogs are unwidespread in Australian cities. Canberra is the foggiest funding city, via roughly forty-6 each year, complied with by Brisbane (22), Melbourne (20), Sydney (17) and also Perth (8). There has been a striking reduction in the number of fogs near Sydney, from thirty-one every year throughout 1931-5, to only 5 in the duration 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 deserve to endure if shielded from the Sun by cloud.

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Having thus taken into consideration cloud near the ground, we continue to comment on the kinds of stratidevelop and also cumuliform clouds.