Clouds And Rain

This chapter links cloud, discussed in the previous chapter, with rainfall, considered in the next. Some correlation between cloud and rainfall is shown by Figure 9.1, with a weaker association in southern latitudes than northern. One reason is that areas between 15-30°S are frequently covered by stratus clouds which produce no rain. But there is a fair connection between cloud and rain in Australia (Figure 9.2, Note 9.A).

The various kinds of rainfall can be classified in various ways. For instance, according to either (i) the intensity of precipitation, (ii) the intensity of uplift, or (iii) the mechanism of cloud formation. As regards the first, we distinguish rain showers (which come from cumulus clouds) from drizzle (which involves rain droplets as small as 0.1 mm (Table 9.1) and comes from nimbostratus low in the sky). Drizzle occurs when surface relative humidities are high, i.e. the difference between air and dewpoint temperatures is 2 K or less. This is most likely at night, in the early morning or in winter. Usually there is a good breeze during drizzle, creating the turbulence which lifts surface air to dewpoint temperature, to replenish the cloud. Also there may be gentle uplift by hills or a weak front in the vicinity.

As regards classification in terms of the intensity of uplift, there are 'convective' and 'stratiform' rainfalls. Convective rainfall involves a vertical velocity maintained at 1 m/s or more, unlike stratiform rainfall, which is lighter because updraughts within stratus clouds are much weaker (Table 8.1, Note 9.B). Convective rain occurs when the atmosphere becomes unstable (Section 7.3) and is predominant in low latitudes, and in midlatitudes during the summer. It includes rain from thunderstorms, caused by the isolated convection of huge volumes of moist air over ground heated by the Sun (Section 9.5). In addition, convective rainfall may be caused by the passage of cold moist air over a warm surface, which happens off the coast of New South Wales when cold air from high latitudes flows over the warm East Australian Current (Section 7.4; Chapter 11). Convective rain is almost always accompanied by stratiform rain in the final stage of a thunderstorm (Section 9.5).

Figure 9.1 The association between annual mean cloudiness and rainfall between 30°N and 30°S.
Figure 9.2 The association of monthly mean cloudiness (as a fraction of unity) and the logarithm of the monthly total rainfall at 263 places in Australia from more than twenty years of data.

Alternatively, rainfalls can be classified in terms of the mechanism of uplift. Thus there is orographic, frontal and convergence rainfall. Orographic rain comes from stratus or stratocumulus created by hills, as when onshore winds rise over high ground at the coast (Figure 8.8). Rainfall from the consequent stratiform clouds occurs at about the rate of condensation, and so there is drizzle. This can continue for days on end, as long as the onshore wind persists (Note 9.B).

The term frontal rain is applied to midlatitude precipitation associated with the uplift occurring when a mass of cold air wedges under a warmer air mass (Chapter 13). It is sometimes called cyclonic rainfall, though it has no connection with low-latitude tropical cyclones discussed in Chapter 13. This rainfall is usually stratiform.

Convergence rainfall occurs, for instance, when wind is funnelled into a narrowing valley, or land breezes (Chapter 14) converge onto a large lake at night, e.g. Lake Victoria in East Africa. Convergence rainfall is usually convective. It takes place on a large scale near the equator, due to the confluence of air from the two hemispheres (Chapter 12). This happens near Darwin in the summer, producing copious rain (Chapter 10). A further kind of convergence rainfall results from tropical cyclones (Chapter 13) and is of particularly high intensity, often responsible for record measurements. For instance, a tropical cyclone near Broome in Western Australia caused a 24-hour rainfall of 351 mm on 18-19 January 1974, with an intensity reaching 122 mm/h.

A last type of rain is virga. This is the name of visible fallstreaks of rain from cloud, failing to reach ground level because the cloud base is too high and the atmosphere beneath so dry that all the rain evaporates before reaching the surface. Usually, less than 10 per cent of the rain from an isolated thunderstorm in the desert reaches the ground, about 30 per cent with large areas of convective cloud in hot and humid climes, but over 70 per cent in the case of stratiform rainfall at middle and high latitudes. The percentage is more on high land, which is nearer the cloud base.

Table 9. 1 Typical properties of cloud droplets, raindrops, rainfall and hail

Diameter (mm) d*

Number of drops/m3

Nt

Terminal velocity Mass of drops (m/s) (g/m3) V* M*

Deposition rate (mm/h)

Pt

Cloud/fog

0.01

107§

0.003

0.006

0.00006

Drizzle II

0.2

2 X 10§

0.75

0.09

0.24

Typical rain

If

600

4

0.3

4

Downpour

3

300

8

4

115

Small hail

5

-

5

-

-

Large hail

20

-

16

-

-

* This is from Parker (1980:373), Zachar (1982:215) and Mcllveen (1992:149). However, a wide range of cloud-droplet concentrations occurs. For instance, 1 g/m3 has been measured in cumulus cloud (Table 8.3).

t That is, 2,000-M/d3, where d3/2,000 is the weight (grams) of each drop.

§ Hence the separation of adjacent droplets is about 5 mm, i.e. the cube root of {109/107}, there being 109 cubic millimetres in a cubic metre. This is about 500 times their own diameter, so spontaneous collisions between cloud droplets are rare.

|| There is a convention that a cloud droplet has a diameter below 0.1 mm; drizzle between 0.1-0.5 mm; and a rain drop over 0.5 mm.

^ This implies that about a million (i.e. (1/0.01)3) cloud droplets need to collide with each other before they become a typical raindrop.

* This is from Parker (1980:373), Zachar (1982:215) and Mcllveen (1992:149). However, a wide range of cloud-droplet concentrations occurs. For instance, 1 g/m3 has been measured in cumulus cloud (Table 8.3).

t That is, 2,000-M/d3, where d3/2,000 is the weight (grams) of each drop.

§ Hence the separation of adjacent droplets is about 5 mm, i.e. the cube root of {109/107}, there being 109 cubic millimetres in a cubic metre. This is about 500 times their own diameter, so spontaneous collisions between cloud droplets are rare.

|| There is a convention that a cloud droplet has a diameter below 0.1 mm; drizzle between 0.1-0.5 mm; and a rain drop over 0.5 mm.

^ This implies that about a million (i.e. (1/0.01)3) cloud droplets need to collide with each other before they become a typical raindrop.

Renewable Energy Eco Friendly

Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

Get My Free Ebook


Post a comment