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Meteorology

Cloud and Precipitation

Cloud and Precipitation

When air is cooled below its dew point the water vapour therein starts to condense out into water droplets.

The water droplets form either fog or cloud depending on the process by which the air is cooled.

It is generally understood that the fog is formed when the cooling of the air takes place at the surface by conductive processes, whereas cloud generally forms above the surface due to the adiabatic cooling of the rising air.

The cooling of the air occurs as the air is forced to rise as follows

(a) At a warm front

(b) At a cold front

(c) By convectional processes

(d) By high ground

(e) By turbulence

Contributory causes in cloud formation may be

(i) Radiation of heat by water vapour in the atmosphere

 (ii) Mixing of two masses of nearly saturated air at different temperatures.

Clouds are classed as being either high above 6000 metres, medium 2500 - 6000 metres or low below 2500 metres.

There are several types in each class, the main ones being shown in the following table:

The water vapour mentioned earlier cannot condense into large size water do unless condensation nuclei are present. These nuclei, which may be salt or dust particles, are always present although the numbers will vary from about 1000 per cm3 over the sea to about 150,000 per cm3 over industrial areas.

Water drops of less than 0.1mm diameter cannot reach the ground as they evaporate on the way. These drops are cloud drops and their average diameter is 0.01mm.

Drops over 0.1mm are drizzle drops and over 0.5mm are raindrops. The maximum size to which a drop can grow is 5.5mm.

The process of growth of these water drops has brought forth many theories one of which is that in the original stages of condensation taking place, some drops form larger than the rest. Their growth due to the attraction of the smaller drops is then assured.

Rain is usually described. as being either Frontal, Convectional or Orthographic. This is in accordance with the process of cloud formation; Appreciable rainfall is unlikely with turbulent cloud. FRONTAL RAIN occurs in a warm front. The precipitation commences from Altostratus cloud and increases in intensity as the lower nimbostratus cloud comes in.

Where the front is not so marked occasional precipitation, might occur. This has the same characteristics as frontal rain and lasts less than 30 minutes.

CONVECTIONAL RAIN falls when the strong up currents, within the cumulus type cloud, cease or are so reduced that they can no longer keep the rain drops within the cloud. It may be noted that an up current of 8m/s will keep the maximum sized raindrops from falling. If this velocity is slightly reduced, only the largest such drops will fall, and the smaller drops will remain in the cloud.

A feature of conventional type rain is that rainfall always starts heavily and often with little warning.

Another name for convectional rain is showers.

These last less than 30 minutes although showers frequently merge to give a period of prolonged rainfall.

ORTHOGRAPHIC RAIN. This falls on the weather side of mountain ranges and gives some of the heaviest rain known.

SNOW. When water vapour condenses at a temperature below freezing point ice crystals form. These join together to form snowflakes. If the air temperature is much above freezing point the flakes will melt. In general the temperature near the surface must not exceed 3°C for snow to fall. It should be understood that at temperatures less than 3°C rain is still very likely and is not necessarily replaced by snow.

SLEET. This is partially melted snow.

THE CUMULONIMBUS CLOUD (Cb)

This is a cloud of great vertical development formed in unstable conditions and may extend up to 13000m from a base which may be as low as 500m.

The Cb cloud may be formed

At a cold front,

At a mountain range,

When there is considerable heating of the ground, or

When cold air flows over a warm surface.

However, by whichever method it is formed it will still have its characteristic appearance.

Air currents at high levels may, and often do, produce in ‘anvil’. Within the cloud will be water drops and ice crystals and in between the two there may be super cooled water drops.

These latter are water drops in their liquid state at a temperature below freezing. As long as they do not contact anything they can remain liquid at temperatures down to (-) 40°C. If, however, they touch anything, they immediately freeze into globules of clear ice.

HAIL. Due to the turbulence within the Cb cloud ice crystals frequently contact the Super cooled water drops.

When this occurs the super cooled drop freezes to give a layer of clear ice round the ice crystal. This is the start of the hailstone, which will then grow as convection currents carry it through the cloud.

During this process the stone acquires alternate cores, one of clear ice (from the super cooled drops) and one opaque ice (from the ice crystals). If a hailstone is cut open the various layers can be seen, and the number of times it has been carried up and down can be estimated.

When the hailstone can no longer be supported within the cloud it will fall to the ground. The hailstone varies considerably in size being anything from 5mm to 50mm in diameter.

THUNDERSTORM

When raindrops reach their maximum size of 5.5mm they break up. This splitting gives the air a negative electrical charge whilst the raindrop attains a positive electrical charge.

This action takes place within the Cb cloud and it is found that there is a large positive charge near the top of the cloud. At the bottom there is a large negative charge with a small positive charge due to the raindrops.

When there is sufficient difference of potential between the top and bottom of the cloud, or between two clouds, or between a cloud and the earth, there is a visible electrical discharge, which is called LIGHTNING.

If air is cooled below its dew point by conduction (i.e. Contact with a cold surface) then the water vapour will condense into droplets on or near that surface. The radiation from the earth on cloudless nights will cause considerable surface cooling with the possible deposit of water droplets if the air near the earth is cooled until saturated.

DEW. The water droplets deposited on the surface by the above process. Clear skies and practically no wind are necessary conditions for its formation.

HOAR FROST. Formation is the same as for dew except that the dew point temperature is below freezing. The freezing of water droplets, which were deposited as dew, may also form hoar frost.

MIST is similar to fog but here the visibility is 1000m to 2000m. It should be noted that HAZE giving the same visibility as mist is formed of dust particles not water droplets.