39 HUMIDITY, CLOUDS AND FOG

145. HUMIDITY AND DEW POINT

Water is present in the atmosphere in all three of its states — gas, liquid and solid. In the gaseous state, it is known as water vapour, in the liquid state as rain or dew, and in the solid state as ice crystals or snow. There is some water present in the air in the invisible form of vapour at all times and in all places, but the amount varies considerably; air blowing offshore usually contains less water vapour than air blowing onshore in the same continental locality, and warm air is capable of holding considerably more than cold air. This capacity of the air for holding water vapour is important, for without it there would be neither clouds nor rain and day temperatures would be much colder everywhere than they are.

A measure of the amount of water vapour present in the air is known as the humidity. The maximum amount of water vapour air can contain at various temperatures has been ascertained by experiment, and when the atmosphere at a certain place contains this largest possible quantity of water vapour it can contain without increase of temperature, it is said to be saturated.

The proportion of moisture present in the atmosphere at any time to the amount of moisture necessary for saturation is known as the relative humidity, and is expressed as a percentage. As the warmer air is, the more water vapour it can contain, it will be seen that the relative humidity increases with a decrease in temperature. For example, the maximum amount of water vapour that can be contained in a sample of air at a temperature of 27°C. is about 2.4%. If a sample of air at 27°C. were indeed to contain this amount, its relative humidity would be 100%; if, however, it contained only 1.8% of water vapour at 27ºC., its relative humidity would be only 75%. Supposing this latter sample of air containing 1.8% water vapour at 27°C. were now cooled to 21°C., the temperature at which air can only contain about 1.8% of water vapour. The sample would now have become saturated, and its relative humidity increased by the cooling process from 75% to 100%.

The temperature to which a sample or mass of air must be cooled in order to reach saturation point (at a constant pressure) is called the dew point, and from the example above it will be seen that the dew point of air of 75% relative humidity at 27°C. is 21°C. If air is further cooled to a temperature below its dew point, it has to surrender its water vapour while doing so. Now the air contains myriads of minute salt particles which originate from sea spray, together with other tiny particles which are the products of combustion. As the water vapour in a mass of air cooled below its dew point is surrendered, it condenses in the form of visible water drops on these minute particles, and cloud is formed in the sky. If the mass of air is near the Earth’s surface it is called mist or fog, and if it is actually on the surface, the water vapour condenses on blades of grass, or on the deck of a ship, in the form of dew. If the dew point is below freezing point, the moisture is deposited in the form of hoar frost.

Both the relative humidity and the dew point of the atmosphere can be  obtained in practice by means of an instrument called a hygrometer, (fig. 39-1) which consists simply of two thermometers, one of which has its bulb permanently moist – the wet bulb, while the other thermometer is left dry – the dry bulb. The wet bulb is covered with muslin which is kept continually moist by means of a strand of cotton wick immersed in a small glass reservoir of clean distilled water. Both thermometers are housed in a slatted screen through which the air can pass freely but so that the thermometers are not affected by radiation of heat from nearby objects. So long as the air is not saturated, evaporation will take place from the muslin round the wet-bulb, and the drier the air the more rapid will be this evaporation so that the wet-bulb will read lower than the dry-bulb. From the difference between the two temperatures, meteorological tables immediately give the relative humidity and the dew point of the atmosphere. When the wet-bulb reads the same as the dry-bulb, the air is saturated and the relative humidity is 100%.

146. THE FORMATION, DISPERSAL AND CLASSIFICATION OF CLOUDS

The formation of clouds results from the condensation of water vapour in the free atmosphere on minute particles or nuclei in the air, this condensation is caused by the cooling of rising moist air to a temperature below the dew point or frost point. Clouds may be composed of water drops, ice crystals or a mixture of the two, according to the temperature of the air layer. High cloud is, mainly composed of ice crystals, medium clouds are mainly of water drops, and low cloud are entire of water drops. There are four ways in which the initial rising and cooling of the air may be brought about.

(i) By turbulence in strong winds, especially when moist air is blown over a cooler surface. In such winds the vigorous stirring causes a more uniform distribution of water vapour in the surface layers and, if the surface is a cold one, a general cooling of these layers, with the result that condensation may occur to form a cloud at a fairly low level. A necessary condition with turbulent clouds is that the temperature of the air above the condensation level is higher than that of the rising air. This warm air prevents the turbulent air from rising further and the resultant cloud spreads out horizontally. The cloud forms at the height at which the dew point is reached, and since turbulent mixing cannot extend to any great height, the base of the cloud is low, generally below 600 metres (2000ft.). The process is illustrated in fig. 39-2.

(ii) By Orographic Ascent, caused by obstruction to the wind on a larger scale than that described as turbulence. When moist air is forced to rise upon meeting a range of mountains it cools, becomes saturated and further cools as the air continues to rise resulting in condensation and the formation of clouds (see fig. 39-3.). Orographic clouds may be observed at sea over high islands or over mountains near the coast.

(iii) By Convection, when the localised heating at the surface within a relatively cold air mass causes rising currents of air to cool and produce the well—known cumuliform clouds (fig. 39-4). The depths of these clouds may vary between 500 and 10,000 metres (1,000-35,000ft.).
The convection currents may be caused by local heating of air in contact with the ground (as during a warm summer day on land), or in contact with a warm sea in cool air coming from higher latitudes. The whole of this warmed air ashore or at sea cannot rise as a single mass over a large area, as other air must fall to take its place. The air thus ascends in separate columns and, wherever a column has penetrated the condensation level, the space occupied by the rising air above that level is filled with cumuliform clouds.

(iv) By General Ascent of Air over a wide area, occurring mainly in the frontal areas of depression when a warm air mass rises over a colder air mass or is undercut by a colder air mass. This process is described in detail earlier in this Chapter. In these conditions, the ascent of air can occur over considerable areas and can be associated with rather extensive areas of cloud and precipitation (rain, hail or snow).

The dispersal of cloud can be brought about either by evaporation, resulting from a rise of temperature causing the relative humidity to fall below 100%, or by mixing with its dry surroundings, or by fall—out as precipitation, where the removal of the water eliminates the fundamental element of the cloud.

It is possible to define ten basic shapes of clouds according to the international system of cloud classification. These ten basic shapes are divided into groups according to the height at which the cloud base is usually found, as shown in the following table: –

Before going on to give a brief description of each of these ten types of cloud, it is worth noting that there are really only two basic forms or shapes of the cloud: “stratiform”, or layer cloud, i.e. cloud formed in horizontal sheets; and ‘Cumuliform’ (woolpack or cauliflower) cloud, or clouds with a much greater vertical development than horizontal extent. Cumuliform clouds are formed when moist air is caused to rise quickly, for example during periods of vigorous convection or when warm air is forced rapidly above cold air. Stratiform clouds are formed when the warm air rises at a moderate speed or is forced to rise above mountain ranges or elevated coasts. They are also formed when surface friction causes turbulence sufficient to produce saturation and condensation in the upper part of the turbulent layer.

In the descriptions of the various cloud types which follow, the common abbreviation for each type follows the name of the cloud. All these clouds are illustrated in fig. 39-5.

CIRRUS (Ci) has a fibrous or feathery appearance, is generally white in colour without shading, and is composed of ice crystals. Tufted cirrus clouds are popularly called “Mares tails”.

CIRROSTRATUS (Cs) is a thin whitish veil sometimes covering the whole sky and often producing a halo around the sun (or the moon at night).

CIRROCUMULUS (Cc) appears as small white flakes or globular masses arranged in groups or ripples.

ALTOSTRATUS (As) is a grey or bluish cloud sheet or layer of striated, fibrous or uniform appearance, totally or partly covering the sky, often varying in density and having parts thin enough to reveal a vague image of the sun, but no halo. It is a darker and lower version of Cs.

ALTOCUMULUS (Ac) is similar in appearance to Cirrocumulus and lower in the sky, but the globules are larger, flatter and darker, sufficient to cause a shadow, whereas Cc is entirely white.

NIMBOSTRATUS (Ns) a grey cloud layer, often dark and diffused by falling rain or snow. It is thick enough to blot out the sun.

STRATUS (St) a generally grey uniform low cloud layer with a fairly flat uniform base and an appearance like fog.

STRATOCUMULUS (Sc) a cloud layer consisting of irregular masses or rolls of grey and whitish patches. Softer and more irregular than Cumulus.

CUMULUS (Cu) – detached clouds, generally dense and dome-shaped with sharp outlines developing vertically in the form of rising mounds. The sunlit parts are brilliant white while the bases are relatively dark and nearly horizontal.

CUMULONIMBUS (Cb) heavy dense cloud with considerable vertical development in the form of mountains or huge towers, the upper parts being of a smooth fibrous texture often spreading out in the shape of an anvil. The base is usually very dark, from which rain is falling.

147. THE FORMATION OF FOG

Just as the formation of clouds results from the condensation of water vapour in the free atmosphere, when such condensation happens near the surface, mist or fog is produced, which can thicken upwards to a depth of up to 150 metres (500ft.). Like clouds, fog is formed by the condensation of water vapour on minute particles, which are always present in the atmosphere (notably salt particles over the sea). These particles have such an affinity for water that they start to pick up moisture when the relative humidity is only about 75%. The formation of fog, therefore, depends on the number of particles in the air, and upon the relative humidity. If air of relatively high humidity is progressively cooled, it will become saturated, and further cooling will cause condensation of the water vapour on the particles into visible drops of water. These seriously reduce visibility by shutting off the light coming from distant objects, and by reflecting towards the eye light that falls on them from many directions, in effect interposing a whitish screen between the observer and the object.

The cooling of damp air is thus the primary cause of fog. Advection or Sea Fog is caused by comparatively warm, moist air being cooled by flowing over a colder sea surface. The lower layers of the air are cooled and are prevented from rising by warmer air above so that the same patch of air is kept in contact with the cold sea. 85% to 95% of fogs experienced in the open sea are formed in this manner. Sea fog is dissipated only by a complete change in the source of air, a considerable increase in wind strength, or a rise in sea temperature.

Radiation Fog forms over land on quiet nights with clear or nearly clear skies when the land rapidly radiates its heat into space and becomes cool, thus chilling the air in contact with it to its dew point, which a gentle breeze will fan into a fog. Such fog may drift out to seaward and interfere with coastal navigation. Radiation fog is most frequent and persistent in temperate and high latitudes in winter, particularly in the vicinity of large industrial areas, and is generally thickest during the latter part of the night or early part of the day.

Mist is similar to fog except that the water droplets held in suspension are smaller and more sparse, and the visibility is reduced by not less than one kilometre (1100 yards). Haze is a suspension of dry, solid particles in the atmosphere, e.g. smoke, sand, and dust, causing reduced visibility.

Advection fog can form at any time of the day or night over land or sea; when it forms over the sea it is alternatively called sea fog. One condition essential for its formation is that the air must be carried onto a surface whose temperature is below the dew point of the air. Sea fog occurs most frequently in winds between 4 and 16 knots (force 2 to 4 in the Beaufort Scale)., although such fogs can, on occasions, persist in winds of force 5 or even 6, because sea water does not warm readily and the stirring action of the wind on the sea surface brings up colder water from below.

Situations in which sea fog is likely are:—

Air moving from warm land onto a cold sea surface, occurring mainly in spring and early summer, e.g. a warm easterly air stream flowing from the European continent onto the cold North Sea in spring;

Air moving from a warm sea onto a cold sea current, e.g. a warm, moist southerly air stream flowing over the cold Labrador Current frequently causes the notorious fogs on the Grand Banks of Newfoundland;

 Warm air in advancing over a surface which becomes progressively colder, occurring most commonly with Tropical Maritime air moving into higher latitudes, e.g. south-westerly winds reaching the British Isles from the Azores in spring and early summer sometimes bring fog into the English Channel.