Humidity, Clouds, Rainfall and Cyclones

Every cloud, every shower and every foggy morning begins with one ingredient: water vapour, the invisible gaseous form of water. It enters the air by evaporation from oceans, rivers, soil and plants (plants release it through transpiration). Evaporation is faster when the air is warm, dry and moving, and when the water surface is wide. Wind keeps removing the vapour that has just formed, so washed clothes dry faster on a hot, windy, dry day than on a cool, still, humid one. And evaporation is a cooling process: the molecules that escape carry heat away, which is why acetone on the palm or water sprinkled on a hot roof feels cool.

Buoyancy has nothing to do with evaporation. The main factors that change the rate of evaporation are temperature (more heat, faster), humidity (drier air takes up more vapour), wind speed (carries vapour away), surface area (wider surface, more molecules escape) and the nature of the liquid. Buoyancy (the upward push of a liquid on an object) belongs to a different topic, floating and sinking, and is not a factor here.

Humidity is the amount of water vapour in the air. There are three ways to state it, and exams ask you to tell them apart. Warm air can hold much more vapour than cold air, and this one fact explains most of the chapter.

Relative humidity is a 'how full is the bucket' number. Imagine air as a bucket whose size depends on temperature: warm air is a big bucket, cold air a small one. If the amount of vapour stays the same and the air cools, the bucket shrinks and the relative humidity rises. If the air warms, the bucket grows and the relative humidity falls. That is why RH is highest at dawn, when the day is coolest, and lowest in the early afternoon. When the bucket is exactly full (RH 100 per cent) the air is saturated, and the temperature at which this happens is the dew point. Cool it any further and the extra vapour must condense.

Weather elements and weather versus climate. The elements that describe the weather of a place are temperature, pressure, wind, humidity, cloudiness, precipitation and visibility. Things such as soil pH belong to soil science, not to the weather elements. Weather is the state of these elements at a given place and time (today it is hot and humid); climate is the average weather of a place over a long period, usually about 30 years. Weather changes by the hour; climate changes only over decades.

Condensation happens when air is cooled to its dew point. What you see depends on where it happens and on how cold the air is. Near the ground on a cool night we get dew, frost, fog or mist; high up in the sky we get clouds. The first four are all 'ground-level' forms, and the key to separating them is the temperature at which the vapour turns back into water.

Frost is not frozen dew; it is a direct step from vapour to ice. When the surface is colder than 0 degrees Celsius, vapour in the air touching it skips the liquid stage and settles as ice crystals. This direct change from gas to solid is called deposition (many Indian books call it sublimation; the opposite change, solid to gas, is sublimation, as in camphor or dry ice). Dew, in contrast, forms first as water and may freeze afterwards.

Smog and the Great Smog of London. The word smog joins smoke and fog. In December 1952 London was covered by a thick smog for several days: a temperature inversion and windless cold weather trapped the sooty smoke from coal fires and industry under a lid of cold air near the ground. Thousands of people died, and the disaster led to the Clean Air Act of 1956. This smog came mainly from burning coal. Los Angeles shows another type, photochemical smog, formed in sunshine when vehicle exhaust (nitrogen oxides and hydrocarbons) reacts to make ozone at ground level. Delhi's winter smog is similar to London's in the way an inversion traps smoke, and it is worsened by vehicle and crop-residue smoke.

Clouds are collections of tiny water droplets or ice crystals formed by condensation (or deposition) around nuclei, high in the air. Clouds are classified by height and shape. The English chemist Luke Howard gave them Latin names in 1803, and the names are easy once you know the roots: cirrus = a curl of hair, cumulus = a heap, stratus = a layer, nimbus = rain-bearing, and the prefix alto- = middle height. The international system has ten basic cloud genera.

A cloud droplet is so small (a few hundredths of a millimetre) that the slightest upward air current keeps it floating. To fall as precipitation, droplets must grow, by colliding and merging or by ice crystals collecting vapour, until a single raindrop holds roughly a million cloud droplets and becomes too heavy to stay up. Precipitation is all forms of water that reach the ground from clouds; dew and frost are not precipitation because they are not falling from clouds. Rain is only one kind.

Rainfall is classified by what makes the moist air rise and cool. There are three types, and every rainfall question is really asking you which lifting method is at work.

Rain shadow in one line: the windward side wins the rain, the leeward side pays for it with dryness. The same hill can therefore have a rainforest on one slope and a desert or dry plateau on the other. Famous rain-shadow regions are the Deccan interior (behind the Western Ghats), Patagonia (behind the Andes), the Great Basin of the USA (behind the Sierra Nevada) and the Tibetan Plateau and Ladakh (behind the Himalaya).

Where does it rain most? Annual rainfall is highest in the equatorial belt (Amazon and Congo basins, Indonesia) and on monsoon coasts, because warm oceans, rising air and mountains all work together. It is also heavy on the western coasts of temperate lands, where the westerlies bring ocean air. Rainfall is lowest in the hot deserts under the subtropical highs (the Sahara, Arabia, the Thar, the Atacama) and in the cold polar regions, where the air holds almost no vapour. Rainfall is measured with a rain gauge, usually in millimetres or centimetres, and lines joining places of equal rainfall on a map are isohyets. Cloud seeding (artificial rain) sprays silver iodide or dry ice into suitable clouds to give the droplets nuclei to grow on, but it cannot make rain from a clear sky.

An air mass is a huge body of air, often thousands of kilometres across, in which temperature and humidity are nearly the same at any given height. It gets these properties by sitting for days over a uniform surface, called its source region, such as a frozen plain, a desert or a warm ocean. The best source regions lie under high-pressure areas, where the air is calm and settles down. When the air mass moves, it carries its character to the new place and changes the weather there: a cold dry air mass from Siberia brings a cold wave, and a warm moist one from the tropical sea brings humid heat or rain.

A front is the sloping boundary where two air masses of different temperature and density meet. They do not mix easily, so the lighter warm air is lifted over the heavier cold air, and this lifting makes clouds and rain. The formation of a front is frontogenesis and its dissolving is frontolysis. Fronts are typical of the middle latitudes where polar and tropical air meet; they are absent in the tropics, where the air masses are all warm, which is why tropical cyclones are non-frontal storms. There are four types of front.

Why does a cold front give stormy weather but a warm front gives long drizzle? Because of the slope. A cold front is steep, so the warm air is thrown upward quickly and forms tall storm clouds that drop their rain in a short, heavy burst. A warm front is gentle, so the warm air glides up slowly over a long distance and forms a long, layered cloud sequence that drizzles for hours. Also, the cold front moves faster, so it eventually catches the warm front, and that is how an occluded front forms.

Temperate (extra-tropical or mid-latitude) cyclones are low-pressure systems that form along the polar front between about 35 and 65 degrees latitude in both hemispheres. The scientists of the Norwegian school, led by Vilhelm Bjerknes (about 1918 to 1922), described their life cycle in the polar front theory. They are very large (often 1,000 km or more across), they have fronts, no calm 'eye', and they travel from west to east with the westerlies and the jet stream. They are strongest in winter and bring most of the winter rain and snow of temperate lands. In the northern hemisphere, the winds blow anticlockwise into the centre; in the southern hemisphere, clockwise.

Western disturbances (WD) are temperate-type low-pressure systems born over the Mediterranean region (some also from the Black Sea and Caspian Sea area). The subtropical westerly jet stream steers them east across Iran, Afghanistan and Pakistan into north-west India, mainly from December to March. They bring winter rain to Punjab, Haryana, Delhi and western Uttar Pradesh, and snow to the Himalaya (Jammu and Kashmir, Himachal Pradesh, Uttarakhand). This rain is a boon for the rabi crops, especially wheat. Too much of it can cause hailstorms and floods, and the cloudiness before and the cold air after it can bring fog and a cold wave to the plains.

An anticyclone is the opposite of a cyclone: a system of high pressure in which the isobars are closed and the pressure is highest at the centre. Air sinks at the centre and spreads outward near the ground, turning clockwise in the northern hemisphere and anticlockwise in the southern. Sinking air warms and dries, so anticyclones bring clear skies, light winds and settled, dry weather. In winter, however, the clear skies and calm air cause cold nights, frost, radiation fog and temperature inversion; in summer they give hot, dry spells. Anticyclones are larger and move more slowly than cyclones. The subtropical highs and the Siberian High are semi-permanent anticyclones.

A tropical cyclone is an intense, compact, spiralling low-pressure system that forms over warm tropical oceans. It is a heat engine: warm sea water evaporates, the vapour rises and condenses, and the latent heat released warms the air further and keeps it rising. This is why a tropical cyclone dies when it moves over cold water or over land, where the supply of vapour is cut off.

Conditions for formation. (1) Sea surface temperature of at least about 26.5 degrees Celsius, through a good depth of water (about 50 m). (2) A location at least about 5 degrees away from the equator, so that the Coriolis force is strong enough to make the air spin. (3) High humidity in the lower and middle troposphere. (4) Low vertical wind shear, so that the storm is not torn apart. (5) A pre-existing disturbance, such as a trough or a cluster of thunderstorms in the ITCZ, to start the circulation.

Structure. At the centre is the eye, a roughly circular area of tens of kilometres across where air sinks, the sky is nearly clear, the wind is light and the pressure is the lowest. Around it is the eyewall, a ring of towering cumulonimbus where the strongest winds and the heaviest rain are found. Outside are spiralling rainbands. The cyclone spins anticlockwise in the northern hemisphere and clockwise in the southern hemisphere, and winds blow inward and upward near the surface, then outward at the top.

Names by region. The same kind of storm has different local names: hurricane in the North Atlantic and the north-east Pacific, typhoon in the north-west Pacific (China Sea and Japan), cyclone in the Indian Ocean and the south-west Pacific (the name used for India), willy-willy in north-west Australia, and baguio in the Philippines. The word 'cyclone' for the tropical type in India is the same storm as the American hurricane and the Chinese typhoon.

Tropical cyclones in the Indian region. They form mostly in the Bay of Bengal, which gives several times more cyclones than the Arabian Sea. They come in two seasons: the pre-monsoon (April to June) and the post-monsoon (October to December), with October and November the most active. Most Bay of Bengal storms hit the coasts of Odisha, Andhra Pradesh, Tamil Nadu and West Bengal (and also Bangladesh and Myanmar); Arabian Sea storms hit Gujarat, Maharashtra or move towards Oman and Pakistan. Storms usually move west or north-west with the trade winds and may later turn poleward.

How the Indian Ocean cyclones are named. Since 2004 the cyclones of the North Indian Ocean (Bay of Bengal and Arabian Sea) are named from a list made by the countries of the WMO/ESCAP Panel on Tropical Cyclones, which now has 13 member countries: Bangladesh, India, Iran, Maldives, Myanmar, Oman, Pakistan, Qatar, Saudi Arabia, Sri Lanka, Thailand, the UAE and Yemen. Each country gives names, and they are used in order. The name is given by the country whose turn it is, which is why the name of a storm hitting India may not be an Indian word. IMD (New Delhi) is the Regional Specialised Meteorological Centre that names and warns for this region.

IMD intensity scale. IMD classifies a system by its maximum sustained wind speed, in knots: low-pressure area (below 17), depression (17 to 27), deep depression (28 to 33), cyclonic storm (34 to 47), severe cyclonic storm (48 to 63), very severe cyclonic storm (64 to 89), extremely severe cyclonic storm (90 to 119) and super cyclonic storm (120 and above). A system is named once it reaches the cyclonic storm stage (34 knots).

Damage and warning. The greatest killer in a landfalling tropical cyclone is usually the storm surge, a rise of sea level pushed onto the coast by the winds and the low pressure, followed by flooding from very heavy rain and the destruction caused by wind. Low-lying, funnel-shaped and densely populated deltas, such as the Bengal-Odisha coast, are very exposed. IMD issues the warnings in stages, and the cyclone warning uses colour codes (yellow, orange and red for watch, alert and warning, and green for no warning). Satellite and radar tracking, evacuation, shelters and coastal shelterbelts of mangroves and casuarina reduce loss of life.

A thunderstorm is a local storm produced by a cumulonimbus cloud, with thunder, lightning, heavy rain and often gusty wind or hail. Three things are needed: plenty of moisture, an unstable atmosphere (warm air near the ground that keeps rising once lifted), and a trigger that lifts the air (strong surface heating, a front, or a mountain slope). It is the commonest example of violent convection.

Lightning and thunder. Inside a cumulonimbus, ice crystals and soft hail collide as they are carried up and down, and this separates electric charge: the upper part of the cloud becomes mostly positive and the lower part mostly negative. When the charge difference becomes too great the air breaks down and a huge spark jumps, as lightning, within the cloud, between clouds, or from cloud to ground. The lightning suddenly heats the air in its path, which expands explosively and we hear thunder. Light travels almost instantly while sound is slow (about 3 seconds per kilometre), so we see the flash first; counting the seconds between flash and thunder gives the distance.

Hail. In a strong cumulonimbus the updraught carries raindrops far above the freezing level, where they freeze. As the lumps of ice are tossed up and down, they gather more layers of water that freeze on them, until they are too heavy for the updraught and fall as hail. Hail can badly damage standing crops, and it is commonest in spring and early summer storms, in the middle latitudes and over high ground in the tropics. It is different from sleet (frozen raindrops that fall as small pellets in winter) and snow.

Thunderstorms of India. The commonest are the pre-monsoon (March to May) afternoon and evening storms, born of intense heating and moist air from the sea. Famous examples are the Kalbaisakhi (Nor'westers) of Bengal, Odisha and Assam, the Aandhi (dust storms) of the north-west and the Gangetic plain, and the mango showers and blossom showers of the south. These bring short but heavy rain, hail and strong squalls. The hot dry Loo of the northern plains is not a thunderstorm: it is a hot wind.

Tornado. A tornado is a narrow, violently rotating column of air that hangs from a cumulonimbus and touches the ground, shown by a funnel cloud. It is only tens to hundreds of metres wide and short-lived, but its winds are among the strongest on earth, so it is the most destructive storm for its size. It is born mainly from severe rotating thunderstorms (supercells). The best known region is the Great Plains of the USA, called Tornado Alley, where cold dry air from Canada meets warm moist air from the Gulf of Mexico. Tornadoes are rated on the Fujita scale (F0 to F5; now the Enhanced Fujita scale). Over water the same thing is a waterspout.