Frequently Asked Questions About Weather With Special Emphasis on Pakistan’s Weather

Earth is surrounded by gases known as the atmosphere. Air is composed of roughly 21 % oxygen, 78 % nitrogen, and less then 1 % carbon dioxide and other gases. This atmosphere stretches upto 100 km above Earth’s surface. The lowest 10 km which is known as troposphere consists of moisture which combined with temperatures, pressures and topography creates conditions called weather.

Formation of rainfall is due to condensation of water vapours held in the atmosphere. This condensation may occur due to the following processes:

The existence of low pressure area causes surrounding air with high pressure to move into the depression, displacing low pressure upwards, which may cool moist air to dew point. If cold air is replaced by warm air (the process known as warm front), the frontal zone is usually large and the rainfall of low intensity and longer duration may occur.

A cold front slopes much more steeply than the warm front as illustrated in Fig. 1, strong updraughts can stir up violent storms. Cold polar air sharply undercuts the rapidly rising warm tropical air and usually result in rainfall of shorter duration and high intensity.

When a moist air stream meets a mountain range, it is forced upwards towards the summit as shown in Fig. 2. As it rises, it cools and may condense into clouds around the summit of the mountains. Higher-level clouds can then act as "feeder" clouds, letting a little rainfall onto the rising clouds below resulting into heavy rainfall. Fronts too, may be disrupted by mountains. Warm fronts may be broken up when they run up against a mountain ridge. Cold fronts may deposit so much rain that they die out quickly on the far side. This brings rain to the windward side of mountains, and leaves the leeward side much drier.

High up in the mountains, the pressure drops due to reduced column of air above them. On mountain tops such as Mount Everest’s summit, the air pressure can be as low as 300 mb with ferocious winds blow at about 320 kph and temperature often as low as –70^oc.

During the day, the land heats far more quickly than the sea, and air begins to rise. As warm air rises above the land, cool air form sea is drawn in underneath as a replacement process, creating a strong sea breeze blowing inland as shown in Fig. 3.

At night, the situation is reversed. The land cools more quickly, and air begins to sink. The cool air out under the warm air starts moving to the sea as shown in Fig. 4. This is called land breeze.

The weather in coastal areas are accordingly less extreme than farther inland. Because the sea retains heat well, nights tend to be warmer on the coast, with winters milder, and summers slightly cooler.

Seasons occur because different parts of the Earth are tilted towards the sun as it moves around the sun during the year. In the northern hemisphere when the North Pole tilts away from the sun, as shown in Fig. 5 ,the sun is low in the sky and days are short, bringing in winter. When the North Pole tilts towards the sun, the sun is high and days are long, bringing in summer. Between these two extremes lie spring and autumn. In the southern hemisphere, the seasons are precisely the opposite.

Pakistan is one of the few countries of the world, which undergoes complete meteorological transformation from summer to winter. During winter, the country falls under the "extra-tropical" low pressure systems called the westerly waves (from Mediterranean), while in summer it is under the complete influence of tropical airmass called the monsoons. During spring and autumn (technically called the pre-monsoon and post-monsoon season) the country remains under the influence of both the westerly waves as well as the monsoons.

Toward the end of winter and the approach of summer season, the tropical airmass tends to expand northwards to Southern Pakistan. Rise of temperature over Balochistan, Southern Punjab and adjoining Sindh creates a permanent "seasonal low pressure" in this area which is fully established by the middle of May and is the most significant weather system during the pre-monsoon period. Westerly waves tend to accentuate this low, which causes the warm and moist air to be pulled in from the Arabian Sea all along the Sindh coastal belt. This south westerly air current is largely responsible for rain events that occur in Pakistan in April-June period. Orographic lifting of airmass along the Pirpanjal and the Himalayan ranges causes heavy rainfall along the windward slopes.

Post-monsoon season (October-November) is the driest of all the seasons. By this time monsoon has withdrawn completely while the westerly waves are moving farther north of Pakistan mostly along the central Asian States. Prolonged dry spell with only some isolated rains over the northern areas, is the most common characteristics of this period.

The weather systems in winter approach from west and can be traced as far as Mediterranean and at times further west to the Atlantic. The country is under the typically temperate weather conditions, similar to those prevailing in Europe and USA. These waves are the low pressure systems which continue to arrive from west and move eastwards across the country. Most of the westerlies move north of 30 degree latitude. A secondary low pressure at times forms within the trough of the primary that remains as a cutoff situated over Balochistan and adjoining Sindh areas. This secondary is quite often the cause of prolonged winter rains spread over a period of three to seven days. Winter rains are characterized by the uniform and relatively less intense precipitation, except when an active cold front is moving, in which case thunder-showers occur along a north-south axis. Since the main axis of westerly waves remains north of 30 degree latitude, the northern parts of Pakistan (Northern Punjab, Kashmir and NWFP) receive much more rains in winter and remain frequently under cloud cover.

Monsoons are like giant sea breezes, the rains begin when summer sun heats up tropical continents much faster than the oceans around. Warm air rising overland draws in cool, moist air from the sea, and rain-bearing winds gradually pushed farther inland. In the northern hemisphere for example, for about six months, showers of torrential rain sweep north over the country, right upto the Himalayas, until in October the southwest wind dies down and the rains slacken. A monsoon’s onset is hard to predict, and sometimes it fails to bring any rain to the hot, draught-stricken lands that year. Then crops fail, posing great danger of famine. Some of the major monsoon winds that occur in Asia are following:

The hot, dry lands of Asia draw warm air, laden with moisture, in from the Indian ocean during the early summer.

The cold, dry winter air spreads out from central Asia, bringing chilly, dusty conditions to the lands around.

Monsoons affect large areas of the tropics and the sub-tropics from northeast Australia to the Caribbean. Asian monsoons are the most marked because of Asia’s vastness.

During summer, pressure becomes low over Asia where there is intense surface heating, but pressures remains relatively high over the cool seas to the south. Most moisture laden equatorial air moves north due to this pressure gradient as well as earth’s diminishing rotational velocity as we move from the equator to the north pole. The moving air masses with an anti-clockwise rotation reach Indo-subcontinent as the southwest monsoon. Air moves north towards the shortest route, away from the equator and the Cariolis force exerted due to rotation of earth from west to east causes these winds to move south westerly over south India and almost westerly over Pakistan.

The permanent low pressure zone over Pakistan is located over its Balochistan Province and neighbourhood during summer season. Mountains force the monsoon upward causing even more rain. Large cumulonimbus clouds pile up against high ground as the monsoon blows inland. The monsoon thus brings some of the strongest torrential rains. Monsoon rain can be so intense that floods are frequent. Asian monsoons may also be triggered off when westerly jet streams in the upper air swing north over the Himalayas.

Monsoon Season is the main rainy season in the country marked by heavy downpours and frequent floods. Inter Tropical Zone of Convergence (ITZC) moves northwards to reach its north most position south of Himalayan Range, which generates the typical south-easterly wind commonly called as "Monsoon". The southeast air currents meet the southwesterly system described earlier along the east and northeastern regions of Pakistan. This phenomenon affects the upper catchments of the eastern rivers of Pakistan including Sutlej, Ravi, Chenab and Jhelum Rivers.

Apart from the southeast and southwest monsoon currents, very unique weather systems called the monsoon depressions also arrive in Pakistan from Bay of Bengal moving across India generally along west/northwest direction. A number of such monsoon depressions are able to reach Pakistan and of these a few turn north/north-eastwards to enter the upper catchments of Sutlej. Ravi Chenab and Jhelum Rivers.

Chances of monsoon depressions affecting the upper catchments of eastern rivers increase with the advance of monsoon season, specially from middle of August till the end of September. One reason for this is the intensification of the high pressure over Tibetan Plateau which due to its elevation remains cold as compared to the surrounding atmosphere. This creates a high pressure core with a clockwise movement of air around it. Tibetan high is thus is an added factor to facilitate the northeast turning of the monsoon depressions. The westerly waves continue to move from west to east along the northern latitudes even during summer season while strong monsoon winds (south-easterlies) are moving from east to west. These phenomena are responsible for heavy monsoon rains in Pakistan causing extremely high flood peaks in rivers and general flooding in Pakistan.

In recent years, scientists have become increasingly worried about the effects of human activities on the world’s weather. Most meteorologists are now convinced that the world is getting warmer, due to increased "greenhouse" gases in the atmosphere but there is no consensus on how much warmer it will get. Like the panes of glass in a green house, they trap heat and keep the Earth snug and warm. Carbon dioxide is the main greenhouse gas, and most of the increase comes from burning coal, oil, and wood, but methane from rice fields and rubbish dumps, and choloro-flourocarbons(CFC) from aerosol sprays and refrigerators also contribute to the greenhouse effect. If the Earth becomes a few degree warmer, polar ice will melt, drowning such low-lying cities as Sydney, London, New York. Certain knowledgeable scientific sources now believe that global warming is of cyclic nature, is due to solar hyper activity over certain periods and that effect due to green house gases yet to be confirmed.

Most meteorologists predict the world will warm between 2-4^oC by the year 2030, unless we take drastic steps to cut down the greenhouse gases. Some estimates are even more threatening as shown in Fig 7.

The World’s weather has not always been the same. Since the Earth and its atmosphere cooled some four billion years ago, its climate has gone through many changes, some lasting a few years, others lasting hundreds of thousands of years. By far the most dramatic changes occured between cold periods (Ice Ages or glacials), and warm periods (interglacials). In the last Ice Age, the weather was so cold that polar ice sheets grew to cover a third of the Earth in ice over 240 m thick. We now live in an interglacial period following the end of last Ice Age, some 10,000 years ago. Since then, there have been many minor fluctuations in the weather as shown in Fig 8.

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