Climate and Weather
Pakistan lies on the western margin of one of the major climatic regions of the earth, the monsoon region. The winds in winter are north-east to south-west, and the reverse, south-west to north-east, in summer. The causes of the reversal of the wind system, and of the pulsating character of the monsoons, are varied and complex. The summer monsoon brings maritime influences and rain, but there are annual fluctuations or pulsations in the strength of the monsoon current. Cyclones in the monsoons cause rainfall, but their frequency is variable. Similarly, the paths of the cyclones vary with the position and strength of the Inter-tropical Front, and this again results in variable rainfall.
The climate of Pakistan is more ‘continental’ than that of other parts of the sub-continent, which come under a more typical monsoon regime. The rainfall in most parts of Pakistan is insufficient and its usefulness for agriculture is further reduced by its variable nature. Moreover, the efficiency of the rainfall is reduced because it takes place in the late summer months when, because of the high temperatures, much of it is evaporated.
Pakistan has the same four seasons found in the rest of the sub-continent, but their duration is somewhat different. In Pakistan the seasons may be distinguished as follows: Cold Weather Season (mid-December to March); Hot Weather Season (April to June); Monsoon Season (July to September); and Post-Monsoon Season (October to min-December).
Cold Weather Season
The cold weather season is charaterized by high barometric pressure, somewhat low temperatures, and small precipitation from shallow western disturbances. In the month of January, which typifies the climatic conditions of this season, mean pressure generally decreases from 1035 millibars in the north-west at Drosh to 1015.2 millibars in the south. The mean monthly temperature is below 40oF in the mountainous areas and varies from about 65oF in the south.
The generally fine weather of this season is occasionally affected by disturbances from the west, which form along the Mediterranean Front, and reach Pakistan after travelling across Iraq, Iran and Afghanistan. With the advent of the cool season, the incidence and intensity of these disturbances increase, and they move southward. The winds along the cold fronts of these depressions lower the temperature. The minimum temperature occasionally falls below freezing even in the parts of the plain, to 28oF in Lahore, for example. It is still lower in the Potwar Plateau, the cis-Indus plains, and the hilly areas.
Rainfall during the months December-March increases northwards and westwards. Over the middle and lower Indus Plain it is 1 inch or less, in the upper Indus Plain it ranges from 3-5 inches, and in the north and north-west, it rises to 10 inches or more.
Hot Weather Season
High temperature and aridity are the main characteristics of the hot weather season. With the approach of the season, the day temperature begins to rise. In May-June, it reaches its peak when, over large areas, the mean maximum daily temperature varies between 105oF and 114oF. Southern and south-western parts of Pakistan register higher temperatures than elsewhere. Jacobabad is the hottest place in the sub-continent with the highest recorded temperatures of 126oF in May and 127oF in June. The hill resorts and the Sind-Makran coast are areas of comparatively low temperature, but Karachi experiences short spells of high temperatures (108oF.) when winds from the Rajasthan Desert are drawn to low pressure troughs in the north Arabian Sea.
As the season approaches, pressure falls. A trough of low pressure begins to appear in April, when most of the Indus Plain has a mean pressure of 996 millibars. Relative humidity drops from about 50 per cent in the early morning to 25 percent or less in the afternoon. Rainfall is small, varying from 1 to 3 inches over the plains to 4 to 5 inches in the Himalayan sub-montane areas and parts of the Potwar Plateau. The rainfall is associated with the westerly disturbances, which have by now swung to more northerly latitudes, causing thunderstorms over the hills and widespread dust-storms over the plains.
The Monsoon Season
The establishment of low pressure over the Indo-Pakistan sub-continent in May and June attracts winds from the Indian Ocean, which ‘burst blowing’ over the land about the middle of June as the south-west monsoons. The monsoons gain in strength until July, remain constant to the end of August, and then begin to slacken. The monsoon-current reaches Pakistan about the beginning of July and is well established by the middle of that month. In some years, the monsoon remains active even in September.
The tropical cyclones or ‘lows’ formed along the Inter-tropical Front at the head of the Bay of Bengal move in a north-westerly direction over northern India and enter Pakistan. Their tracks vary with the position of the Inter-tropical Front. Some, after reaching the central parts of India or Rajisthan, re-curve north and north-westward. Others continue westward and bring rains to the lower Indus Plain.
The effect of the Arabian Sea branch of the monsoons over Pakistan is felt from the end of June. However, these monsoons penetrate only the coastal areas, and result in the formation of status clouds, with very little rain.
During the month of July, the mean monthly temperature exceeds 90oF over most of the Indus Plain and western Baluchistan. Pressure in the low, centered on Multan-Jacobabad, is 996 millibars. Average rainfall during the monsoon season in the Indus Plain decreases from 25 inches in the north, to 5 inches or less in the south.
The Post Monsoon Season
This season is a transitional period between the monsoonal regime and cool-season conditions, and is also known as “the season of retreating monsoons.” In October, maximum temperatures range from 94o to 99oF with 60oF as the normal minimum. There is a further fall of about 10oF in maxima and minima in November. The high pressure begins to establish itself over Pakistan in mid-November. The absence of any active wind system results in general dryness, and October and November are the driest months.
The Climatic Elements
Pakistan extends north-south over a considerable expanse of latitude (24oN to 37oN). This, together with the diversity of terrain, result in a diversity of temperatures at any given time. Seasonal differences in temperature are also substantial, due more to the high temperatures of summer than extreme cold in winter.
Temperatures in the hottest month are very high, except in mountainous localities. In the plains the hottest month is June, in the hill stations, July. In Hilly areas summer temperatures like winter temperatures, are influenced by altitude and the “face” of the terrain, and thus vary considerably from place to place. For example, the mean temperature of the hottest month is 7.8oF. at Drosh (4,806 ft.) 78.5oF. at Murree (7,445 ft.), 87.8oF. at Quetta (5,213 ft.) and 100.4oF. at Nushki (3,416 ft.). The difference between the extreme maximum and extreme minimum of the month is great.
In the PLAINS temperatures in the hottest months are uncomfortably high. The mean monthly temperature for June at most stations in the plains is 100oF or more. The extreme maximum generally rises above 117oF. Jacobabad is the thermal-pole of the sub-continent, where the mean maximum for June is 119.4oF., and the mean minimum, 90.7oF. It is probable that nowhere else in the world are there agricultural populations cultivating crops in such intense heat. The day-time heat is sometimes relieved by cooler nights, and the temperature at Jacobabasd can fall to 70oF. at night in June.
In the COASTAL AREAS the summers are milder. Karachi has a June mean maximum of 95oF. and a mean monthly minimum of 85oF. Stratus cloud cover reduces the daily maxima.
| Absolute Maximum and Minimum Temperatures
for the Hottest Month, June/July, in degrees F.
|Station||Absolute Maximum||Absolute Minimum|
Temperatures in January, the coldest month, are low in the Northern and North-Western mountains. Chitral has a mean maximum of 47.7oF. and a mean minimum of 29.7oF. in January the coldest month. These areas are snowbound until April. Stations in the western mountainous areas experience somewhat similar temperature in January.
|January Temperatures at Mountain Stations (degrees F.)|
|Station||Absolute Max.|| Mean Max.
||Absolute Min.|| Mean Min.
January temperatures in the upper Indus Plain are moderate and pleasant. At Lahore, for example, the mean maximum is 75.2oF. and the mean minimum is 50.7oF. These figures increase in the lower Indus Plain, reaching 87.3oF. and 64.8oF. in Karachi.
Pressure and Winds
In summer, the land becomes heated and a low pressure area is created in south-western Pakistan. In the month of July, atmospheric pressure (reduced to 32oF and mean sea-level) is lowest (994.7 millibars.) in the vicinity of Multan, and rises north-wards (Lahore, 996.0 millibars) and southwards (Karachi, 997.7 millibars). This low pressure area attracts winds from the Indian Ocean. As previously explained, some cyclonic storms migrate to this low all the way across northern India from the Bay of Bengal. Although their moisture content decreases as they move westward, it is these storms which bring most of Pakistan’s rainfall. Winds sucked in from the Arabian Sea bring less moisture because these air streams have originated over Arabia, and have a lower moisture content. Nevertheless, they do produce some rain in the western mountains.
In winter the temperatures over the land are relatively low, and a high pressure area is established. The pressure generally decreases from north to south. In January, the pressure is 1022.6 millibars at Peshawar, 1017.4 millibars at Lahore, and 1017.3 millibars at Karachi. Thus, while the prevailing direction of the winter monsoons over the sub-continent as a whole is almost from north to south. Because these winds blow from the land toward the sea, they are generally dry.
The mean annual rainfall of Pakistan is 40 inches or more in the northern mountainous region (Murree, 64.6 inches). Local variations, characteristic of highly differentiated terrain, are recorded in this area. Areas in the extreme north-west, largely sheltered from the monsoonal effect, receive only 20-25 inches (Chitral, 23.1 inches; Drosh, 25.9 inches). The Himalayan Piedmont receives 30-40 inches and the 20-inches isohyet (line joining places receiving the same amount of rainfall) lies somewhat north of Lahore (19.3 inches), veering north-west. The amount of rainfall decreases sharply toward the southern part of the upper Indus Plain. It is less than 5 inches in the Indus corridor, and the northern parts of the lower Indus Plain (Sukkur, 3.6 inches). The Kachhi-Sibi re-entrant is one of the driest areas (Jacobabad, 3.5 inches; Sibi, 5.6 inches). The rainfall again increases southward toward the coast (Hyderabad, 6.1 inches; Karachi, 8.2 inches). On the Makran Coast, it is over 5 inches (Pasni, 5.2 inches; Lasbela, 7.7 inches), increasing over the central ranges of Baluchistan (Quetta, 7.7 inches; Fort Sandeman, 15.0 inches). Elsewhere in Baluchistan, it varies from less than 5 inches (Nokkundi, 1.95 inches) to about 10 inches (Panjgur, 4.8 inches, Kalat, 9.2 inches).
With the exception of some areas in the north and north-west, rainfall is concentrated in the three months (July to September) of the summer monsoon. Table 4 shows summer monsoon rain as a percentage of total rainfall for selected stations. Winter rainfall (December to March) is much smaller in amount.
| Summer Monsoon Rainfall
(July – September)
as a Percentage of Total Rainfall
Variability of Rainfall
The rainfall of Pakistan, like that of some other marginal areas of monsoon climate, is markedly variable in the amount and timing of its incidence, and in its areal distributions. Over a large area of Pakistan most of the rainfall is associated with monsoon depressions. A secondary source is the passage of western disturbances. In the coastal areas, tropical storms from the Arabian Sea, and thunderstorms associated with thermal instability produce some rainfall. Each of these sources of rainfall is in itself of a variable character.
The monsoon activity is of a ‘pulsating’ nature. The monsoon blows in ‘intermittent bursts’. Areas of Pakistan receiving 50-75 percent of their rainfall from the monsoon have above normal variability. The above-normal variability increases from north-east to south-west over Indus Plain. Below-normal variability is indicative of the comparatively steady influence of the western disturbances, and occurs where winter precipitation is 50-100 percent of the annual total. The tropical storms from the Arabian Sea area are markedly variable in their incidence, and have erratic paths. They contribute to high variability of the coastal strip. Jacobabad, the area of greatest in temperature, also registers the greatest fluctuation between average and absolute maximum rainfall.
The mean annual number of rainy days varies from 89 at Murree to less than 10 (Sukkur, 6.5). It is obviously larger at wetter places, and in localities receiving rainfall in both the winter and summer months.
Efficiency of Precipitation
It has been shown that rainfall over most of Pakistan is low, markedly variable in character, and occurs mainly in the summer months when temperatures are high. High temperatures cause great evaporation and transpiration (giving out of moisture by the leaves of trees and plants). Thus, the unusefulness or efficiency of the rainfall for plant growth is reduced.
A measure of the efficiency of precipitation, known as the ‘moisture index’, has been devised by Thornthwaite. When applied to Pakistan, Thornthwaite’s formula reveals that, with the exception of a narrow strip of land along the N.W.F.P. – Kashmir border and a small area around Parachinar. Pakistan has a negative moisture index. This indicates that aridity or insufficiency of moisture for plant growth is a basic characteristic of Pakistan’s physical environment.
A critical isopleth (line joining, places having the same moisture index, temperature and rainfall) dividing semi-arid from arid areas, runs from south of Lahore to north of Peshawar, and thence to a point north-west of Quetta. To the north of this line, moisture conditions are semi-arid to dry sub-humid. The vast area to the south of this line is arid. Aridity is highest in two distinct areas, the desert lowland of north-western Baluchistan, and the area around Jacobabad.
A more detailed examination of the moisture balance, by months, indicates that only in a few rainy hill areas, such as Murree, there is no month of water deficiency. In the dry areas, water deficiency is commonly experienced for 9 to 10 months and, in some cases, 12 months.
A detailed scheme of climatic divisions for Pakistan is as such: The four major divisions are : Sub-Tropical Continental Highlands, Sub-Tropical Continental Plateau, Sub-Tropical Continental Lowlands, and Tropical Coastlands.
Sub-Tropical Continental Highlands
Sub-tropical continental highlands include the outer and middle Himalayas, the north-western hills (including Chitral, Swat, Waziristan, Zhob and Loralai), and the Baluchistan hills (Quetta, Sarawan, Central Makran and Jhalawan). They are characterized by cold, snowy winters, cool summers, winter and spring rains, and frequent fog.
The outer Himalayan area, which includes Murree and Hazara, receives rainfall throughout the year, with two maxima, one in late summer, and the other in spring. In the rain-shadow of the outer Himalayas, precipitation decrease. In Chitral and Swat, the summer rains become scanty, and bout two-thirds of the annual total falls from December to April. In the Kohat and Waziristan areas, the rainfall generally decreases toward the south. In the Zhob-Loralai area, both winter-spring and summer rains diminish, producing a total of about 10 inches. Quetta and the Sarawan area have a dry climate, with a mean annual total of 5-10 inches, occurring mostly in winter and spring. North and north-west winds, known as gorich, blow from October to February, and are piercingly cold. In the Makran-Jhalawan area rainfall is still lower, under 5 inches a year. From Kohat south, the annual temperature range is pronounced : the winters are cold and the summer hot.
Sub-Tropical Continental Plateau
This embraces north-western Baluchistan and is markedly dry and hot. Hot and dusty winds prevail almost continuously from mid-May to mid-September. Most of the scanty rainfall takes place in January and February (Nokkundi, 1.95 inches). Extreme heat, dryness, and dust are the chief characteristics of this climatic division.
Sub-Tropical Continental Lowlands
These include the entire Indus Plain, with the exception of the coastal areas. The climate is characterized by high summer temperatures, aridity, and late summer monsoon rains. The annual range of temperature is high. The northern submontane area and the Potwar Plateau are wetter than the rest of the Indus Plain and receive more winter rain. The Thal Desert, the Kacchi-Sibbi Plain and the south-eastern desert are the driest areas. Thunderstorms are a prominent feature, especially in the Peshawar Plain, and dust storms are frequent during summer.
The tropical coastlands are dominated by sea breezes throughout the summer. The annual and diurnal temperature ranges are low and humidity is high. The mean annual temperature is over 90oF., and the rainfall over 7 inches. May and June are the hottest months, with a secondary maximum after the cloud cover dissipates in October. At Karachi, relative humidity exceeds 50 percent throughout the year, and 80 percent at night from April to October. From May to September, it is least 60 percent during the day. The Lasbela coastal plain with rainfall maxima in both summer and winter, is the transitional area between the Makran coast and the Karachi-Sind coastal belt. Westward, most of the rain takes place in winter, while from Karachi north-eastward most of the rain occurs in summer.