The climate is typical of an arid or semiarid steppe, with cold winters anddry summers. The mountain regions of the northeast are subarctic with dry andcold winters. In the mountains bordering Pakistan, a divergent fringe effect ofthe monsoon, generally coming from the southeast, brings tropical air massesthat determine the climate between July and September. At times, these airmasses advance into central and southern Afghanistan, bringing increasedhumidity and some rain.
On the intermountain plateaus the winds do not blow very strongly, but in theSistan Basin there are severe blizzards that occur during the winter, generallyDecember through February. In the western and southern regions a northerly wind,known as the "wind of 120 days," blows during the summer months ofJune to September. This wind is usually accompanied by intense heat, drought,and sand storms, bringing much hardship to the inhabitants of the desert andsteppe lands. Dust and whirlwinds frequently occur during the summer months onthe flats in the southern part of the country. Rising at midday or in the earlyafternoon, these "dust winds" advance at velocities ranging between 97and 177 kilometers per hour, raising high clouds of dust.
Temperature and precipitation are controlled by the exchange of air masses.The highest temperatures and the lowest precipitation prevail in thedrought-ridden, poorly watered southern plateau region, which extends over theboundaries with Iran and Pakistan.
The Central Mountains, with higher peaks ascending toward the Pamir Knot,represent another distinct climatic region. From the Koh-e Baba Range to thePamir Knot, January temperatures may drop to -15 C or lower in the highestmountain areas; July temperatures vary between 0 and 26 C depending on altitude.In the mountains the annual mean precipitation, much of which is snowfall,increases eastward and is highest in the Koh-e Baba Range, the western part ofthe Pamir Knot, and the Eastern Hindukush. Precipitation in these regions andthe eastern monsoon area is about forty centimeters per year. The easternmonsoon area encompasses patches in the eastern border area with Pakistan, inirregular areas in eastern Afghanistan from north of Asmar to just north ofDarkh-e Yahya, and occasionally as far west as the Kabul Valley. The WakhanCorridor, however, which has temperatures ranging from 9 C in the summer tobelow -21 C in the winter, receives fewer than ten centimeters of rainfallannually. Permanent snow covers the highest mountain peaks. In the mountainousregion adjacent to northern Pakistan, the snow is often more than two metersdeep during the winter months. Valleys often become snow traps as the high windssweep much of the snow from mountain peaks and ridges.
Precipitation generally fluctuates greatly during the course of the year inall parts of the country. Surprise rainstorms often transform the episodicallyflowing rivers and streams from puddles to torrents; unwary invading armies havebeen trapped in such flooding more than once in Afghanistan's history. Nomadicand seminomadic Afghans have also succumbed to the sudden flooding of theircamps.
The climate of the Turkistan Plains, which extend northward from the NorthernFoothills, represents a transition between mountain and steppe climates. Aridityincreases and temperatures rise with descending altitudes, becoming the highestalong the lower Amu Darya and in the western parts of the plains.
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