Iran under influence of various atmospheric systems
Special geographic location of Iran affects its climate condition. As a result, weather condition of Iran is stable in summer, when it is under the influence of subtropical high-pressure system, and unstable in winter, when it is under the influence of extratropical atmospheric systems.
About 35.2% of the area of Iran has hyperarid, 29% arid, and 19.9% semi-arid climatic condition. According to climate classification of Iran, aridity has an increasing spatial trend from north to south and west to east. Precipitation is also decreased in a spatial trend from north to south and west to east as temperature is increasing to the east and south, except over high mountainous areas.
Air masses affecting Iran
Dependent on different seasonal conditions, Iran is influenced by various air masses from different directions.
Some of these air masses perform more actively and affect the climatic conditions. One of the parameters affecting the air masses is orographic features and alignment of topography. This can make a rise in precipitation and divert the flow direction. The air masses influencing Iran can be categorized into two types of winterdominated and summer-dominated air masses:
Winter-dominated air masses
Siberian high-pressure system: The subpolar continental air mass flows from the Siberian high-pressure center penetrate over Iran. Some tributaries of the cold airflow come often to Iran from northeast and dominate all over the country. This air mass provides a cold and dry weather condition with clear sky. The Siberian high-pressure system is dominant over Asia from October to March. The pressure center is so extensive that it greatly influences climate of Eurasia in cold months of the year.
Continental polar (cp): The air mass is moving into the country from northern regions of Iran. Expansion of Siberian high-pressure system over northeast Iran is a canal by which the
air mass is moving over Iran territory. It usually brings dry cold or severe cold air for the region.
Continental arctic (ca): The air masses coming from high polar marginal areas are conducted into Iran by troughs of the westerlies and form very cold and dry air. The directions from which these air masses are entering into the region are northeast and northwest. They often reach the middle and south areas of Iran.
Maritime polar (mp): In the north areas of Atlantic Ocean around Iceland, the main sources of the air masses are formed and moved over Iran. Passing over Mediterranean Sea makes the mp air masses warm and more humid. Most of the winter rainfalls on Iran are resulted from these air masses. The directions from which they are coming over Iran are frequently the western borders of Iran.
Maritime tropical (mt): The mt air mass is predominantly originated from the cyclone developed on the Red Sea. It is coming to Iran through southern arms of the westerlies. It gains moisture from the Persian Gulf and strengthened by leeward cyclones of Zagros Mountains. It usually makes orogenic rainfalls as moving from the Zagros Mountains.
Summer air masses
Continental tropical (ct): According to atmospheric general circulation, as the sun moves to the Northern Hemisphere in summer, the westerlies recede. At this time, the subtropical highpressure belt develops to upper latitudes and becomes dominant over the Middle East. As a result of the general circulation pattern, the air descends toward the Earth surface and impedes any kind of air upward movement from the surface.
This makes a high-pressure center called Subtropical High Pressure (STHP) that allows no ascending flow of air. The performance of adiabatic heating and stability provides a warm, sunny, and dry weather conditions for the system.
The heat and dry situation is the most prominent characteristic of the air mass that generates stable condition over Iran during summer time. Warming of the Earth surface in summer makes a shallow layer of local low pressure on the lands of Iran, the local pressure center can never ascend because it is repressed by the descending
STHP. These circumstances make the occurrence of any kind of precipitation impossible in the region.
The clear sky in summer and general circulation patterns in the region create a particular energy balance. The balance of energy in warm periods of the year accompanied with the warming of continental areas generates an extensive low-pressure system called Asian low pressure. The low pressure covers Pakistan, southern Iran, Persian Gulf, and Saudi Arabia. At the same time, a prevailing low-pressure pattern is developed on Indian Ocean and Arabian Sea. Therefore, weather conditions in the Iranian Plateau in spring and summer are affected by the two pressure systems.
Air masses from Saudi Arabia and Sahara:
Warm and dry unstable air masses from Saudi Arabia and Sahara are flowing toward the arid areas of Iran. It is sometimes called local flows.
The weather over African Sahara is heated by perpendicular sunshines in summer and the warm air is flowing toward Iran due to pressure differences and general circulation effects. The essential properties of the air mass are warm and
dry weather conditions.
Factors affecting aridity of Iran
Aridity in the Iran is resulted from some parameters including the location of the Iranian
Plateau, general atmospheric circulation, rain shadow, continental condition, and distance from ocean as the source of moisture. Most areas of Iran are affected under the dominance of STHP during the warm season. The dominance of such weather systems prevents westerlies to enter into the area. In other words, the STHP in summer in Northern Hemisphere is shifted to higher latitudes.
This shifting replaces the position of the westerlies as the major factor for bringing the humidity and instability to temperate regions. In winter, STHP goes back to lower latitudes and westerlies can penetrate into interior of Iran with seasonal precipitation. The northern part of Alborz Mountains (southern part of the Caspian Sea) has a completely different climatic condition and mainly covered by forests and shrubs. The stretched strip coastal shore has usually humid climate.
Topographic condition in west and rain shadow are the major factors that help develop arid conditions in Iran, especially those areas in the eastern part of mountainous areas with less moisture from west or southwest. Zagros and Alborz mountains are very high and extensive mountains which act like a natural barrier preventing the moisture reaching central Iran. Thus, since the mountains serve as high walls in the western and northern boundaries of Iran, respectively, the air flows moving toward the interior areas of Iran leave their moisture in windward section of these natural walls and the dry air masses descend on the interior Iran. This can increase the aridity of the eastern and central parts of Iran by rain shadow effects.
Distance from ocean and major water bodies can also magnify the aridity in the region. Most areas of Iran are far from oceans and moisture sources, especially those in the eastern side of Zagros Mountains and southern side of Alborz Mountains. Combination of these two factors of rain shadow and long distance from moisture sources can overwhelmingly intensify the aridity condition in central Iran, especially in Yazd, Kerman, Sistan and Baluchestan and Semnan provinces. Arid climate of the area is strongly affecting flora and fauna in the region as well as human activities.
The above is a lightly edited version of a chapter of ‘Desert Landscapes and Landforms of Iran’ written by Mehran Maghsoudi and published by Springer in 2020. The photos originally appeared in the book.