The horizontal and vertical extension of the Persian Gulf pressure trough

The Persian Gulf Barometric Trough is an extension of the south‐west Asian Monsoon. It starts in the middle of June, stabilizing by the end of the month, remaining dominant during July and August and disappears very quickly in mid‐September. Because of its thermal origin its height is limited to about 1500 m. Above it dominates, during the summer months, the subtropical high‐pressure system with its Azores extension covering the Mediterranean region. The transitional zone between these two pressure systems is an inversion zone, whose lower base sometimes reaches the mountain tops of Israel and even lower. On such occasions the daily maximum temperatures are higher at the stations in the mountains than in the coastal plain. Because of its thermal origin, it was expected that diurnal changes of temperature must affect the horizontal and vertical extension of the Persian Gulf Trough. However, the diurnal fluctuations of the pressure are mainly remarkable at stations at the margin of the trough, while at those stations affected directly by it, the daily fluctuations are minimized by irregular periodic changes in the intensity of the Persian Trough occurring every few days. These changes are due mainly to fluctuations of the upper level high‐pressure system. Changes in the position of the Persian Trough influence greatly the daily weather in the Levant.