Now, the Intertropical Convergence Zone, or ITCZ, just reaches Oman in the… — Page 439, Lesson 407

I want to walk you through the summer weather pattern over the Asian continent, specifically the area around the Arabian Sea and the Middle East. This is a region of intense heat and very distinct seasonal changes. We start with a thermal low-pressure system that centres on the area of Baluchistan, which lies across the Iran/Pakistan border. The reason this low forms here is due to the mainly south-facing rocky nature of the surface, combined with intense solar heating from the high noonday sun. That rocky surface absorbs heat very efficiently, and the sun is nearly overhead, so the ground gets extremely hot, heating the air above it and causing it to rise, creating a thermal low. Now, the Intertropical Convergence Zone, or ITCZ, just reaches Oman in the west. In the east, its movement is seasonal: it traverses the northern Arabian Sea northwards in June and July, and then southwards during September. So the ITCZ migrates north in early summer and retreats south in early autumn. Let's talk about the thermal low pressure in more detail. Over Eastern Pakistan and Northwest India, the period from March to June is dry. As spring advances, the land mass begins to warm, so pressure begins to fall. This falling pressure draws in warm, moist air from the Arabian Sea in response to the pressure gradient. First, this inflow happens at lower latitudes, then progressively further north. The ITCZ follows this movement. Cyclones occur over the Arabian Sea during the advance and retreat of the ITCZ, between June and September. So the period when the ITCZ is moving north and then back south is the cyclone season over the water. Now, let's look at the surface winds. In the west, there is anticlockwise rotation around the Baluchistan Low. That anticlockwise rotation is modified by the Zagros mountains, which have a northwest-to-southeast orientation. This gives a northerly or northwesterly wind. In particular, the shamal wind originates as dry and dusty convection currents from Iraq. It is northwesterly and blows the length of the Gulf, picking up moisture and bringing a dusty, humid wind to facing coasts around Bahrain and Dubai. So the shamal starts dry and dusty, but as it travels over the Gulf, it picks up moisture, making it humid by the time it reaches those coastal areas. Moving on to cloud and precipitation. Most of the region is almost rainless, and temperatures can exceed 50°C. Inland areas are very dry, but Gulf coastal regions facing the north or northwesterly onshore winds can be oppressively humid. An exception is the southeast of Oman. The ITCZ reaches the coast there, and where it does, the desert terrain temporarily "bursts into bloom." Further east, towards the Indian coast, the northward movement of the ITCZ is followed by the onset of the southwest monsoon. The monsoon is very moist and convectively unstable. Orographic and convection cloud and heavy rain are widespread. Regarding visibility, the north and northwesterly winds can cause much dust in the desert regions. The shamal will bring dusty, moist air to coastal areas. Visibility will also be reduced in regions affected by the ITCZ, likely due to the cloud and precipitation. For upper winds, above 20,000 feet, winds are light easterly. So at typical cruising altitudes in summer, you'd expect light winds from the east. Icing should not be a problem in summer. Finally, let's look at the average tropopause and freezing level heights. In January, the tropopause is at 38,000 feet, and the freezing level is at 11,000 feet. In July, the tropopause rises to 56,000 feet, and the freezing level rises to 16,000 feet. So in summer, both the tropopause and the freezing level are significantly higher, which makes sense given the intense heating.