Area Climatology — Page 439, Lesson 405

I want to walk you through the winter climatology of Arabia, the Gulf Area, the Arabian Sea, and the borders enclosed within 15°N to 35°N and 35°E to 75°E. This is a defined region, and we’ll look at its geographical setting first, then the pressure systems, cloud and precipitation, surface winds, visibility, and upper winds — all for the winter season. Geographically, the inland areas of Iraq, Saudi Arabia, and Oman are largely desert. The Tropic of Cancer at 23½°N almost bisects the region, meaning that in summer the noon sun is virtually overhead, but even in winter the geography matters. The daytime interior is extremely dry and hot in summer, but the key point here is that surface wind direction is generally governed in the west by the northwest-to-southeast axis of the Zagros mountains in western Iran, and in the east by the Himalayas. Now, let’s move into winter pressure systems. The Siberian High is established over Asia to the northeast, but its surface outflow affects this region. That means cold, dense air from the Siberian High pushes outward and influences our area. Additionally, thermal lows — areas of low pressure caused by heating — often with associated cold fronts, travel eastward from the Mediterranean across Arabia into Iran and Afghanistan. So we have two main players: the outflow from the Siberian High, and travelling lows from the Mediterranean. There’s an important detail about the Siberian air: when Siberian dry cold frontal air passes over the relatively warm Caspian Sea to the north, it can initiate considerable thermal instability. That means the cold air sitting over warmer water becomes unstable, which can lead to cloud development and precipitation. Cloud and precipitation in winter are restricted to the north and west only. The travelling lows from the Mediterranean will produce cumulus and cumulonimbus — CU/CB — heavy showers, and thunderstorms. Their associated cold fronts may bring some weather and duststorms to the more southern interior of Arabia. And that thermal instability over the Caspian Sea I just mentioned can bring thunderstorms, hail, and possibly snow to high ground to the north. Surface winds in winter are shaped by the terrain. In the west, the Zagros mountain range and parallel ridges are oriented northwest-to-southeast. These block and deflect the cold Siberian outflow so that the surface winds become north or northwesterly. In the east, the Siberian outflow escaping round the western end of the Himalayan air block will again be northerly. So the dominant winter surface wind direction across the region is from the north or northwest. The exception is a temporary southerly wind which occurs ahead of the travelling depressions from the Mediterranean — so before those lows arrive, you get a brief southerly flow. Visibility in winter is generally much better than in summer convection currents. But in the northwest of the region, rising dust can occur with any wind direction, and especially in the southerly winds ahead of the travelling ex-Mediterranean lows. Violent but short-lived duststorms may accompany the passage of associated cold fronts. Overland near coasts where humidity is high, radiation fog may form, but dispersal is quick after sunrise. Upper winds in winter are westerly. The 200 hPa subtropical jet covers the west of the region and may extend as low as 300 hPa, which is about 30,000 feet. Core speeds frequently exceed 100 knots. Icing is not a problem except when climbing or descending through large cumulus or cumulonimbus clouds. Let me show you the winter chart so you can see how these features come together on the map. That covers the winter picture for this region. In the next part we’ll look at summer, where the pressure systems and winds change dramatically.