Area Climatology — Page 433, Lesson 401

I want to walk you through the area climatology of the Mediterranean region, starting with how depressions form there. Let's begin with thermal depressions. When cold air from the Siberian High flows over a relatively warm landlocked sea area — like the Mediterranean — instability or thermal lows are created. Over the Mediterranean, these form particularly in central and eastern areas and then move eastwards toward Arabia, the Arabian Gulf, Iran, and Afghanistan. Next we have polar front depressions. Polar front lows — and sometimes secondary lows — can enter the region via SW France or Gibraltar. After they enter, they tend to become absorbed by other depressions. Now let's look at cloud and precipitation. Cold fronts associated with Mediterranean front depressions — also orographic and thermal depressions — produce CU and CB, which are cumulus and cumulonimbus clouds. These bring attendant heavy rain or hail showers and thunderstorms. There is also some layer-type cloud and more continuous rain in association with the few polar front depressions in the west. Moving to visibility: radiation fog is less common here than in NW Europe, but it can be persistent in the Po Valley in North Italy. Otherwise, visibility is excellent between showers — except when air blows from the south, bringing dust-laden air from the Sahara Desert. These southerly winds have different local names: they're called the Sirocco in Algeria, the Chili in Tunisia, the Ghibli in Libya, and the Khamsin in Egypt. They blow ahead of depressions travelling east over the sea. Now for surface winds. Surface winds blow in accord with the location of depressions, but there are some named winds blowing into the Mediterranean from surrounding land areas that you should note. First is the Mistral. This is a strong northerly wind up to 70 knots, blowing down the Rhône valley in SE France. It occurs especially when high pressure is to the north. It's a valley wind, normally stronger at night and in winter, and it brings cold air from the north. The Mistral helps form the Genoa orographic low. Next is the Bora. This is a stronger, dry, gusty NE wind up to 100 knots, which is part valley wind and part katabatic wind. The wind blows through the mountain passes into the Northern Adriatic and can be reinforced by high pressure to the NE. It can bring snow and is strongest at night. It can set in suddenly and is therefore dangerous. It can help form the Adriatic orographic low. Then we have the Gregale. This is similar to the Bora but less strong, further south, and more moist. It blows from the NE near southern Italy and Malta. It's due to continental relatively high pressure to the north and low pressure over the Mediterranean Sea. It occurs in 1-2 day spells in association with Mediterranean depressions to the south which are moving eastwards. Finally, the Sirocco — also spelled Scirocco. This blows out of Algeria and the Sahara Desert into the western Mediterranean ahead of travelling Mediterranean lows, and can carry dust up to 10,000 feet. The Sirocco can sometimes continue northwards to France; while in transit it will be cooled and humidified by the sea and can thus cause advection fog and/or low stratus along the South French Coast.