Air Masses — Page 313, Lesson 298

I want to walk you through the air masses chapter now, and we're picking up with the Mediterranean Front. The Mediterranean Front is the boundary between two specific air masses: Polar Continental or Polar Maritime air coming down from Europe, and Tropical Continental air coming up from North Africa. So it's a frontal zone that separates cold or cool air from the north from hot, dry desert air from the south. This front extends west to east across the middle of the Mediterranean Sea, all the way to the Caspian Sea. One important thing to note: the Mediterranean Front disappears entirely in summer — it's a seasonal feature. Next we have the Intertropical Convergence Zone, which you'll often see abbreviated as ITCZ. The ITCZ is the broad zone of separation between the air masses on either side of the heat equator. In other words, it's the region where the northeast trade winds and the southeast trade winds converge — they come together from opposite hemispheres. The ITCZ is subject to large seasonal movement over land, but much less movement over the sea. It's sometimes called the Thermal Equator or the Equatorial Trough. We'll discuss the ITCZ in much more detail in Chapter 20, but for now, understand it as the major global boundary between tropical air masses. Now let's move into frontal factors — the general principles that govern how fronts behave. Fronts in a locality are named either warm or cold, depending on whether warm air or cold air is replacing the other at that location. All fronts have a slope with height, meaning that if you look at a front from the side, it's a sloping surface — the cold air sits like a wedge underneath the warm air. Now, while fronts are normally associated with convergence and ascending air, which gives a lot of cloud and bad weather, it is actually possible for air masses to flow side by side with very little interaction. So not every front produces significant weather. The factors that determine whether a front is active or inactive are equilibrium and convergence. Let's start with equilibrium. The Pressure Gradient Force, or PGF, is directed towards the front from both the cold side and the warm side. Under these conditions, the wind would be geostrophic — that means it blows parallel to the isobars, and therefore parallel to the front. When that happens, the frontal surfaces are in equilibrium, with no tendency for the cold air to undercut the warm air. shows this equilibrium state, where the polar front is parallel to the isobars and the geostrophic winds are parallel to the front. In this case, the front is called a quasi-stationary front. These fronts are relatively inactive because there is little convergence — the air masses are just sliding alongside each other without much lifting. Now for convergence. There is always some convergence in any depression, but normally this convergence is small and gives only light precipitation and thin cloud. So for extensive cloud and heavy precipitation to occur, there must be an unbalancing of the equilibrium. That unbalancing causes lifting of the warm air and undercutting by the cold air. In other words, the cold air actively pushes underneath the warm air, forcing it to rise vigorously — and that's what produces the thick cloud and heavy rain we associate with active fronts.