365 Days of Climate Awareness 35 - Air and Water Masses

Air masses, or air parcels, and and water masses are large volumes or air or water which move as a coherent body, changing shape but still maintaining observable boundaries. Air masses can range from hundreds to thousands of miles in extent, while water masses range from tens to thousands of miles in extent. Both air and water are fluids, so they behave in broadly similar manners, but there are very important differences.

Examples of air masses.

Water is roughly 800 times as dense as air, which affects nearly every aspect of its behavior, starting with the typical speeds it moves with. High winds, such as in hurricanes, can move 32 m/s (74 mph), while a rapid ocean current, such as the tides of Cook Inlet, Alaska, move at 5 m/s (11 mph). Despite that, the much denser water has far greater momentum. A cubic meter of air at hurricane velocity has the same momentum as a cubic meter of water moving 4 cm/s!

A parcel of air or water tends to move with a collective momentum. Being a fluid, the mass can change shape if impacted by topography or a neighboring air or water mass, but it does not collapse. Two air or water masses which encounter each other will not merge and blend properties. They will by and large maintain their separate qualities, with some turbulence and mixing along the boundary, or front, between then.

In the atmosphere, the front between a warm and cold mass of air is frequently marked by thunderstorms, or precipitation of some sort. In the ocean, such a front is the site of some mixing and formation of eddies, or swirls. Another common process in the ocean is "entrainment", when a moving parcel of water--for example, the warm Gulf Stream moving north through the cold Atlantic--drags, or entrains, a large amount of neighboring water along with it, through friction.

That entrained water over the course of many hundreds of miles does mix with and become part of the original water of the current. The current becomes cooler, the entrained water warms up, and the overall volume of the former Gulf Stream becomes the North Atlantic Current, bringing five times as much water as it started with.

The Gulf Stream and North Atlantic Current, via thermal satellite imagery.

An air or water mass generally dissipates, except for stable, long-term formations such as the Polar High, a dome of cold, high pressure air over the Arctic, or Antarctic Bottom Water, a layer of very cold, very dense water which spreads northward through the Atlantic from its Antarctic source. After a low-pressure, warm storm system has spent its heat energy, the low pressure rises and the air temperature is equalized with its surroundings. Simiarly, the North Atlantic Current ends its life as cold water which is as cold or colder than the water around it (another topic we will revisit later). Friction and mixing processes along the front will gradually erode and eliminate it.

Weather and current systems are dominated by the dynamics of these very large bodies of air and water which move in more or less unitary fashion. We have already seen a bit of this in the post on thunderstorms, and will continue to observe it in future posts on ocean currents and atmospheric features such as the Polar Vortex.

Tomorrow: wind circulation

Be well!