One of the problems that I have with learning (as opposed to just memorizing) weather is that it can sometimes be difficult to pick out the principles when looking at the real-world complexity of it all. Take today for example. On the SFC Prog Chart, I see a pretty strong region of lows over the Great Lakes, one off the East Coast, and a couple of weaker systems over the Midwest and West Coast. What’s the first thing we learn about lows? That they draw surrounding air into them, and that the inflow is deflected to the right because of Coriolis effect, causing counter-clockwise circulation around the system. Now take a look at the Visible Satellite loop. The system over the Great Lakes is a decent illustration of what happens around an area of low pressure. But from the perspective of a student just learning, how strong an impression would that low off of the East Coast make?
That’s where I got to thinking about how I’d teach this. Why not pick out an extreme example rather than trying to pick out something lame from the day-to-day weather? Take the weather picture from 4 years ago today–right about the time Hurricane Katrina was making landfall near New Orleans. A hurricane (aka, cyclone) is basically just a low pressure system taken to an extreme. They dominate the weather around them, and it’s easy to see what all a low does by looking at a hurricane. The pressure at the center of a hurricane (usually in the 800-1000 mB range) is well below any of the examples on today’s weather map. The first thing you’ll notice from the Visible Satellite image is the circulation around Katrina: air moves in along the isobars and breaks to the right to create the counter-clockwise swirl. This wind flow pattern is driven by warm, humid air rising from the surface at the center of the low.
From what I hear, stuck-wing drivers like departing out of a low since the rising air improves their climb performance…whether this is relevant for those of us who never climb above 1500 AGL, I couldn’t say. This rising air around a low also drives cloud formation as the moist air is pulled aloft and cools. Once again, Katrina illustrates this to the extreme: the spiraling bands moving toward the eye are lines of cumulus and cumulonimbus clouds. This is where the rule of thumb that bad weather is usually associated with low pressure systems comes from. (Although I’d call bullshit that the opposite is true–that highs mean good weather. I’ll post more on that this winter when there’s a high sitting over the Columbia River Basin, locking us into low IFR and icing conditions.)
So what happens to all the air that is pulled into the low? The rising air erupts out of the top of the low and spreads out from there. Apparently, with a hurricane, how the rising air is drawn away is an important factor in how powerful the storm will be–if the rising air isn’t being drawn off, it acts as a cap on the column of air at the center of the low, weakening the storm. In the upper atmosphere above the low, there is actually a reversal in the circulation. If you look on the northwest corner of the Katrina image, you can actually see this: there are high clouds that are spiralling clockwise. These are cirrus clouds that are formed from from moist air lofted into the upper atmosphere.
Any other thoughts or learning points for teaching about low pressure systems? Add them to the comments–I’ll be setting this up as my first community ground lesson soon.