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Student Dude: But wait a minute. Way back when, you told us that for a surface low, winds converge into the low center because of friction. Remember that? I do. low center surface winds converging into a low center Student Dude: But wait a minute. Way back when, you told us that for a surface low, winds converge into the low center because of friction. Remember that? I do. Wouldn't this convergence increase the mass in the column? And wouldn't that prevent the low pressure from strengthening? What's the deal, Professor Dude? Student Dude: But wait a minute. Way back when, you told us that for a surface low, winds converge into the low center because of friction. Remember that? I do. Wouldn't this convergence increase the mass in the column? And wouldn't that prevent the low pressure from strengthening? What's the deal, Professor Dude? Professor Dude: Excellent questions, student dude. But remember, it's really the net convergence or divergence that matters. Which means we also have to think about upper levels......maybe there's divergence up there. ● ● Surface high and low pressure systems are associated with ridges and troughs at upper levels And these ridges and troughs are associated with convergence and divergence patterns. Specifically..... upper-level trough surface low pressure system ● ● ● To the east of an upper-level trough, we generally find divergence And to the west of the trough, we typically have convergence So the question is: Where is the surface low relative to the ridge and trough? divergence ● ● ● To the east of an upper-level trough, we generally find divergence And to the west of the trough, we typically have convergence So the question is: Where is the surface low relative to the ridge and trough? convergence ● ● ● To the east of an upper-level trough, we generally find divergence And to the west of the trough, we typically have convergence So the question is: Where is the surface low relative to the ridge and trough? ● ● Well, as it turns out, for a developing system the upper-level trough is typically to the west of the growing surface low And this puts the low center directly below the region of upper-level divergence ● ● Well, as it turns out, for a developing system the upper-level trough is typically to the west of the growing surface low And this puts the low center directly below the region of upper-level divergence ● ● ● So.....the surface low center is more or less directly below the region of upper-level divergence...... …..which means the divergence at upper levels must be countering the convergence at the ground..... …..and net divergence means decreasing surface pressure and a strengthening system! ● ● ● …..which means the divergence at upper levels must be countering the convergence at the ground..... …..and net divergence means decreasing surface pressure and a strengthening system! ● So.....the surface low center is more or less directly below the region of upper-level divergence...... How conveeenient, you say. ● ● ● …..which means the divergence at upper levels must be countering the convergence at the ground..... …..and net divergence means decreasing surface pressure and a strengthening system! ● So.....the surface low center is more or less directly below the region of upper-level divergence...... How conveeenient, you say. But there's a good reason why the divergence is where it is. And it goes like this..... ● Remember that an upper-level trough is a region of lower heights on an upper-level pressure surface, which in turn means that the layer of air below the surface is contracted ● And a contracted air layer means colder temperatures ● So where in the cyclone are the temperatures the coldest? ridge trough WARM COLD ● Well.....the temperatures are coldest on the west (or back) side of the cyclone, where northerly winds bring colder air southward behind the cold front. The coldest temperatures are on the back side of the low center...... cold ● And as you might expect, the upper-level trough sits directly above this region of colder surface air, to the west of the low center..... ....which means the trough must be on the back side of the low center as well! cold ● …..which, in turn, puts the upper-level divergence directly above the low pressure at the ground! ....which, in turn, means the divergence must be directly over the low! cold ● And the more the cold air plunges southward behind the front, the deeper the upper-level trough gets. And the deeper the trough gets, the greater the divergence above the surface low. cold ● And the more the cold air plunges southward behind the front, the deeper the upper-level trough gets. And the deeper the trough gets, the greater the divergence above the surface low. cold Etc, etc, etc...... cold Summary: Development of a Mid-Latitude Cyclone upper-level pressure surface (heights) cold surface map (pressure & temperature) warm It all starts with a stationary front at the ground, separating warmer air to the south from colder air to the north. Summary: Development of a Mid-Latitude Cyclone DIV upper-level pressure surface (heights) cold surface map (pressure & temperature) warm Along comes an upper-level disturbance, bringing divergence on the front side of the trough. Summary: Development of a Mid-Latitude Cyclone DIV upper-level pressure surface (heights) cold surface map (pressure & temperature) warm The upper-level divergence reduces the mass in the column, causing lower pressure at the ground. Summary: Development of a Mid-Latitude Cyclone DIV upper-level pressure surface (heights) cold surface map (pressure & temperature) warm The surface air begins to rotate around the new low center, organizing into cold and warm fronts. Summary: Development of a Mid-Latitude Cyclone upper-level pressure surface (heights) DIV surface map (pressure & temperature) COLD warm As the cold front plunges southward, it brings even colder northern air on the back (or west) side of the low, directly below the trough. Summary: Development of a Mid-Latitude Cyclone upper-level pressure surface (heights) DIV surface map (pressure & temperature) COLD warm As this cold air moves in behind the front, the whole air layer contracts, and the upper-level trough gets deeper. Summary: Development of a Mid-Latitude Cyclone upper-level pressure surface (heights) DIV surface map (pressure & temperature) COLD warm And a deeper trough means stronger divergence at upper-levels. Summary: Development of a Mid-Latitude Cyclone upper-level pressure surface (heights) DIV surface map (pressure & temperature) COLD warm And as the upper-level divergence gets stronger, the low at the ground must get stronger also. Summary: Development of a Mid-Latitude Cyclone upper-level pressure surface (heights) DIV surface map (pressure & temperature) COLD warm And on it goes, amplifying through these internal feedbacks, until the system finally occludes. Once the system occludes, the warm air is cut off entirely from the low center at the ground. The trough at upper levels then moves directly over the low..... Once the system occludes, the warm air is cut off entirely from the low center at the ground. The trough at upper levels then moves directly over the low.....and the system starts to dissipate.