Question about height falls

[David Draun]

I've noticed in forecast threads for today's soon-to-be historical tornado outbreak, some posts mention height falls. When I think of descent in the atmosphere, I think of stable conditions. So my question is, what is meant by "height falls" and how do they contribute to thunderstorm development?

 

[Bob Schafer]

Heya David!

Others could answer this better, but I'll give my interpretation anyway:

My understanding is that height falls refer to the mandatory pressures falling with an approaching cyclone. i.e., as cyclone approaches (any given geographic location), the height of 700mb may drop from, say, 3150 meters to 3000 meters, 500mb from 5800m to 5700m, etc.

I'll leave the question of how that causes severe wx to someone else who can surely answer it better than me.

Bob

 

[Michael P. Morris]

Bob's right - height falls mean exactly that, heights of mandatory levels falling. It implies ascent ahead of an approaching upper level disturbance, though, not descent (it seemed kind of counterintuitive when I was first learning QG theory so I understand your confusion David). Recall the definition of an upper level trough - it's a center or axis of low heights. Therefore if heights are falling, the trough is approaching and you will get large scale ascent and frequently, surface cyclogenesis. In the case of today, when the height falls are as intense as they are (210 meters at H5!) rapid cyclogenesis can occur. Similarly, if heights are rising it implies high pressure is building in.