A tropical storm (barotropical) is identified because of spiral rain bands radiating from the center, caused by the latent energy of the warm waters and the counterclockwise winds; in addition, the temperatures warm up as you approach the center, consistent with the above. A non-tropical low (baroclinical) is caused by temperature differences between two air masses, and the uplifting of the warmer air results in condensation (mostly ahead of the cold front on which the low has formed, or in back of a warm front; in addition, temperatures decrease as you approach the center). So, what is a "sub-tropical low"?
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How does an extra-tropical storm differ either from a baroclinic or sub-tropical storm? My guess is that it was once a tropical storm but has become baroclinic.