Unless you're a weather geek you probably haven't heard about the subtropical depression that formed in the Atlantic Ocean today. It didn't make it in any of my weather segments today as it sure didn't pass the, "who the heck cares" test.
A subtropical depression is like a hybrid tropical-non tropical storm. Our nor'easters in the winter are non-tropical low pressures. Hurricanes like Gloria, Carol, 1938, and Bob are tropical lows. But like so much in meteorology it's really not cut and dry and there's a continuum or spectrum of lows - fully tropical, fully non-tropical, and somewhere inbetween. This is a somewhere inbetween storm. The biggest difference between tropical and non-tropical storms is where they derive their energy with the former gaining energy from warm ocean waters and the latter from processes including fronts, jet stream disturbances, and other such things.
Here's an official definitition of a subtropical system from the National Hurricane Center:
A non-frontal low-pressure system that has characteristics of both tropical and extratropical cyclones. Like tropical cyclones, they are non-frontal, synoptic-scale cyclones that originate over tropical or subtropical waters, and have a closed surface wind circulation about a well-defined center. In addition, they have organized moderate to deep convection, but lack a central dense overcast. Unlike tropical cyclones, subtropical cyclones derive a significant proportion of their energy from baroclinic sources, and are generally cold-core in the upper troposphere, often being associated with an upper-level low or trough. In comparison to tropical cyclones, these systems generally have a radius of maximum winds occurring relatively far from the center (usually greater than 60 n mi), and generally have a less symmetric wind field and distribution of convection.
If Subtropical Depression #1 manages to strengthen a bit more it will even get a name - Arlene! It's a nice looking swirl 830 miles west of the Azores that's no threat to land and expected to dissipate shortly. Besides the fish this thing is impacting no one.
As usual, there was a bit of complaining this morning on weather twitter about whether this system should have been declared, whether it would have been declared one in the past, and whether this whole thing is a waste of time. I fall on the side of sure - let's call it what it is a "subtropical depression" and even name it if it strengthens. But that doesn't mean it's worth spending time talking about - it's nothing more than a swirl of clouds in the middle of nowhere that manages to get a fancy web page and graphics built for it because the National Hurricane Center called it something.
So now you know what a subtropical depression is whether you've been curious or not.