Is this (image to the right) a hurricane or a Nor’easter? Your gut reaction may be “hurricane.” My gut reaction was hurricane! But we have been duped by Mother Nature - it is not a hurricane. It is a specific type of mid-latitude cyclone, called a Nor’easter - the very one that impacted the Northeast this past weekend on February 8-9.
If hurricanes and mid-latitude cyclones often physically resemble each other so closely and are both characterized by areas of low pressure and cyclonic (counterclockwise in the northern hemisphere) winds, what makes them different? Let’s have a look.
Where they originate: It all has to do with geography. Hurricanes most commonly are born in the tropical latitudes over the Atlantic Ocean, typically between 0-30° north latitude (in the northern hemisphere). They often begin as a small storm thunderstorm cluster moving off the coast of Africa or an area of low pressure known as an easterly wave in the Atlantic. They eventually find their way into the Caribbean, the Gulf of Mexico, the North America mainland or swing out to sea in the open Atlantic. Hurricanes are tropical.
If hurricanes are tropical, mid-latitude cyclones are extra-tropical meaning they form outside of the tropics. It’s in the name: “mid-latitude” cyclones meaning they form in the mid-latitudes, or 30-60° north latitude. You’ve probably heard of these before: the Alberta Clipper, the Gulf Low, the Cape Hatteras Low, the Colorado low (off the Rockies), and off course the Nor’easter. All of these names describe locations in the mid-latitudes.
Internal structure: While the physical structure of these two types of storm systems can be very similar, their internal structure is what makes them so different. Kind of like twins: they may look the same on the outside but can be very different on the inside (I can say this because I’m a twin! But I digress…) Hurricanes derive their energy through latent heat from the warm ocean making them warm-core lows.
Mid-latitude cyclones derive their energy from horizontal temperature contrasts when cold arctic air clashes with warm moist air, from the subtropics and/or tropics. They are inherently cold-core lows. Those who followed Hurricane Sandy closely remember the cold core vs. warm core debate as it made landfall (yesterday, the National Hurricane Center settled the debate - stating Sandy had transitioned to a more of a cold core or “post-tropical” system just over two hours prior to landfall).
Other important structural characteristics: Hurricane winds are strongest near the surface and decrease with height. Mid-latitude cyclone winds are strongest aloft, and decrease toward the surface.
Hurricane centers of circulation are characterized by sinking air which is why they often develop a well-defined and clear eye, whereas air rises in the centers of mid-latitude cyclones.
Finally, mid-latitude cyclones have associated surface boundaries such as cold fronts, warm fronts, occluded fronts, and stationary fronts often occurring simultaneously. Hurricanes have no obvious associated surface fronts.
Finally I want to comment on size: While hurricanes understandably get a lot of attention due to their ferocity and impact on popular coastal destinations, their size often pales in comparison to mid-latitude cyclones, or Nor’easters. Mid-latitude cyclones are the largest weather systems in the world often double, triple, or even quadruple the size of an average hurricane. Whereas hurricanes can impact multiple states at one time, a mid-latitude cyclone can effectively impact a third or more of the United States at one time.
Are there exceptions to the rule? Absolutely. Hurricane/Superstorm Sandy was a hybrid storm; as she transitioned from tropical to extra-tropical she retained hurricane characteristics but grew to the size more comparable to a mid-latitude cyclone.
Zoomed in (above), the Alaskan cyclone and Typhoon Jelawat look similar in terms of size. Zoomed out (below, not to scale), you can see the Alaskan cyclone stretching down the eastern Pacific Ocean while in comparison Typhoon Jelawat occupies a relatively small portion of the western Pacific Ocean east of the Philippines:
Conclusions:While mid-latitude cyclones may sometimes closely resemble hurricanes on the outside, their internal structure and origin sets them apart. It’s important to remember that even though mid-latitude cyclones are naturally much larger than hurricanes, we know they don’t always pack a bigger punch. Both types of storm systems embody wonders of nature that can be immense and beautiful but also destructive and deadly. Just remember: when looking at a satellite image, don’t judge a book by its cover!