This is a sample lesson web page from the Certificate of Achievement in Weather Forecasting offered by the Penn State Department of Meteorology. Any inquiries about this regime can be directed to: Steve Seman


You are watching: In the northern hemisphere, hurricanes and middle latitude cyclones are similar in that both:

Upon completing this web page, you must have the ability to compare and comparison the standard framework and development of tropical and mid-latitude cyclones. Specifically, you must be able to talk about distinctions in vertical movement over the centers of mid-latitude cyclones and also hurricanes, and the implications of these differences in regards to temperatures, family member humidity, and surchallenge pressure. You must likewise be have the ability to specify vital parts of a hurricane"s structure (such as eye, eyewall, spiral bands, and also second circulation). Finally, you need to leave this page being able to summarize the standard feedback process that reasons tropical cyclones to intensify.


With the excellent potential for loss of life and also home posed by tropical cyclones, they absolutely garner great attention from weather forecasters and also the public at big. But, why perform effective tropical cyclones even more generally steal national and worldwide headlines, while mid-latitude cyclones rarely do? The first factor is likely that mid-latitude cyclones are more plenty of. Hundreds of them trek throughout the globe every year. At the same time, only about 80 tropical cyclones develop yearly.

Secondly, a tropical cyclone have the right to acquire a a lot higher intensity in regards to both sea-level push and also wind rate (some even contact hurricanes the "kings" of all low-pressure systems). For instance, the the majority of intense tropical cyclones deserve to have actually sea-level pressures listed below 900 mb. Typhoon Tip (1979) had the all-time lowest at 870 mb, but other storms such as Hurricane Wilma (2005) and Super Typhoon Haiyan (2013) have had central pressures below 900 mb. On the other hand, the sea-level press at the facility of a mid-latitude cyclone seldom drops listed below 950 mb. For instance, the Superstorm of 1993 (aka the "Storm of the Century"), had actually a central press of 963 mb at its optimal.


(Left) A large, sprawling mid-latitude cyclone centered near Lake Michigan demonstrated a acquainted comma shape on this visible satellite photo from May 11, 2003. (Right) Hurricane Rita (approaching Category 5 status) at 1610Z on September 21, 2005, lacked the well-characterized comma form of a mid-latitude cyclone. The visual distinctions of these 2 storms provides a clue that mid-latiude and also tropical cyclones run a bit differently.
Credit: NASA

With these monitorings in mind, a herbal question can be, "Why carry out strong tropical cyclones regularly achieve sea-level pressures that are notably reduced than those connected with mid-latitude cyclones?" While both forms of cyclones are low-press systems, the answer to that question deserve to discovered by researching the distinctions in framework and strengthening mechanisms characteristic of each type of low-push mechanism.

For starters, respeak to that mid-latitude cyclones undergo the process of self-development. This process requires the cyclone to build in an area of strong horizontal temperature gradients (a baroclinic zone, or front) and under an area of strong upper-level divergence. In short, divergence downwind of a 500-mb shortwave tstormy reduces the weight of air columns, forming an area of low pressure at the surface, approximately which winds rotate counterclockwise (in the Northern Hemisphere). Decreasing surchallenge pressures bring about a stronger press gradient pressure, which causes quicker winds. As winds about the cyclone increase, cold-air advection southwest of the low boosts, bring about 500-mb heights to fall and the 500-mb trough and vorticity maximum to strengthen. In turn, upper-air divergence increases over the center of the low, causing surface pressures to even more decrease and surface winds to increase further. This positive feedearlier loop continues uninterrupted till the late stperiods of occlusion, when the low moves back right into the cold air (away from the baroclinic zone) and also upper-level divergence over the low weakens (the low starts to "fill" -- surface push rises).

In a nutshell, the magnitude of the divergence aloft (which is higher than the magnitude of the convergence at lower altitudes) drives the intensity of the mid-latitude cyclone. Also note, but, that the divergence aloft in addition to low-level convergence drives upward movement over the center of the low. You"ll sometimes read or hear explanations that suggest that rising causes lower surchallenge pressures, but that"s just not true. In reality, just the opposite is true. Rising air actually functions versus the all at once reduction in surface push.

Recall that increasing air cools using growth, and when clouds and also precipitation build, deserve to likewise yield evaporational cooling (assuming the setting is not already at saturation). In revolve, cooling by compelled climb boosts the suppose density in the column of air that extends from the ground to the tropopausage (low-level convergence and upper-level divergence are still at work). Assuming a almost hydrostatic setting (in which the pressure of gravity is well balanced by the upward push gradient force), this boost in intend column density serves to add column weight. During the advancement stage of a mid-latitude cyclone, leading weight-loss processes, such as net column divergence and also warm advection near 200 mb overwhelmingly balance out the tendency for air columns to gain weight from adiabatic and moist adiabatic cooling. But my allude should now be clear: Rising air tends to make surface pressures greater, not reduced. In various other words, climbing air actually works against the deepening of a mid-latitude cyclone; it serves as a "inspect and balance" on the all at once intensity of the device.

Strong tropical cyclones, on the other hand, don"t have actually this "examine and also balance" over their centers. Undoubtedly, the predominant vertical motion over the center of a hurricane is downward. It"s that downward activity that creates the eye of the storm, as presented in the visible satellite image of Hurricane Isabel from 1404Z on September 11, 2003 (below).


Visible satellite picture showing the eye of Hurricane Isabel at 1404Z on September 11, 2003. Keep in mind that some areas of the eye are clear, although some clouds are present.

For the document, the eye is a around circular, fair-weather zone at the center of a hurricane. By "fair weather", I intend that bit or no precipitation occurs in the eye and also an observer looking upward in the eye can frequently check out some blue skies or stars. The diameter of the typical eye varieties from around 30 to 60 kilometers (about 16 to 32 nautical miles across), however eye diameters as small as four kilometers (roughly two nautical miles) and as huge as 200 kilometers (around 110 nautical miles) have actually been oboffered. For the record, Hurricane Wilma"s "pinhole eye" was the smallest rebuilt up by forecasters at the National Hurricane Center (two nautical miles) as the storm deepened to 882 mb (the lowest on record in the Atlantic Basin).

The "fair weather" in the eye deserve to mostly be attributed to the sinking air over the facility of the storm. The downward motion in the eye is just on the order of a few centimeters per second, which suggests that the main core of strong tropical cyclones is roughly hydrostatic. Given that the compressional warming in the eye decreases the mean density of the main column of air in the eye (and therefore its weight), we have the right to deduce that subsidence contributes to the low main pressures observed in hurricanes. Of course, as the sinking air warms, relative humidity decreases within the sinking parcels, which promotes the clearing observed within the eye.

I have to point out but, that the air does not uniformly sink within the eye of a hurricane. Dropwindsonde observations taken from the eye of a hurricane frequently disclose an invariation at an altitude of about one to 3 kilometers. To see what I intend, check out the representative temperature and also dew-point soundings retrieved from dropwindsonde measurements in the eye of Hurricane Jimena (1991). The subsidence invariation close to 850 mb is the telltale sign of downward activity in the eye of a hurricane, however the existence of this inversion suggests that air does not sink all the method to the ocean surconfront. The truth that air does not sink all the way to the surconfront defines why low clouds frequently exist in the eyes of hurricanes (although skies may not be completely overcast).

Regardmuch less of the truth that air does not uniformly sink throughout the entire eye, the compressional warming connected with the subsidence in the eye is one contributor to the "warmth core" of a hurricane. At the same time, deep, moist convection external of the eye (in the eyewall--the partial or complete ring of powerful thunderstorms around the eye, and spiral bands--reasonably lengthy and thin bands of convective rains) also contributes to the heat core.


A radar photo of Hurricane Ivan on September 7, 2004. Hurricane Hunters gathered the radar information in between 1920Z and also 1940Z throughout a reconnaissance trip. Keep in mind the bands of convection (yellow, oarray, and red shadings) spiraling inward towards Ivan"s eye.

The photo over offers you a "bird"s-eye" view of the standard framework of a hurricane on radar. Keep in mind the spiral bands (yellow, ovariety, and also red shadings) curving in towards the facility of the storm, and the eyewall nearly completely encircling the much lower reflectivity worths (dark green and blue) in the eye. How does the deep, moist convection in the eyewall and spiral bands contribute to the heat core of the storm? Ssuggest put, the air parcels increasing in thunderstorm updrafts are initially incredibly warm and also moist (as a result of evaporation from warm tropical seas). As these parcels rise in thunderstorm updrafts, expensive amounts of latent warm of condensation are released. Yes, air parcels cool as they climb, however the release of latent warm keeps them warmer than they otherwise would certainly be, which keeps the air within a hurricane warmer than air at the exact same altitudes outside of the affect of the hurricane. Weaker tropical cyclones are likewise heat core units because of the release of abundant latent warmth (also though weaker devices don"t have eyes--there"s no arranged compressional warming in the facility of the storm).

All in all, within a strong tropical cyclone, the heat core generated by latent warmth release and also compressional warming deserve to be fairly extensive. For example, inspect out the cross-area of satellite-detected temperature anomalies from Super Typhoon Haiyan at 1726Z on November 7, 2013 (below).


Cross-area of satellite-detected temperatures reflecting the heat core of Super Typhoon Haiyan on November 7, 2013 at 1726Z. The maximum warmth anomaly corresponds with the eye of the storm, via lesser warm anomalies extending hundreds of miles in either direction.

The core of the warm anomaly roughly corresponds via the eye of Haiyan, and at its top in the middle and also upper troposphere, temperatures were as a lot as 7 levels Celsius better than the atmosphere bordering the storm. Beyond the eye, the warm anomaly is weaker, yet still spans thousands of miles across the storm. Given the maximized warm core near the facility of the storm, it becomes clear that hurricanes create big horizontal temperature gradients internally (especially at the interconfront of the eye and also eyewall) in the time of their advancement, also though they initially develop in the weak horizontal temperature gradients that characterize the tropics. As you"ve learned, mid-latitude cyclones are just the opposite: They create in locations through huge horizontal temperature gradients, and also their circulations eventually act to minimize horizontal temperature gradients over time.

Sustaining Tropical Cyclones

Now that we"ve establiburned an essential difference in between tropical cyclones (which have actually a warmth core) and also mid-latitude cyclones (which carry out not, given that they are characterized by increasing activity over their centers and also commonly absence deep, moist convection near their cores), let"s revolve our attention to an additional vital aspect in the intensification of both mid-latitude and tropical cyclones--divergence aloft. You"re already familiar with the duty of divergence aloft in mid-latitude cyclones, gave primarily by 500-mb shortwave troughs and 300-mb jet streaks, yet divergence aloft plays an important role in tropical cyclones, too.

In order to help you visualize divergence aloft in tropical cyclones, enable me to present the second circulation of a tropical cyclone. As the name implies, tropical cyclones have actually two distinctive circulations. The primary circulation, as you could suppose, describes rotation of air roughly the center of the storm. But, there"s one more circulation going on at the very same time. In a simple feeling, low-level air flows in towards the center of the storm, rises in thunderstorms within the eyewall and also spiral bands, and also flows (mostly) outward aloft, sinking around the periphery of the storm. This basic circulation (in at the bottom of the storm, up, out at the top, and down around the storm"s periphery) is the additional circulation. To visualize this "in, up, and also, out" procedure in the context of a strengthening hurricane, check out the slidedisplay animation below.

The divergence aloft in a healthy tropical cyclone acts to better mitigate surface pressure by removing mass from air columns near the center of the storm. Eventually, hurricanes intensify as an outcome of a positive feedago loop, albeit a totally different one than the self advancement process for mid-latitude cyclones. One of the salient functions in the positive feedback loop for hurricanes is "scale interaction." In a nutshell, processes on the spatial range of convection (thunderstorms, for example) job-related to amplify changes on a larger spatial range (such as lowering surconfront air press in the eye of a hurricane). In revolve, amplification on the larger spatial scale amplifies convection (thunderstorms), and also the feedago loop is off to the races.

We"ll delve much deeper into the details later in the course, but below are the basics of the feedback: As eye-wall thunderstorms mushroom upwards and also intensify, the magnitude of the second circulation (and divergence aloft) becomes higher, as does subsidence and compressional warming in the eye. This all sets the stage for negative press tendencies close to the sea surconfront (surchallenge press decreases with time), which draws more low-level air inward to climb in thunderstorm updrafts, and the cycle continues. The key to keeping the whole procedure is sustaining organized deep convection approximately the core of the storm.

As you currently recognize, tropical cyclones operate fairly a little in different ways from mid-latitude cyclone, so make sure that you understand also the primary contrasts in between the 2 kinds of storms. To help you keep track of the major differences, below is a quick summary, highlighting the essential distinctions between mid-latitude and tropical cyclones.

Key Differences Between Mid-Latitude and Tropical CyclonesMid-latitude cyclones create in atmospheres via solid horizontal temperature gradients, while tropical cyclones create in atmospheres via weak horizontal temperature gradients (yet they create strong horizontal temperature gradients internally).Air rises over the facility of a mid-latitude cyclone, and also hence, cools, which functions against falling surconfront pressures. Over the centers of solid tropical cyclones, however, air sinks and warms via compression, which helps surchallenge pressures decrease.The release of latent warmth from deep, moist convection, and compressional warming from subsidence reasons tropical cyclones to have actually a warmth core. Mid-latitude cyclones, on the other hand also, lack a heat core.Mid-latitude cyclones depend on divergence aloft to drive decreases in surface push. Low surconfront pressures in tropical cyclones, on the other hand also, result from significant contributions from the warmth core of the storm (low column density) and divergence aloft using the second circulation.

By now, I hope you"re beginning to appreciate the distinctions in between the mid-latitudes and the tropics. But, we"re not done quite yet. Even the devices that tropical forecasters use are different! We"ll start via map projections following. You"ll quickly view that the map projections commonly used in the mid-latitudes do not work so well in the tropics!

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Mid-Latitude Cyclones with Eyes?

The centers of mid-latitude cyclones are typically fairly cloudy because of the upward activity that occurs there. However, some mid-latitude cyclones (specifically those over the oceans), actually exhilittle bit "eye-like" functions in the time of their mature phases. Such features sometimes come to be apparent once intense mid-latitude cyclones spin-up off the East Coastand aren"t actually true "eyes" prefer those in tropical cyclones. Instead, these cloud-complimentary regions in the facility of a mid-latitude cyclone are referred to as "warmth air seclusions." For example, an intense mid-latitude low off the shore of Long Island, NY, developed an eye-prefer, warm air seclusion at 15Z on April 16, 2007 (inspect out the 1515Z visible satellite picture below).


Warm-air seclusions deserve to resemble the eyes of tropical cyclones, however they absence deep convection bordering them. This effective mid-latitude cyclone off the East Coast of the U.S. included such a function, seen right here on visible satellite imagery from 1515Z on April 16, 2007.

Keep in mind that unprefer tropical cyclones, no thunderstorms were present about the facility of this eye-favor attribute (inspect out the 1515Z magnified infrared image for confirmation -- high cloud-tops indicative of deep convection were absolutely lacking). While the details of the formation of such attributes are well beyond the scope of this course, in a nutshell, air wraps cyclonically approximately the western flank of the low and also traps heat air at the facility of circulation, producing a warm air seclusion. The cyclone design, which describes the advancement of these types of cyclones, is called the Shapiro-Keyser Cyclone Model, and it differs somewhat from the timeless "Norwegian" cyclone model you"re acquainted via. If you"re interested in the Shapiro-Keyser Cyclone Model and also heat air seclusions, here"s one of the digestible study records on this topic. Enjoy!

Can cyclones ever adjust type?

In order to grow, tropical cyclones need arranged thunderstorms about their centers. In contrast, mid-latitude cyclones need huge horizontal temperature contrasts in order to intensify. With these contrasting attributes in mind, you could assume that tropical cyclones deserve to never crossover into the realm of mid-latitude cyclones, however that"s not really true.

As tropical cyclones move poleward, they inevitably enter an setting wright here there are horizontal temperature gradients. Before dissipating, a tropical cyclone occasionally becomes "extratropical" or "post-tropical," transitioning from a system with thunderstorms about its facility to a mid-latitude low-push system that derives its energy from synoptic-scale temperature gradients.

A excellent instance of an "extratropical transition" deserve to be viewed through Hurricane Noel. Early on November 2, 2007, Hurricane Noel started to move poleward off the coast of Florida. To get a sense of the as a whole weather pattern, inspect out the 06Z surface evaluation. Noel"s position is marked by the hurricane icon and also note the cold front coming off the East Coast. On the 0615Z magnified water vapor photo, it"s straightforward to watch the high cloud tops and also high concentrations of water vapor in the top tropospbelow concentrated around Noel"s center. As Noel progressed north-northeastward (examine out Noel"s track) toward the cold front later that night, the tropical cyclone ended up being post-tropical as it became embedded in the temperature gradients associated via the front. The 09Z surface analysis on November 3, 2007, shows the remnants of Noel (noted by the red "L") on the verge of merging with the front.

Another clue to Noel"s post-tropical transition is the comma-shaped configuration of the high cloud tops on this intensified water vapor picture at 1515Z the following morning (November 3, 2007). This satellite presentation is continuous through the timeless conceptual design of mid-latitude cyclones that you"ve learned. The National Hurricane Center listed Noel"s transition in its last Noel advisory (5 P.M. EDT on November 2).

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For the record, "tropical transitions" deserve to take place, too, in which non-tropical cyclones change into tropical cyclones. Often, such cyclones at the same time exhilittle attributes of both mid-latitude and tropical cyclones for a time (and are dubbed "subtropical cyclones"), but we"ll touch on these topics later in the course.