Upper Atmospheric Clouds

Weather is caused by the atmosphere. However, most of what we know as weather is limited to the atmosphere's lowest layer, the troposphere. The height of the troposphere varies with latitude on the Earth, from under 4 miles (6 km) in the coldest polar regions to over 10 miles (16 km) at the equator. The distinguishing feature of this layer that allows weather to occur is cloud formation.

Cloud formation occurs when water, evaporated from the ground into gaseous water vapor, condenses into small water droplets in the atmosphere. However, the water vapor-laden air must have some means by which it is transported upward. One such means is orthographic cloud formation.

This occurs when the wind direction pushes air up the slope of a topographical feature such as a mountain. As air rises, it expands and cools. Since cooler air holds less water vapor, condensation begins, clouds form, and precipitation can follow. Another impact of this process is that the opposite side of the mountain from the prevailing wind can be very dry.

However, clouds of course also form in the absence of mountainous terrain. This occurs when a parcel of air near the surface, containing evaporated water, is heated by the warm sunlit ground. Warmer than the adjacent atmosphere, it rises. As the air rises, its pressure drops in accord with that of its surroundings (air pressure decreases with altitude, a phenomenon mountain climbers are familiar with). By the ideal gas law, the pressure of a gas is proportional to its temperature. Therefore, a rising parcel of air cools.

The tallest clouds form in the case of absolute instability, illustrated above (click to enlarge). The parcel of air begins at the ground at a temperature of 40° C. As it rises, it cools. However, in the unstable case, the air temperature of the ambient atmosphere drops more quickly than the temperature in the rising parcel of air (the rate of its cooling is known as the dry adiabatic lapse rate). As a result, the cooling air nevertheless remains warmer than its surroundings and continues to rise. Eventually, its becomes too cold to hold all of the water vapor and condensation begins at the condensation level. Condensation releases heat to the atmosphere, so the temperature decrease of the parcel slows (to what is known as the wet adiabatic lapse rate). The resulting condensation forms a cloud, whose lower extremity is the condensation level.

At the top of the above diagram, the parcel temperature is still much higher than the atmospheric temperature, so the air rises yet more, continuing to increase the height of the cloud on the way. In situations such as this, a cumulonimbus cloud may form.

Cumulonimbus can extend from a few thousand feet from the ground over 10 miles (16 km) upward. It is these clouds that are responsible for many types of extreme weather, including severe thunderstorms, hail, and even tornadoes. However, even for these enormous clouds, there is an limit to their upward growth: the tropopause. The tropopause is the boundary between the troposphere below and the next layer of the atmosphere, the stratosphere, above. As indicated at the beginning of the post, the height of the tropopause varies with latitude. It is characterized by a reversal of the decline in temperature with height that takes place up to this point. Beginning at the tropopause and into the stratosphere, temperature again begins to increase with height (as shown near the bottom of the figure below).

Unlike in the troposphere, where the only significant source of heat was the sun-heated surface below, the stratosphere derives heat from another source: the absorption of solar ultraviolet radiation by the ozone layer. Oxygen and ozone (O₃) molecules interact with incoming ultraviolet rays, impeding their passage to the Earth's surface and therefore protecting it from most of this harmful radiation. In the process, they absorb heat. The result of all this is that air just above the tropopause is always stable: temperature increases with height. Rising parcels of air do not penetrate much past this point. Rather, they spread out laterally, forming the anvil top characteristic of cumulonimbus clouds and illustrated above. By virtue of their momentum, a few parcels enter a bit into the stratosphere; this is the "overshooting top" in the diagram.

However, in rare cases, clouds can form outside the troposphere. For example, polar stratospheric clouds (PSCs) may sometimes form in high latitude regions. They occur in the lower stratosphere, at altitudes varying from 6 to 15 miles (10-25 km), where temperatures plummet to below -100° F. At these especially low temperatures, water vapor can condense directly into ice crystals. The more spectacular PSCs are the nacreous clouds, which are composed entirely of these crystals. They are most visible just after sunset when the Sun has disappeared below the horizon on the ground but still illuminates the stratosphere. The ice crystals diffract light, producing beautiful iridescent displays.

This image shows nacreous clouds over McMurdo Station, Antarctica. Though they are most often seen over that continent, they also appear occasionally over northern Europe, North America, and Russia. PSCs are of scientific interest because they can serve as sites for chemical reactions that destroy ozone. Understanding the climatology of these unusual clouds helps to monitor the regional variation of ozone in polar regions.

In addition to PSCs, there is another type of upper atmospheric cloud that occurs even at even higher altitudes. Noctilucent clouds, so named because they are only visible at night well after the Sun has passed below the horizon, form in the mesosphere. This layer is defined as being the second region where temperature decreases with height. The clouds form generally around 50 miles (80 km) above the surface. Though not fully understood, it is thought the formation of noctilucent clouds is due to meteoric dust. This dust is the result of small objects from space breaking up upon entering Earth's atmosphere and serves as a site for the formation of ice crystals. Though the amount of moisture water vapor this high is minuscule, the temperature here can plummet to a frigid -180° F at high latitudes, supporting ice crystal formation.

The above image, taken from Nunivak Island, Alaska, shows noctilucent clouds after sunset. These clouds are at such a high altitude that they reflect sunlight from below the horizon back down to Earth. Studying the locations where these clouds form can reveal concentrations of dust (meteoric or from other sources such as volcanoes) in the mesosphere as well as shed light on how the upper atmosphere changes with the climate. Apart from their scientific value, upper atmospheric clouds join the aurorae as some of the most beautiful displays to be found in polar skies.

Sources: The Encyclopedia of Weather and Climate Change by Juliane Fry, https://www.wunderground.com/cat6/Methane-Giving-Noctilucent-Clouds-Boost?cm_ven=hp-slot-1, http://www.richhoffmanclass.com/chapter4.html, https://www.nasa.gov/mission_pages/sunearth/science/atmosphere-layers2.html, https://www.atoptics.co.uk, https://www.nasa.gov/multimedia/imagegallery/image_feature_680.html, https://www.sciencedaily.com/releases/2014/04/140411091939.htm

2019 Season Summary

The 2019 Atlantic hurricane season had above-average activity, with a total of

20 cyclones attaining tropical depression status,
18 cyclones attaining tropical storm status,
6 cyclones attaining hurricane status, and
3 cyclones attaining major hurricane status.

Before the beginning of the season, I predicted that there would be

15 cyclones attaining tropical depression status,
14 cyclones attaining tropical storm status,
6 cyclones attaining hurricane status, and
3 cyclones attaining major hurricane status.



The average number of named storms, hurricanes, and major hurricanes for an Atlantic hurricane season (over the 30-year period 1981-2010) are 12.1, 6.4, and 2.7, respectively. The 2019 season was much above average in the number of tropical storms (in fact, it was only the sixth year since the advent of the modern naming system that the "S" name was used), but near average in the hurricane and major hurricane categories. This stemmed in part from the fact that the season featured many short-lived, weak tropical storms and a few powerful hurricanes. The ACE value for the 2019 season (which accounts for duration and intensity of storms as well as number) was around 130, just above the average. Apart from the addition of a few extra short-lived cyclones, my predictions were on target.

The El Niño event initially forecast to continue (see the first image in this post) through the 2019 summer/fall died out by July, transitioning to an ENSO neutral/slightly positive state that continued through November. Since an El Niño event typically suppresses hurricane activity, this factor alone would suggest an average to slightly below average season. Indeed, 2019 did display some of the hallmarks of an El Niño season, including storm tracks curving away from the north American mainland and wind shear in the main development region (the Caribbean and tropical western Atlantic). However, the "tropical wave train" off of Africa was quite vigorous for most of the season, and even extended later than usual into October (with Tropical Depression Fifteen among the latest forming tropical waves off of Africa ever recorded). Also offsetting the residual El Niño was the continuing ocean warmth of the subtropical Atlantic, which contributed to prolific development; nearly half of the season's storms formed in the subtropical Atlantic.

The most damaging storms of the 2019 season were Hurricane Dorian and Tropical Storm Imelda. Dorian devastated the Bahamas, especially Abacos Island and Grand Bahama, as a category 5 hurricane. It was the strongest landfalling storm on record for the region and was tied for the most intense landfalling hurricane by windspeed ever recorded in the Atlantic (with the Labor Day hurricane of 1935). Imelda was only a tropical storm, but its slow motion over southeast Texas caused a major flooding event similar to, though slightly less severe than, those associated with Harvey and Florence in the previous two years. Some other notable facts and records from the 2019 season include:

• Seven of the eighteen named storms of the 2019 season lasted less than 24 hours, a new record
• Subtropical Storm Andrea formed on May 20, marking the fifth consecutive year in which a named storm developed before the official start of the season on June 1 (also a new record)
• Dorian was the strongest Atlantic hurricane ever recorded so far north at its latitude of peak intensity, 26.6° N
• When Lorenzo became a category 5 hurricane, it marked only the seventh time on record that multiple category 5 hurricanes were recorded in a season; Lorenzo was the easternmost forming category 5 on record
• Pablo strengthened into a hurricane farther north and east than any cyclone previously observed, at 42.8° N, 18.3° W, only a few hundred miles west of the coast of Spain

Overall, the 2019 season was somewhat above average (though near average in number of hurricanes and major hurricanes), with Hurricane Dorian the strongest and most damaging.

Tropical Storm Sebastien (2019)

Storm Active: November 19-24

Around November 15, a trough of low pressure formed over the central Atlantic, generating a diffuse area of shower activity well to the east-northeast of the Lesser Antilles. The system moved steadily northwestward over the next few days and developed a weak low pressure center on November 17. Convective activity concentrated some near the center soon after, though upper-level winds and dry air out of the west confined most thunderstorm activity to the disturbance’s eastern side. During the morning of November 19, satellite data indicated a closed circulation was present and the low was upgraded to Tropical Storm Sebastien.

The next day, the cyclone turned northward as it rounded the edge of a subtropical ridge and began to feel the influence of an approaching front from the west. Sebastien faced heavy shear but still managed to strengthen some due to favorable upper-level divergence. That day, it continued its turn to the northeast, accelerating some as it did so. Convection was rather disorganized, but the system maintained its identity as it moved ahead of the advancing frontal boundary.

By November 22, Sebastien was beginning to exhibit some extratropical characteristics, but its inner core was still clearly that of a tropical system as it sped up even further. Under the cyclone, ocean heat content rapidly diminished, but it did not weaken, and in fact reached its peak intensity of 65 mph winds and a pressure of 994 mb the next day. Sebastien was already bringing heavy surf and high winds to the Azores by this time, and it rocketed across the westernmost islands during the evening of November 24. At last, its quick forward speed, coupled with cold ocean waters and frontal interaction, caused the storm to fully transition to extratropical later that night. The remnants of Sebastien merged with another system near Ireland a little over a day later.

The above image shows Tropical Storm Sebastien on November 24 shortly before extratropical transition.

The track and evolution of Sebastien were not well anticipated by computer models. While the general recurvature was agreed upon, initial model runs did not indicate Sebastien would remain tropical for as long as it did, nor that it would be able to maintain strong tropical storm intensity.

Subtropical Storm Rebekah (2019)

Storm Active: October 30-November 1

During the last week of October, another nontropical low over the northern Atlantic slowed to a stop west of the Azores islands. The powerful low weakened some as it drifted generally southeastward over the next couple of days, but it moved over slightly warmer water. By October 30, the low had enough thunderstorm activity to be classified Subtropical Storm Rebekah. Though convection was concentrated in a band wrapping halfway around the center, the center was located under an upper-level low, an indicator of subtropical characteristics. The cyclone moved eastward at a good clip over the next day and changed little in intensity. On October 31, conditions near Rebekah degraded as it encountered cooler waters and strong upper-level winds. Deep convection vanished by that evening, and the storm became post-tropical early on November 1. By this time, the remnants were nearing the Azores, but impacts on the islands were minimal.

Lacking deep convection and colocated with an upper-level low, Rebekah was classified as a subtropical storm.

The above image shows the meandering track of Rebekah over the northern Atlantic. Square points represent times at which Rebekah was subtropical.

Hurricane Pablo (2019)

Storm Active: October 25-28

On October 23, a non-tropical low centered several hundred miles west of the Azores began to deepen and produced strong winds and scattered rains over a large area of the northeast Atlantic. The low moved generally east-southeastward over the following days. On October 25, a small pocket of deep convection formed about the center of circulation. That afternoon, the system became Tropical Storm Pablo, a very small tropical storm in the middle of what appeared on a larger scale as an extratropical cyclone. Pablo was named at the same time as Tropical Storm Olga in the Gulf of Mexico, a new record for the latest in a season that two storms were named simultaneously. Though ocean temperatures were below the ordinary threshold for tropical cyclone development, a cool upper atmosphere gave the system enough instability to support tropical development.

Upon formation, the tiny Pablo had an even tinier eye feature on satellite imagery. Gale force winds associated with the tropical cyclone extended only a few dozen miles from the center, even though the parent extratropical system still was generating comparable winds in its much bigger northwest quadrant. On October 26, Pablo turned east and then northeast, strengthening a bit as it did so. Early that evening, the center passed close to the easternmost Azores islands, bringing additional rain, strong winds, and high waves. Defying expectations, the cyclone intensified further as the eye became better defined. During the morning on October 27, Pablo achieved hurricane status. At 42.8° N, 18.3° W, this was the furthest northeast any hurricane had ever formed on record. Pablo reached its peak intensity of 80 mph winds and a central pressure of 977 mb a few hours later. Meanwhile, the storm turned back toward the north and slowed down somewhat.

Soon after, even colder waters along Pablo's track at last caused convection to degrade. The small system suffered a rapid demise overnight, weakening to a low-end tropical storm and then becoming post-tropical early on October 28 when it degenerated into a swirl of low clouds. The remnant low drifted slowly northwest before it was absorbed by another low pressure system.

The above image shows Pablo near hurricane strength on October 27.

Pablo became a hurricane farther to the northeast than any previous cyclone on record.

Tropical Storm Olga (2019)

Storm Active: October 25

During the latter part of October, a tropical wave moved across the Caribbean and toward central America. As it passed near Belize around October 22, it began to exhibit disorganized thunderstorm activity. The system moved west-northwestward over the following days but did not develop further until emerging into the Bay of Campeche. On October 24, a circulation began to spin up in earnest over water west of the Yucatan Peninsula and convection increased markedly. Already, a autumn cold front approaching from the northwest was beginning to interact with the developing low. Nevertheless, it managed to become Tropical Depression Seventeen during the morning of October 25.

A few hours later it strengthened into Tropical Storm Olga. The front accelerated the newly formed Olga north-northeast that afternoon and its circulation elongated. Aircraft data collected during the evening suggested that the center had become embedded within the frontal boundary, and that a clear temperature gradient existed across the two sides of the circulation separated by the front axis. Thus, Olga was deemed post-tropical, only 12 hours after initially becoming a tropical cyclone. That night, ex-Olga made landfall in Louisiana. The combined system brought very heavy rain and widespread wind gusts to gale force, with some reaching 70 mph. Olga's tropical moisture drove up rain totals with the storm for the next few days as it pushed eastward.

The above image shows Tropical Storm Olga on October 25 interacting with the front.

Just like Nestor before it, Olga was a short-lived tropical cyclone in the Gulf of Mexico whose primary impacts occurred after merging with a front.

Tropical Storm Nestor (2019)

Storm Active: October 18-19

Around October 12, a large low pressure area formed over the southwestern Caribbean sea. The system moved slowly northwest over the following days but land interaction with central America stifled any chance at development initially. It emerged into the Bay of Campeche on October 16 and began a gradual turn toward the northeast. The disturbance deepened over water, but it also began to interact with a trough of low pressure to its northeast across the northern Gulf of Mexico. This interaction spawned a cyclone with some tropical characteristics, but which was also highly asymmetrical. The pull of the front also caused the system to accelerate northeastward. Finally, during the afternoon of October 18, the cyclone began sufficiently tropical to be classified Tropical Storm Nestor.

Though rather disorganized and not resembling a classical tropical cyclone, Nestor gained a boost in strength from the nearby trough, pushing it to its peak intensity of 60 mph winds and a pressure of 996 mb late that evening. By this time, the storm was approaching the panhandle of Florida, already bringing rain and gale force wind gusts. Nestor lost its tropical characteristics the morning of October 19 as convection retreated well to the east of the circulation and became post-tropical. The post-tropical storm made landfall a few hours later. Soon after, it crossed the U.S. southeast and exited toward the open Atlantic waters.

The above shows the disorganized Nestor shortly after classification as a tropical cyclone.

Nestor's remnants spawned a few tornadoes over the U.S. southeast as the system passed through.

Tropical Depression Fifteen (2019)

Storm Active: October 14-16

On October 13, a large tropical wave emerged into the Atlantic, exiting western Africa. Ordinarily, tropical waves do not organize so far east by mid-October, but unusually warm waters and low wind shear allowed the disturbance to consolidate. During the afternoon of October 14, the wave developed into Tropical Depression Fifteen. The formation took place southeast of the Cabo Verde islands, making the depression one of the easternmost forming tropical cyclone ever observed so late in the year. It tracked northwest over the following day but changed little in organization. Having a very broad circulation, Fifteen struggled to develop deep convection. However, this did not prevent the cyclone from bringing locally heavy rains and gusty winds to the Cabo Verde islands on the 15th. The system's circulation became elongated soon after as atmospheric conditions began to deteriorate. Fifteen dissipated during the morning of October 16.

The above image shows Tropical Depression Fifteen near the coastline of West Africa.

Though very short-lived, Fifteen was notable for forming so close to Africa exceptionally late in the calendar year.

Tropical Storm Melissa (2019)

Storm Active: October 11-14

On October 8, a non-tropical low pressure center formed along the western edge of a dissipating frontal boundary situated west-southwest to east-northeast across the western Atlantic ocean. When the low formed, it was located a few hundred miles off the North Carolina coastline. It moved north-northeastward over the next few days, deepened some, and absorbed another disturbance approaching from the south. Steering currents collapsed on October 10 and the system became almost stationary east of the mid-Atlantic coastline. Even as a non-tropical system, it brought dangerous ocean conditions and strong winds to the coastline, especially southern New England. There was not much in the way of rainfall associated with the low at first, but convection increased in a curved band north of the center early on October 11. Shortly afterward, the disturbance was classified Subtropical Storm Melissa, already with maximum winds of 65 mph and a central pressure of 995 mb.

That day, the cyclone drifted slowly southward, but soon westerly flow steered Melissa east and caused it to accelerate some away from the U.S. coastline. The structure changed some by October 12, with convection moving closer to the center of circulation. The structural change necessitated a reclassification of Melissa as a tropical storm. As the cyclone moved east, it encountered more hostile atmospheric conditions, which stripped away most of the thunderstorm activity. Melissa weakened into October 13. These trends continued until the cyclone became extratropical on October 14.

The above image shows Melissa after transitioning to a tropical storm on October 12. During this transition, the windfield contracted significantly, leaving a small area of central convection

Melissa formed near the east coast but moved away without affecting land.

Hurricane Lorenzo (2019)

Storm Active: September 22-October 2

On September 22, a vigorous tropical wave over Africa emerged into the far east Atlantic. Immediately, it showed signs of organization, and was classified Tropical Depression Thirteen that night while located southeast of the Cabo Verde Islands. The next day, it developed spiral bands and steadily strengthened, earning the name Tropical Storm Lorenzo that afternoon. Upper-level winds, sea surface temperatures, and humidity were all in Lorenzo's favor as it continued to intensify. The cyclone turned west-northwest on September 24 as it followed the boundary of a mid-level ridge. Later that day, Lorenzo achieved hurricane status. The cyclone was also quite a large hurricane, with tropical storm force winds extending over 200 miles from the center of circulation.

On September 25, an eye feature began to form on satellite imagery. This led to more rapid strengthening shortly thereafter. Lorenzo was able to overcome a small dry air intrusion and stabilize its eyewall overnight. By mid-morning on the 26th, it had rocketed to major hurricane strength, becoming the 3rd of the 2019 season. Nor did the rapid increase in strength stop there. By later in the day, Lorenzo was a category 4 hurricane. At that point, it was the second easternmost forming category 4 on record, behind only Hurricane Julia of 2010. In fact, by that evening, the system was brushing up against the maximum theoretical intensity for a hurricane forming in that region, given the sea temperatures and atmospheric conditions. Overnight, it peaked with 145 mph winds and a pressure of 937 mb. Meanwhile, the cyclone followed a very well-forecast curve toward the north into a weakness in the aforementioned ridge.

An eyewall replacement cycle commenced on September 27, weakening the hurricane back to a category 3 as the inner eyewall convection became ragged and asymmetric but expanding its windfield. The eye itself also clouded over on visible imagery. This weakening may have been exacerbated by southwesterly shear, but Lorenzo came back with a vengeance on the 28th, completing the replacement and developing very cold cloud tops in the new eyewall. With this organization came a truly extraordinary burst of strengthening in which the cyclone's winds increased 40 kt in just 12 hours. At its peak that night, Lorenzo was a category 5 hurricane with sustained winds of 160 mph and a minimum pressure of 925 mb, making it, by far, the easternmost category 5 ever observed in the Atlantic.

By September 29, the storm was curving northward and then north-northeastward into cooler waters and a more stable atmosphere, causing a steady decline of the maximum winds. The large storm continued to grow in size, however, as it gained latitude. On the 30th, Lorenzo, now a category 2 storm, began to accelerate northeastward toward the Azores islands. Some signs of extratropical transition were evident by October 1, but the storm still had a vigorous core of convection. That night, the center passed near the western Azores, by which time the wind radii were huge: hurricane force winds extended up to 150 miles from the center and tropical storm force winds up to 390 miles. Several of the islands experienced damaging winds and very high surf from the massive circulation. After passing to the northeast, Lorenzo became extratropical. Late the next night, the storm slammed into Ireland, bringing hurricane force wind gusts. The storm later passed over the UK before finally dissipating.

The above image shows Lorenzo as a category 4 hurricane over the eastern Atlantic.

Lorenzo did not affect any land for most of its life, but ultimately became one of the most powerful cyclones on record to affect the Azores when it passed over the western islands as a large category 2 hurricane.