Roll Clouds: The Most Unusual Cloud in the Skies

Turning towards the sky, one can’t help but become mesmerized by beauty and movement of the different clouds that flow across the atmosphere.  One of the most peculiar looking clouds, arcus clouds, are horizontal elongated tube-like clouds that can occur all over the world.  A subgenre of arcus clouds known as roll clouds, are even more atypical as they are detached from any other cloud features.

Roll cloud seen on January 25th 2009, on “Las Olas Beach”, located in “Punta del Este”, Uruguay, by Daniela Mirner Eberl.

While roll clouds can occur in many places, such as Germany, Canada, South Africa, Brazil, Uruguay, and even Florida, they are regionally known as “Morning Glory” clouds along the North Australian Coast, more specifically over the Cape York Peninsula and Gulf of Carpentaria.  The clouds are so named as a result of their early morning appearance, and frequently occur during late September and through early October in this region.  These phenomenal clouds may be on the order of 400-600 miles in length, ½ – 1 mile high, and may move as fast as 40 miles per hour.

As with any cloud, moisture must be present in order for water vapor to condense into water droplets.  Morning Glories tend to occur when humidity values are elevated and a clash of different air masses.  Once moisture levels are adequate, these clouds may form as a result of drastic temperature changes in air masses ahead of a thunderstorm, frontal boundary, or sea breeze.

Morning glory cloud formation taken from a plane near Burketown (plane heading to Normanton) in QLD, Australia, 11 August 2009, by Mick Petroff

To understand the physical nature of a cloud, let’s first take a look at the relationship between air density and temperature.  Cold air is heavier than warm air as a result of more molecules per volume.


To better understand this, imagine a 10’ x 10’ unheated room in the middle of a Siberian winter.  For a person to keep warm, they would want to fill this room with as many other people as possible, capitalizing on generated body heat.  Now imagine that same 10’ x 10’ room located in the middle of hot Texas summer day with no available air conditioning.  In this scenario, a person may want to remove a majority of the heat generating bodies.  So if we exchange molecules for people in the above example, cold air has more molecules than warm air in the same amount of space, therefore making cold air denser (heavier) than warm air. This is what makes cold air sink downwards and warmer air upwards by nature.

A sudden influx of cold air can also force warm surface air to rapidly rise, which is often the case of what happens when cold air rushes out ahead of a thunderstorm or when sea breezes occur from differential daytime heating.  A gust front is the downward and outward rush of the colder/heavier air from within a thunderstorm, usually followed by strong winds, heavy rain, and possible hail within minutes.  An extremely strong gust front rush out faster, detaching from the parent storm, and creating a roll cloud.  Sea breeze circulations occur as the sun heats land and sea surfaces differently, creating an onshore flow during the day and offshore flow during the night hours.  When an extremely strong sea breeze occurs in the evening, the elevated chances of a Morning Glory cloud occurs the following morning.

While there’s no shortage of atmospheric phenomena to excite the average observer, it is without doubt that encountering a roll cloud is an incredible sight and definitely on any weather lover’s bucket list.


This article was published in our monthly column within The Triton  newspaper (Nautical News for Captain and Crews), and can additionally be found here.


Leonid Meteor Shower This Weekend

Meteor shower alert this weekend!
WHEN: This weekend marks the occurrence of the #Leonid #meteor shower, which tends to be one of the best and brightest every year. The peak of this shower (Friday night into early Saturday morning) also aligns with a new moon, meaning much darker conditions for seeing up to 10-25 meteors per hour! Of course, this is a weather permitting scenario.
WHERE: Face the east, look up, locate the big dipper (the big pot), and follow a “3 fist distance” line towards the constellation Leo (as seen in the image)
WHO: As of now, viewing conditions look to be clear/partly clear for a majority of the United States. The Southeast, Northeast, Southwest, Central and Southern Rockies, and portions of the West Coast and Mid-Atlantic will be in luck. Cloud coverage will likely obstruct views in the Central and Northern Plains, the Great Lakes, and the Middle-to-Upper Mississippi Valley. However check your local weather service forecast by entering your zip code here:
HOW: Exactly what and how does this happen? As our planet orbits the sun, so do comets. This shower is a result of the Earth crossing paths with the debris trail associated with the Temple-Tuttle comet. Much of this debris enters into our atmosphere, but burns up due to frictional effects, hence the meteors that we see!
Sometimes the comets’ approach is closer than other years, turning a meteor shower into a meteor storm. While this year isn’t going to be a closest approach scenario, clear skies and the new moon will aid in producing a memorable event for many.
This show would definitely be worth the drive into a dark(er) location to observe this phenomena. Get out there, send us your pics, and inspire others to appreciate this beautiful event!

Cyclone Cook Aims at New Zealand

Cyclone Cook is currently aiming towards New Zealand, with 30-40 kt winds along the East Coast of the North Island and off the East Coast of the South Island.  Winds will become more northerly as the system continues to slide southward through the remainder Friday and into early Saturday [local NZ time]. Winds in excess of 20kts will remain along the North Island through Saturday afternoon [local NZ time], subsiding below 15 kts by Sunday evening/early Monday [local NZ time]


The Six Ingredients for Hurricane Formation

As we approach the 2017 Atlantic Hurricane Season [Jun 01-Nov 30], observing the sea surface temperatures [SST] indicates the “hot spots”, no pun intended, for hurricane development/intensification.  By the way, the term “hurricane” is synonymous with cyclone and typhoon, with the only difference being where they are geographically located:  Hurricanes refer to tropical cyclones located in the Atlantic and Eastern Pacific Ocean, typhoons are tropical cyclones located in the Central and Western Pacific waters. And the Indian and South Pacific Ocean basins simply refer to them by their general name, cyclones.

While SST values are one of the important determining factors to observe, cyclogenesis [the birth of a cyclone] requires a specific set of conditions to be in place in order for actual development/intensification to occur.  Let’s dissect these 6 major ingredients to fully understand the complexity of one of nature’s most powerful systems.

1.  Sea surface temperatures [SST]:  Minimum SST of 26.5 °C [79.7 °F] is necessary to provide enough heat content to “fuel” the system.  This temperature needs to be distributed through at least 50 meters [164 ft] in ocean depth.  According to Richard A. Dare and John L. McBride of the Centre for Australian Weather and Climate Research, Bureau of Meteorology in Melbourne, Australia, 98.3% of global cyclone formation occurs when SST values exceeding 25.5°C [77.9 °F].  So while meteorologists may watch thunderstorms pumping off the African Coast in anticipation of cyclone development, until there is sufficient water temperatures to fuel future development, the thunderstorm moves offshore and remains just a thunderstorm out to sea.

Atlantic Ocean basin sea surface temperatures [SST} for March 21, 2017.

2. Unstable Atmosphere/Vertical Motion: An unstable atmosphere is defined by one in which warm air continues to rise until it finds itself surrounding by air of an identical temperature. Once it finds its “home base”, this is what is known as equilibrium.  So what causes this warm air to rise?  The answer lies within the density differences between warm and cold air. Say what now?  Did things just get all science-y up in here? Well, imagine you were in a 10’ x 10’ room in the middle of a Siberian winter, with no heat; you would want to fill this room with as many people as possible to keep warm, stuffing person after person into the space to capitalize on the generated body heat.  Now imagine the same 10’ x 10’ room is located in the middle of hot Texas summer day with no A/C available; you would want to kick many of these people out of this room, ultimately to keep as much distance between yourself and any other heat generating individuals.  Now exchange people for molecules, and the idea of air density should be getting clearer; more people (molecules) in the room (air) makes the room weigh more, less people (molecules) in the same room (air) make the room weigh less.   Now you may remember that density was the amount of mass per given volume.  So, while the volume of the room stays the same, the amount of molecules (people) is what differs.  And there you have it, cold air is denser than warm air, therefore explaining why the warm air continues to rise until it achieves equilibrium.  Wow, things are getting heavy around here.

Provided there is adequate moisture present in the atmosphere, this rising warm air and moisture combine work in tandem to develop clouds.  If the rising motion continues unchecked, this will allow the clouds to continue building vertically, which now has the potential for thunderstorms.

Instability_with arrows and explanation

3.  Relative Humidity [RH]: Relative humidity is the amount of moisture available in the atmosphere, compared to how much it could fully hold [100% humidity].  High values of RH need to be present from the lower to middle portions of the atmosphere. So how much is enough?  Low values of RH cannot support cloud/thunderstorm development, and the 50% threshold of RH is borderline at best, whereas 70% and above is considered prime RH values.

4. Preexisting condition: It may begin as a simple thunderstorm, but some form of a disturbance or an area of lower pressure relative to its surroundings is the bullet to the trigger.  If a disturbance has any chance of developing into something more, it must develop or migrate into a region of the above mentioned factors.

5. Wind Shear: Wind shear is defined as the change in wind speed/direction with height.  These changes in wind direction with height must be enough to sustain a counterclockwise flow [low pressure’s spin counterclockwise in the Northern Hemisphere], but not too strong or it may move the heat and moisture away from the center of the system and essentially destroy the vertical integrity of the cloud column.

6.  Coriolis Force: This is a biggie.  This force, as a result of the earth’s rotation, induces motion to the right [Northern Hemisphere] and to the left in the Southern Hemisphere [think of launching missiles.  You don’t aim at the target, but slightly off, to compensate for the earth’s rotation.]


In addition, the amount of Coriolis force increases as the distance from the equator increases.  The sweet spot for adequate force is about 500 km [310 miles] from the equator, although formation outside of that is entirely possible.  It is physically difficult for formation to occur within 5° of the equator, because the amount of Coriolis force is simply too weak. Consequently, once a system rises above 20° latitude, the other above mentioned conditions become harder to maintain/achieve, so the ideal “Goldilocks Zone” for cyclogenesis remains between [5°- 20°].

Hurricane Movement Globally
Global tropical cyclone tracks between 1985-2005.  Photo courtesy of

So while the Atlantic Basin hurricane season is generally characterized by the Jun 01 – November 30 time frame, if the above conditions are met outside of that time frame, hurricane formation/intensification is entirely possible.  In fact, of all the Atlantic storms on record, 97% have formed within the above mentioned time frame.  So what about the other 3%?  The earliest known system has been re-analyzed to have occurred in January [1938] and the latest development has occurred in December [1954], towards the end of the month.  So while unlikely, it’s both historically and statistically conceivable.

Given the position of the earth and the amount of incoming solar radiation [insolation], ocean basins may indeed reach the required temperatures to support a breeding ground and if all other conditions are met, hurricane-a-typhoon-a-cyclone-a-comin’.



While 97% of storms form within the Jun 01-Nov 30 time frame, 6 major factors are required to produce/sustain cyclone development:

  1. SST’s > 79°F
  2. unstable atmosphere
  3. relative humidity > 60%
  4. existing disturbance
  5. adequate wind shear
  6. enough distance from the equator to experience adequate coriolis force

Stop playin’, read the whole article and learn a lil’ something!


Humidity On The Move

Observe the moisture flow in the 1000-500 mb relative humidity [RH] field.

Cool wraparound feature, as the moisture gets transported on the winds toward the departed NE low on the right hand side of the frame. Also notice the connective feature of the departed low tapping into the moisture pool from the Gulf of Mexico [GOMEX].

West coast also seeing an increase in available atmospheric moisture.

Ain’t #weather beautiful?


Evolution Animation of #Blizzard2017.

#Blizzard2017 will be one for the record books.  This system was a powerful combination of two different atmospheric jets, combining in just the right location, at the right position, to induce a rapidily strenghtening low pressure system just offshore.  As the system pulls northward along the coast, its position will enable strong onshore winds bringing larges amounts of moisture.

sfc analysis

Atmospherically speaking, it’s breathtakingly beautiful.

Cat 4 Tropical Cyclone Enawo Affecting Africa

Current IR [infrared] satellite and RH [relative humidity] view of Tropical Cyclone Enawo as it crosses Madagascar and into the Mozambique Straight.

The system made landfall as an intense Cat 4 system, with 137 mph winds. Northerly mid-level flow can be seen via the 200mb (yellow), 500 mb (green), and 850 mb (lavender) wind fields.

Drier air on the southern flank of the system will have a hard time moving northward as storm producing convection over the open water will maintain the healthy moisture field around Enawo.