Kids Corner
6th to 12th Grade
What’s the difference between a hurricane, a cyclone, and a typhoon?
They are all essentially the same thing with respect to both oceanic and atmospheric conditions which are relatively similar in context but different in terms of geographic differences. For example, across the tropical western Pacific Ocean basin, you will often find some of the warmest widespread oceanic sea-surface temperatures anywhere in the world. This correlates with the greatest oceanic heat potential (i.e., the greatest coverage and depth of warmer sea-surface temperatures which correlates to the greatest “fuel” for tropical cyclone genesis and development thereof) which is more conducive for the development of tropical cyclones assuming all other factors remain favorable. However, this is not to say that large numbers of tropical cyclones do not occur in other basins, but rather that there is a great statistical likelihood for more amplified tropical cyclone seasons across the tropical Western and Central Pacific basin than other basins in a given calendar year.
For example, across the tropical Central/Eastern Pacific Ocean basins as well as the tropical North Atlantic Ocean basin, tropical cyclones are referred to as hurricanes. However, across the tropical Western Pacific Ocean basin, tropical cyclones are referred to as typhoons. However, tropical cyclones which form in the Indian Ocean basin are referred to as severe tropical cyclones. In moving to the Southwest Pacific Ocean basin (e.g., in and around the continent of Australia), tropical cyclones are referred to as cyclones.
What is the life cycle of a tropical cyclone?
Most often, tropical cyclones originate as very weak (and often quite disorganized) areas of low pressure which are typically associated with clusters of disturbed weather (e.g., disorganized clusters of thunderstorms). This stage of tropical development is most often recognized as being the tropical wave phase. Tropical waves are the most common type of tropical disturbances which seasonally form and travel across various tropical ocean basins around the world every year. Tropical waves gradually develop deeper convection and often will develop at least one closed pressure line near (or right around) the center of the developing tropical low-pressure system. This is the point at which a tropical wave becomes known as a tropical depression. It is important to note that at tropical depression-strength, the system’s convection is still chiefly driven by diurnal heating (i.e., energy which is received by the Earth from the Sun) rather than being predominantly fueled by latent heat release (i.e., the moisture-based energy which is released from the vicinity of intense convective storms which tend to form within the tropical regions of the world).
Once the system’s maximum sustained winds reach the threshold of 39 miles per hour (or 33.9 knots), the system is then referred to as a tropical storm and is assigned a given name. It is most often found at this stage, that the system’s convection is predominantly driven and influenced by the self-sustaining latent heat release engine which develops at the core of the developing low-pressure system. Thus, it is at this point that a tropical system begins to rely on latent heat release as its primary source of “core energy production.” As the tropical storm’s maximum sustained winds increase up to or above 74 miles per hour, it is upgraded to hurricane status. With further intensification, a strengthening hurricane can often develop an eye toward the center of its circulation due to increasingly more efficient latent heat release and increasingly stronger sinking motion at the center of the intensifying tropical cyclone. This strengthening sinking motion (which is more commonly referred to as subsidence) helps to create an approximately spherical region wherein there are relatively calm conditions which is known as a hurricane’s eye. This process is primarily determined by the net efficiency of both outer ventilation (i.e., the symmetry of the net energy distribution and the upper-level outflow at the periphery of the tropical cyclone) as well as the core latent heat release in and around the eye and/or eyewall of the intensifying tropical cyclone.
Due to variable deep-layer vertical wind shear that can sometimes be present both near and around developing tropical cyclones, a tropical cyclone’s core convection can sometimes become exposed and/or displaced due to less favorable environmental conditions (e.g., increasingly stronger advection of a drier low/mid-level air mass, cooler regional sea-surface temperatures, periodic land-mass interactions, etc.). When such core convection displacement occurs (i.e., either at the developing stages and/or near the peak of given tropical cyclone’s lifecycle), this often halts any further intensification or begins to gradually weaken the tropical cyclone due to less favorable environmental factors in place. When instances of core convective displacement occur, this often generates a more ragged visible and/or infrared satellite appearance which is often visualized as more evident structural asymmetries associated with the weakening and more disorganized tropical cyclone.
The processes described in the paragraph above are very-well explained in a series of graphical animations and text pieces developed by the University of Illinois at Urbana-Champaign meteorology department. In said graphical animations, the critical genesis and development processes tied to tropical cyclone development are illustrated.
Once the system’s maximum sustained winds reach the threshold of 39 miles per hour (or 33.9 knots), the system is then referred to as a tropical storm and is assigned a given name. It is most often found at this stage, that the system’s convection is predominantly driven and influenced by the self-sustaining latent heat release engine which develops at the core of the developing low-pressure system. Thus, it is at this point that a tropical system begins to rely on latent heat release as its primary source of “core energy production.” As the tropical storm’s maximum sustained winds increase up to or above 74 miles per hour, it is upgraded to hurricane status. With further intensification, a strengthening hurricane can often develop an eye toward the center of its circulation due to increasingly more efficient latent heat release and increasingly stronger sinking motion at the center of the intensifying tropical cyclone. This strengthening sinking motion (which is more commonly referred to as subsidence) helps to create an approximately spherical region wherein there are relatively calm conditions which is known as a hurricane’s eye. This process is primarily determined by the net efficiency of both outer ventilation (i.e., the symmetry of the net energy distribution and the upper-level outflow at the periphery of the tropical cyclone) as well as the core latent heat release in and around the eye and/or eyewall of the intensifying tropical cyclone.
Due to variable deep-layer vertical wind shear that can sometimes be present both near and around developing tropical cyclones, a tropical cyclone’s core convection can sometimes become exposed and/or displaced due to less favorable environmental conditions (e.g., increasingly stronger advection of a drier low/mid-level air mass, cooler regional sea-surface temperatures, periodic land-mass interactions, etc.). When such core convection displacement occurs (i.e., either at the developing stages and/or near the peak of given tropical cyclone’s lifecycle), this often halts any further intensification or begins to gradually weaken the tropical cyclone due to less favorable environmental factors in place. When instances of core convective displacement occur, this often generates a more ragged visible and/or infrared satellite appearance which is often visualized as more evident structural asymmetries associated with the weakening and more disorganized tropical cyclone.
The processes described in the paragraph above are very-well explained in a series of graphical animations and text pieces developed by the University of Illinois at Urbana-Champaign meteorology department. In said graphical animations, the critical genesis and development processes tied to tropical cyclone development are illustrated.
Source: Weather Wiz Kids
"When a cold air mass is located above an organized cluster of tropical thunderstorms, an unstable atmosphere results. This instability increases the likelihood of convection, which leads to strong updrafts (red arrows) that lift the air and moisture upwards, creating an environment favorable for the development of large cumulonimbus clouds. A tropical disturbance is born, the first stage of a developing hurricane.
Surface convergence (pink horizontal arrows in animation below) causes rising motion around a surface cyclone (labeled as "L"). The air cools as it rises (red vertical arrows) and condensation occurs, which releases latent heat into the atmosphere. This heating causes air to expand, creating an area of high pressure aloft. The force resulting from the established pressure gradient causes air to diverge at upper levels (red horizontal arrows).
Since pressure is a measure of the weight of the air above a unit area, removal of air at upper levels subsequently reduces pressure at the surface. A further reduction in surface pressure leads to increasing convergence (due to an intensified pressure gradient), which further intensifies the rising motion, latent heat release, and so on. As long as favorable conditions exist, this process continues to build upon itself, ultimately resulting in the development of a hurricane." - Source: University of Illinois at Urbana-Champaign
Hurricane Formation. Source: Weather Underground
The Science Behind Hurricanes. Source: Ginger Zee, ABC News.
How does the eye of a tropical cyclone form?
As a tropical cyclone located in any tropical oceanic basin (i.e., any part of a given ocean basin where tropical cyclone activity occurs most often during a given calendar year) around the world begins to intensity either gradually or more rapidly, one of the many features which a tropical cyclone often will develop is called an eye. The eye of a tropical cyclone is the circular region located approximately at the center of a strengthening and/or mature tropical cyclone which both has the lowest atmospheric pressure and the calmest conditions throughout the extent of the storm. Though, it is worth noting that stronger winds can sometimes also extend somewhat beyond the eye-wall and into the eye itself to some degree. Another neat fact about the eye of a tropical cyclone is that the region with the warmest temperatures higher up (i.e., approximately between 1 and 2 miles above the surface) within the eye which occur due to compressional warming of air (i.e., that is sinking and warming of air) within the eye can induce localized warming which “can be as much as 10°C [18°F] warmer or more at an altitude of 12 km [8 mi] than the surrounding environment, but only 0-2°C [0-3°F] warmer at the surface (Hawkins and Rubsam 1968) in the tropical cyclone. Eyes range in size from 8 km [5 mi] to over 200 km [120 mi] across, but most are approximately 30-60 km [20-40 mi] in diameter (Weatherford and Gray 1988).” (Credit: Hurricane Research Division’s Atlantic Oceanographic and Meteorological Laboratory)
Source: Encyclopedia Brittanica
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“One feature which is believed to play a role in forming and maintaining the eye is the adjacent eyewall convection. Convection in tropical cyclones is organized into long, narrow rain-bands which are oriented in the same direction as the horizontal wind (i.e., the predominant orientation of the local wind field direction within a given part of a tropical cyclone). Because these rain-bands seem to spiral into the center of a tropical cyclone, they are sometimes called "spiral bands". Along these bands, low-level convergence (i.e., inward flow towards the center of the storm’s circulation) is a maximum, and therefore, upper-level divergence (i.e., the atmospheric process by which mass is removed from a given column of air in the lower/middle levels of the atmosphere) is most pronounced above the top of the storm. Because of the processes noted above, a direct circulation develops in which warm, moist air converges at the surface, ascends through these bands, diverges aloft (i.e., the previously noted air streams split apart at the top of the convective banding features), and descends on both sides of the corresponding rain bands. As the air subsides, adiabatic warming (i.e., warming of the air parcels without any external energy exchange from within the given air parcels) takes place, and the air dries. Because subsidence (i.e., the atmospheric process by which air sinks) is concentrated on the inside of the rain-band, the adiabatic warming is stronger inward from the band causing a sharp contrast in pressure falls across the rain-band since warm air is lighter than cold air. Because of the pressure falls which occur mostly within the confines of the developing or mature eye, the tangential winds (i.e., the winds which flow in direction which is constantly perpendicular to a tropical cyclone’s circulation center) around the tropical cyclone increase due to the increased pressure gradient (i.e., the change in atmospheric pressure over some given straight line distance from the center of a given tropical cyclone). Eventually, the inner-most rain-band moves toward the center and encircles it. It is at this time that the eye and eyewall are most often found to form (Willoughby 1979, 1990a, 1995).” (Credit: Hurricane Research Division’s Atlantic Oceanographic and Meteorological Laboratory)
Animation of Super Typhoon Haiyan 13:00-20:00 UTC November 7th 2013
Source: NOAA
Source: NOAA
How does a storm surge from a tropical cyclone form?
A storm surge is best defined as an abnormal rise in the height of average coastal sea-levels which occur because of strong onshore winds (i.e., onshore flow which are winds whose direction are oriented from over the ocean to over some given landmass) which pushes a tremendous amount of ocean water towards a given coastal region. This abnormal rise in localized sea-level heights in the wake of an approaching tropical cyclone often leads to quite destructive flooding of both coastal and semi-coastal regions in different parts of the world. Typically, the average storm surge magnitude and storm surge impacts will have the greatest impacts near the quadrant of the storm (i.e., the part of the storm) which has the strongest wind. As far as the quadrants of a tropical cyclone are concerned, a typical wind field will consist of a northeast, northwest, southeast, and southwest quadrant.
With respect to the most intense quadrant of a tropical cyclone in the Northern Hemisphere (i.e., through considering the forward movement of a tropical cyclone and what is typically the strongest part of a tropical cyclone’s wind field), this is often found to be the northeast quadrant. To summarize this point, the stronger a given tropical cyclone is, the larger a given storm surge will typically be. The difference between a tropical cyclone’s storm surge and storm tide is that a storm surge is the abnormal rise of water generated by a storm. On the flip side, a storm tide is the water level rise which occurs during impacts from a nearby or landfalling tropical cyclone via the combination of the tropical cyclone’s storm surge and further water level rise contributions which are imposed by the natural regional astronomical tidal cycles. More specifically, the greatest impacts will occur in situations which evolve such that the worst impacts from a landfalling tropical cyclone’s overlap with a given high tide cycle which will act to exacerbate the impacts of a powerful, incoming storm surge.
Source: National Geographic
"Although elevated, this house in North Carolina could not withstand the 15 ft (4.5 m) of storm surge that came with Hurricane Floyd (1999)"
Source: NOAA
Source: NOAA
Another critical factor which contributes to the magnitude of a given storm surge happens to be atmospheric pressure. For the sake of clarity, atmospheric pressure is the force exerted by the weight of air in the Earth’s atmosphere. Since the atmospheric pressure is always higher at the edges of a cyclone than it is at the center, this acts to “push down” on the water located beneath the outer parts of the storm. Consequently, this causes the water to bulge at the eye and eye wall as a direct result of the winds helping to contribute to the rise in sea level heights near the inner core of the tropical cyclone.
Another critical factor which contributes to the magnitude of a given storm surge happens to be atmospheric pressure. For the sake of clarity, atmospheric pressure is the force exerted by the weight of air in the Earth’s atmosphere. Since the atmospheric pressure is always higher at the edges of a cyclone than it is at the center, this acts to “push down” on the water located beneath the outer parts of the storm. Consequently, this causes the water to bulge at the eye and eye wall as a direct result of the winds helping to contribute to the rise in sea level heights near the inner core of the tropical cyclone.
Source: The COMET Program
In addition, among various factors which act to further amplify the strength of a tropical cyclone’s storm surge, two other factors which will act to increase the magnitude of a given storm surge are the size of a tropical cyclone and the shape as well as slope (i.e., the angle of the shoreline in a vertical context) of a given coastline. Regarding tropical cyclone size, the larger a tropical cyclone is, the more prolific a given storm surge will be. Regarding the shape and the slope of a given coastline, when there is a gradual slope angle along the coastal shelf and the immediate coastline, this will often make such coastal regions even more vulnerable to the destructive impact potential from a given tropical cyclone’s storm surge. In addition, in regions where the geography of a coastline is such that there is a concave-shaped coastline (i.e., where the coastline bends inward over some distance), this will often act to help funnel incoming ocean water even more effectively. Thus, amplifying the localized impacts of a given tropical cyclone’s storm surge. Moreover, when there is a gently sloping coastal shelf and immediate coastline, this allows a tropical cyclone’s storm surge to often penetrate much further inland than it otherwise may be able to with a steeper coastline angle. Depending on the exact situation at hand along a given coastal region, the water level associated with a storm surge can reach as high as 10 meters (33 feet) or higher if the storm surge occurs in synchronization with the closest high tide cycle.
In addition, among various factors which act to further amplify the strength of a tropical cyclone’s storm surge, two other factors which will act to increase the magnitude of a given storm surge are the size of a tropical cyclone and the shape as well as slope (i.e., the angle of the shoreline in a vertical context) of a given coastline. Regarding tropical cyclone size, the larger a tropical cyclone is, the more prolific a given storm surge will be. Regarding the shape and the slope of a given coastline, when there is a gradual slope angle along the coastal shelf and the immediate coastline, this will often make such coastal regions even more vulnerable to the destructive impact potential from a given tropical cyclone’s storm surge. In addition, in regions where the geography of a coastline is such that there is a concave-shaped coastline (i.e., where the coastline bends inward over some distance), this will often act to help funnel incoming ocean water even more effectively. Thus, amplifying the localized impacts of a given tropical cyclone’s storm surge. Moreover, when there is a gently sloping coastal shelf and immediate coastline, this allows a tropical cyclone’s storm surge to often penetrate much further inland than it otherwise may be able to with a steeper coastline angle. Depending on the exact situation at hand along a given coastal region, the water level associated with a storm surge can reach as high as 10 meters (33 feet) or higher if the storm surge occurs in synchronization with the closest high tide cycle.
Source: NOAA/The COMET Program
Source: The COMET Program/MetEd
Source: The Weather Channel
What is the monsoon season? What causes it? Where does it occur?
The monsoon season is a period where there is a seasonal shift in the orientation of the prevailing (strongest) winds within a specified region. Monsoonal flow patterns primarily occur over the Indian Ocean and induce both wet and dry seasons throughout many tropical regions around the world. Monsoonal flow induces wind flow regimes which transport air masses from relatively colder to warmer regions in most cases. It is also worth noting that both the Summer and Winter monsoons often will act to modulate and/or influence Southeastern Asia’s and India’s seasonal climate regimes. The Summer Indian monsoon brings heavy rainfall to a good portion of the Subcontinent and generally occurs between the months of April and September. At Winter’s end, warm/moist air is transported across the Indian Ocean and onward towards Southeastern Asia. Summer monsoons often come along with humid weather and torrential, persistent rainfall events.
On the flipside, the Winter monsoon typically occurs between the months of October and April. In this case, the winds blow from the northeast and come from Mongolia and Northwestern China. The Winter monsoon is weaker than the Summer monsoon in Southeastern Asia due to the geographic positioning of the Himalaya Mountains which act to block the wind and moisture from getting further inland from the nearby coastal regions. Winter monsoons are also periodically associated with droughts. For parts of western southeastern Asia, the winter monsoon is their rainy season.
Credit: JeetoBharat
Credit: Bryn Adams
What is the ITCZ?
The ITCZ stands for the Inter-Tropical Convergence Zone. It “appears as a band of clouds consisting of showers, with occasional thunderstorms, that encircles the globe both along and in the immediate vicinity of the equator. The solid band of clouds may extend for many hundreds of miles and is sometimes broken into smaller line segments. It exists because of the convergence of the trade winds from both the outer edge of the Northern and Southern Hemisphere. In the Northern Hemisphere, the northeast trade winds converge and interact with southeast trade winds from the Southern Hemisphere. The point at which the trade winds converge forces the air to ascend higher up into the lower and middle parts of the atmosphere, forming the predominant convection (i.e., the deeper thunderstorms which exist on a day-to-day basis and wrap around nearly the entire extent of the equatorial regions of the world between roughly 15° N and 15°S) which comprises the ITCZ.”
It is worth noting that the ITCZ can also act a periodic catalyst (i.e., a trigger mechanism which sometimes acts as a supporting feature) for a small percentage of global tropical cyclone activity. This is a result of the fact that as the ITCZ exists, there can sometimes be areas of convection associated with more turbulent “atmospheric energy” which can break off and instigate the development of tropical cyclones. This does not happen on a routine basis, but can sometimes catch operational forecasters (i.e., meteorologists who focus on analyzing current weather conditions and how current weather events may affect the evolution of future weather events) by surprise at times when this sort of thing does come to pass.
Source: National Weather Service