Kids Corner
6th to 12th Grade
What causes wind?
Credit: Habby Hints
Wind is caused by differences in air pressure. When a cold air mass (which is more dense) is placed next to a warm air mass (which is less dense), the atmosphere creates wind to help equalize the difference in air pressure at various levels of the atmosphere over some given region. Winds will nearly always flow from areas of high pressure to areas of lower pressure; with some very rare exceptions.
Wind is caused by differences in air pressure. When a cold air mass (which is more dense) is placed next to a warm air mass (which is less dense), the atmosphere creates wind to help equalize the difference in air pressure at various levels of the atmosphere over some given region. Winds will nearly always flow from areas of high pressure to areas of lower pressure; with some very rare exceptions.
Source: NASA
What are the primary differences between types of more common atmospheric frontal boundaries?
A cold front is the somewhat straight boundary (i.e., which often has bends and curves based on the regional temperature and wind field) which often defines and/or represents the leading edge of a colder air mass. On a surface map, it is drawn with a blue line that has blue triangles facing the direction the air mass is headed. On the surface map below, one of the cold fronts extends from just above the Great Lakes and down south through a good portion of the state of Ohio. If you look closely at the temperatures, which are the numbers in red, you’ll notice the temperatures on the left side are cooler than the temperatures on the right side of the front. This is because cold fronts mark the leading edge of a colder air mass and with the passage of such a cold front, the cold air mass typically gets deeper with time in the context of a vertical profile of the lower/middle parts of the atmosphere. A cold front can also bring about differences in winds (changing speeds and directions) and weather conditions (e.g., frontal-enhanced and/or frontal-forced precipitation events).
Map credit: Habby Hints
What are winds like at different levels of the atmosphere?
If you look at a sounding (i.e., a vertical profile of the atmosphere which captures vertical profiles of temperature, dewpoint, wind speed, wind direction, etc), it can give you detailed information about winds on a vertical scale. Here are data from a sounding taken from Norman, OK at 12Z on Nov. 30th, 2017, drawn on a skew-t (i.e., a meteorological diagram which facilitates an effective way of visualizing the data from the vertical profiles of the atmospheric variables listed above): (note that Norman had calm, foggy weather at this time)
Image credit: NOAA Storm Prediction Center
On the left side of a skew-t, you will see at what pressure levels (in millibars (mb)) atmospheric data were measured at; on the right side you will find wind barbs at various elevation heights above the surface of the Earth at Norman, Oklahoma.
At the surface between 1000 and 850 mb, winds were generally calm, ranging from 5 to 20 knots and coming out of the north-northeast through that near-surface layer. Around 700 mb, we reach an inversion (i.e., a layer of the atmosphere in which there is a sudden increase in temperature for a brief time before the temperature profile again continues to decrease with height above a given layer), indicated by the sudden (and brief) increase in air temperature and an abrupt switch in the wind direction. From here, you see winds steadily increasing until they reach a maximum intensity of 90 knots at 200 mb, which is where we might find a jet streak. Above that height, winds begin to back off in intensity which is completely normal as you are approaching the tropopause which defines the boundary between the troposphere and the stratosphere. More specifically, the troposphere is the layer of the atmosphere in which most of the Earth’s weather phenomena occur. Then, the stratosphere is the layer just above the troposphere which is where air temperature stops decreasing with height and gradually begins to increase with height. It is worth noting that it is also at this height that the temperature stops decreasing with height and slowly begins to increase gradually with increasing height.
What is wind shear? What are veering winds?
Wind shear is a measure of the degree to which vertical wind speed and/or direction is changing with height. Wind shear is often referenced as a critical atmospheric wind parameter when assessing the potential for various types of severe, convective storms. Wind shear is also referenced as a proxy by which weather forecasters and research scientists evaluate hurricane activity based on the negative impacts imposed by vertical wind shear on developing, organized convection across large tropical oceanic basins. Veering winds are winds that change in a clockwise direction with increasing height. For example, if winds are southerly at the surface and gradually turn clockwise towards a westerly orientation by around 850 mb, these winds would be veering. Below is a sounding that exemplifies the process of vertically-veering winds occurring in real-life:
Source: National Weather Service
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Directional Shear
"Wind direction changes with height"
"Directional wind shear is the change in wind direction with height. In the image below, the view is looking north. The wind near the surface is blowing from the southeast to the northwest." National Weather Service |
Speed Shear
"Wind speed changes with height."
"Speed shear is the change in wind speed with height. In the illustration below, the wind is increasing with height. This tends to create a rolling affect to the atmosphere and is believed to be a key component in the formation of mesocyclones which can lead to tornadoes." National Weather Service |
"The updraft lifts the rotating column of air created by the speed shear. This provides two different rotations to the supercell; cyclonic or counter clockwise rotation and an anti-cyclonic of clockwise rotation.
The directional shear amplifies the cyclonic rotation and diminishes the anti-cyclonic rotation (the rotation on the right side of the of the updraft)." National Weather Service
The directional shear amplifies the cyclonic rotation and diminishes the anti-cyclonic rotation (the rotation on the right side of the of the updraft)." National Weather Service
What is an atmospheric sounding and how is it used?
A sounding is a vertical profile of the atmosphere. It shows temperature data, wind data, and moisture data at different pressure levels of the atmosphere. These data can be used to calculate or extrapolate other data, such as inversion levels, boundaries, cloud levels, precipitation type, etc. Below is an example of a sounding on a skew-t. To learn more about the skew-t click the following link: http://www.weather.gov/source/zhu/ZHU_Training_Page/convective_parameters/skewt/skewtinfo.html
A sounding is a vertical profile of the atmosphere. It shows temperature data, wind data, and moisture data at different pressure levels of the atmosphere. These data can be used to calculate or extrapolate other data, such as inversion levels, boundaries, cloud levels, precipitation type, etc. Below is an example of a sounding on a skew-t. To learn more about the skew-t click the following link: http://www.weather.gov/source/zhu/ZHU_Training_Page/convective_parameters/skewt/skewtinfo.html
Why do cyclones spin in different directions in the Northern and Southern Hemisphere?
When looking at any form of satellite imagery depending on the time of the day, it is easy to recognize the fact that cyclones of any kind will always spin in opposite directions in the Northern and Southern Hemisphere, respectively. The reason for this different in cyclonic motion is a consequence of Coriolis Effect. On planet Earth, the Coriolis effect most often deflects moving objects to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The Coriolis Effect occurs in such a way that a mass moving in a rotating system experiences a force which is perpendicular to the direction of motion. It is worth noting that the Coriolis Effect is also an important component involved in the formation of cyclonic weather systems.
What are the major ocean currents and how do they affect weather?
Ocean currents are effectively an energy transport system for oceanic basins all over the world. There are surface currents, which are driven by the atmosphere, and there are also thermohaline currents (i.e., deeper ocean currents which are primarily influenced by the constantly evolving changes in the ratio between fresh and saltwater), which are driven by variations in temperature and salinity and typically reside in much deeper sections of various oceans around the world. Ocean currents redirect the ocean’s heat while the Coriolis force pulls water to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This process can also often generate features most commonly referred to as gyres.
A gyre is best defined as the flow of ocean water around the outer edges of an ocean basin and often, there is some degree of rotation of these ocean currents around one another to some extent. There are five major gyres in the world which include (but are certainly not limited to): the North Pacific, North Atlantic, South Atlantic, South Pacific and the Indian Ocean gyre.
Of all the ocean currents which exist, western currents like the Gulf Stream current are the deepest and the fastest. They move warm water towards the Polar regions and carry fewer nutrients than cooler currents that are found near certain coastlines often do. It is also worth noting that ocean currents do not generally flow in a straight path as denoted on idealized map renderings and parts of certain ocean currents can sometimes evolve into swirls, eddies, or other turbulent features. Eastern Hemisphere ocean currents carry cold waters to the equator and typically are broader as well as shallower than the ocean currents of the Western Hemisphere.
What causes lightning and thunder?
Around the world, there are literally thousands of thunderstorms which occur in different places on a near-routine basis. For nearly all thunderstorms, the fundamental reason for why they form is due to warm air rising and cooler air sinking and it is at the point at which the warm and cold air meet up that you will find moisture condensation, cloud formation, and ultimately, potential thunderstorm development. Within thunderstorms, you will mostly find ice crystals in the colder sections of the storm and water droplets in the warmer sections of the storm. It is when the ice crystals and the water droplets collide that you find the generation of static electrical charges in the cloud. Moreover, in nearly all cases you find the positively charged particles towards the top of the thunderstorm and then negatively charged particles towards the bottom of the thunderstorm as well as near and/or right at the surface of the Earth.
Much in the same way that a battery has a positively and a negatively charged side, there is a limit to the extent to which the magnitude of the positively and negatively charged sides can become before the situation becomes unstable in a battery. This is the same concept as that found in association with the positively and negatively charged particles distributions within thunderstorms anywhere on Earth. More specifically, when the oppositely charged particle clusters accumulate enough, the capacity of the air to sustain the particle charge differences collapses and there is a rapid discharge of electricity which is visually observed as lightning. The flash of lightning which is generated in association with a given thunderstorm temporarily equalizes (or stabilizes) the charged regions in the atmosphere until the opposite charges build up again which is when a flash of lightning will occur again. It is also worth noting that lightning strikes can occur between opposite charges within the thunderstorm cloud (which is referred to as intra-cloud lightning) or between opposite charges in the cloud and on the ground (which is referred to as cloud-to-ground lightning).
In short, thunder is caused by lightning indirectly due in part to the fact that lightning which can be as hot as around 50,000 °F often heats the surrounding air to as high as 18,000 °F. As a point of comparison, the average lightning strike temperature noted above is roughly five times hotter than the surface of the Sun which goes to show how hot lightning is. This intense heat associated with a given lightning strike causes the air surrounding the lightning strike to rapidly expand and induces a sound wave which is known as thunder. More specifically, the stepped leader causes the initial “tearing” sound, and the following ground streamer(s) or stroke(s) causes the sharp “crackling” sound which is often heard at a very close range.
Furthermore, it is also worth noting that thunder can sometimes be heard as far as 25 miles away from a given lightning discharge. At these distances, thunder will more often sound like a distant “light rumble” because the higher frequency pitches are more easily absorbed by the surrounding environment. Hence, the different sound waves set off by the source of the lightning discharge have different arrival times.
Much in the same way that a battery has a positively and a negatively charged side, there is a limit to the extent to which the magnitude of the positively and negatively charged sides can become before the situation becomes unstable in a battery. This is the same concept as that found in association with the positively and negatively charged particles distributions within thunderstorms anywhere on Earth. More specifically, when the oppositely charged particle clusters accumulate enough, the capacity of the air to sustain the particle charge differences collapses and there is a rapid discharge of electricity which is visually observed as lightning. The flash of lightning which is generated in association with a given thunderstorm temporarily equalizes (or stabilizes) the charged regions in the atmosphere until the opposite charges build up again which is when a flash of lightning will occur again. It is also worth noting that lightning strikes can occur between opposite charges within the thunderstorm cloud (which is referred to as intra-cloud lightning) or between opposite charges in the cloud and on the ground (which is referred to as cloud-to-ground lightning).
In short, thunder is caused by lightning indirectly due in part to the fact that lightning which can be as hot as around 50,000 °F often heats the surrounding air to as high as 18,000 °F. As a point of comparison, the average lightning strike temperature noted above is roughly five times hotter than the surface of the Sun which goes to show how hot lightning is. This intense heat associated with a given lightning strike causes the air surrounding the lightning strike to rapidly expand and induces a sound wave which is known as thunder. More specifically, the stepped leader causes the initial “tearing” sound, and the following ground streamer(s) or stroke(s) causes the sharp “crackling” sound which is often heard at a very close range.
Furthermore, it is also worth noting that thunder can sometimes be heard as far as 25 miles away from a given lightning discharge. At these distances, thunder will more often sound like a distant “light rumble” because the higher frequency pitches are more easily absorbed by the surrounding environment. Hence, the different sound waves set off by the source of the lightning discharge have different arrival times.
Idealized lightning schematic. Source: SciJinks
"Lightning observed by the GOES-16 Geostationary Lightning Mapper (GLM) illuminates the storms developing over southeast Texas on the morning of February 14, 2017.
Lightning is an important part of weather forecasting. The Geostationary Lightning Mapper instrument on the GOES-R series satellites can detect lightning activity over nearly the whole Western Hemisphere.
Scientists use data from GOES-R series satellites, along with data from the Lightning Imaging Sensor on NASA's Tropical Rainfall Measuring Mission satellite, to study lightning. This complete picture of lightning at any given time will improve "now-casting" of dangerous thunderstorms, tornadoes, hail, and flash floods." - SciJinks
Lightning is an important part of weather forecasting. The Geostationary Lightning Mapper instrument on the GOES-R series satellites can detect lightning activity over nearly the whole Western Hemisphere.
Scientists use data from GOES-R series satellites, along with data from the Lightning Imaging Sensor on NASA's Tropical Rainfall Measuring Mission satellite, to study lightning. This complete picture of lightning at any given time will improve "now-casting" of dangerous thunderstorms, tornadoes, hail, and flash floods." - SciJinks
What are the primary differences between a hurricane and a tornado?
First and foremost, a hurricane generally covers a MUCH larger region spatially (i.e., in terms of the square mileage a hurricane will typically cover from a cloud-cover perspective) than does a typical tornado. In addition, a hurricane typically affects weather on a synoptic (large) scale (i.e., a scale which is best defined as having weather phenomenon which occurs generally anywhere between 3 and 7 days), whereas a tornado is MUCH more localized in terms of its typical coverage zone. While both types of events can have profound, infamous, and memorable impacts on a given region, a hurricane develops over the course of days and/or weeks in most cases, whereas a tornado can be generated by a strong thunderstorm that may develop over the course of a matter of minutes or hours. Having said that, both types of events consistently have the ability to cause a tremendous amount of damage from both an economic and a logistical perspective on a region. However, hurricanes tend to affect coastal areas for the most part as opposed to tornadoes which tend to have the worst impacts on more inland regions of a given continent.
What are the stages of a maturing thunderstorm?
“Thunderstorms have three stages in their life cycle: The developing stage, the mature stage, and the dissipating stage. The developing stage of a thunderstorm is marked by a cumulus cloud which is the stage at which air is being pushed upward by a rising column of air (i.e., the thunderstorm’s primary updraft). The cumulus cloud soon looks like a tower (which is called towering cumulus) as the updraft continues to develop. There is little to no rain during this stage but occasional lightning. Thereafter, the thunderstorm enters the mature stage which is the stage at which the updraft continues to feed the storm, but precipitation begins to fall out of the storm, creating a downdraft (a column of air pushing downward). When the downdraft and rain-cooled air spreads out along the ground, it forms a gust front, or a line of gusty winds. The mature stage is the most likely time for hail, heavy rain, frequent lightning, strong winds, and tornadoes. Eventually, a large amount of precipitation is produced and the updraft is overcome by the downdraft, which often marks the beginning of the dissipating stage. At the ground, the gust front moves out a long distance from the storm and cuts off the warm moist air that was feeding the thunderstorm. This typically marks the point of peak intensity for a given thunderstorm and when rainfall typically decreases in intensity, but lightning can often still remain to be a substantial danger.” NOAA National Severe Storms Laboratory
What is the life cycle of a tornado?
The life cycle of a tornado most often begins with a developing convective storm which forms because of increased low/mid-level moisture convergence developing more organized clusters of low-level cumulus and/or cumulus congestus cloud clusters (i.e., the white puffy clouds we often observe on hot summer days with some decent vertical development tied to them). As a typical afternoon or early evening continues in a region such as the Great Plains of the United States, there often is further vertical convective development which is observed by way of a cumulonimbus cloud which is simply a typical thunderstorm cloud. From there, when a deep convective storm encounters more pronounced deep-layer vertical wind shear, there is often a greater likelihood for the convective storm to develop a deep rotating updraft which is referred to as the mesocyclone portion of the severe thunderstorm.
As this mesocyclone is identified on a given Doppler radar system, the severe thunderstorm is then referred to as a supercell thunderstorm at that point. As the mesocyclone within the supercell thunderstorm continues to ingest warm, moist air from closer to the surface and up into the storm’s primary updraft, a corresponding downdraft known as the rear-flank downdraft will form in a short time as well. As the rear-flank downdraft forms and becomes more pronounced with time, it is often thought that this is the feature which is most often responsible for triggering the formation of tornadoes. Tornadoes typically last so long as the atmospheric conditions which supported the development of the tornado remain in place. Otherwise, they will often shift towards the dissipation stage of a tornado’s lifecycle.
As this mesocyclone is identified on a given Doppler radar system, the severe thunderstorm is then referred to as a supercell thunderstorm at that point. As the mesocyclone within the supercell thunderstorm continues to ingest warm, moist air from closer to the surface and up into the storm’s primary updraft, a corresponding downdraft known as the rear-flank downdraft will form in a short time as well. As the rear-flank downdraft forms and becomes more pronounced with time, it is often thought that this is the feature which is most often responsible for triggering the formation of tornadoes. Tornadoes typically last so long as the atmospheric conditions which supported the development of the tornado remain in place. Otherwise, they will often shift towards the dissipation stage of a tornado’s lifecycle.
Idealized schematic of a supercell thunderstorm.
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Real life example of a supercell thunderstorm.
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An animation showing modeled winds during a May 2011 tornado. Credit: Leigh Orf
"Leigh Orf, a meteorologist at Central Michigan University, presented a model simulation of a what can cause and sustain a violent EF-5 tornado — the most damaging type of tornado that can form — at the Severe Local Storms Conference this week. The visuals show that while we often think of the atmosphere as air, it actually behaves and looks a lot like liquid.
The real-world inspiration for the video was an EF-5 tornado that formed in Oklahoma during a major tornado outbreak in late May 2011. That outbreak spawned 241 tornadoes, including the deadly twister that devastated Joplin, Mo. However, the storm that was modeled was another EF-5 tornado that touched down on May 24 and tore across a nearly 65-mile stretch to the west and north of Oklahoma City. Mobile radar equipment measured winds of at least 210 mph and a local ground station recorded a wind gust of 151 mph, the strongest wind recorded by Oklahoma’s weather monitoring system. The tornado killed nine and left 181 injured while also tearing houses from their foundations, bending metal poles like stalks of corn, and destroying a nearly 2-million pound oil rig." -Climate Central
The real-world inspiration for the video was an EF-5 tornado that formed in Oklahoma during a major tornado outbreak in late May 2011. That outbreak spawned 241 tornadoes, including the deadly twister that devastated Joplin, Mo. However, the storm that was modeled was another EF-5 tornado that touched down on May 24 and tore across a nearly 65-mile stretch to the west and north of Oklahoma City. Mobile radar equipment measured winds of at least 210 mph and a local ground station recorded a wind gust of 151 mph, the strongest wind recorded by Oklahoma’s weather monitoring system. The tornado killed nine and left 181 injured while also tearing houses from their foundations, bending metal poles like stalks of corn, and destroying a nearly 2-million pound oil rig." -Climate Central
Source: Climate Central
5/18/14 Wright to Newcastle, WY Supercell Time-Lapse. Credit: BasehuntersChasing
How Tornado Forms animation. Credit: CuriousVideos
What is a hodograph? How is it used for severe weather forecasting?
A hodograph is a representation of how winds change with height in the atmosphere. The link below gives great detailed instructions on how to plot and interpret hodographs:
http://www.wxonline.info/topics/hodograph.html
By using hodographs, a meteorologist can see how strong wind shear is at a given point in time and whether a given environment is conducive for thunderstorm development. Below is a basic example of a hodograph for a situation with a severe thunderstorm:
What are the components of a classic severe thunderstorm?
A typical severe thunderstorm has several unique features. First off, as a given severe thunderstorm becomes more established, it develops stronger inflow and stronger corresponding outflow channels with time. The strengthening inflow sector can often be identified by way of a feature which is referred to as a “flanking line” or a “beaver’s tail” which is essentially an elongated line of low-level clouds which rapidly condense from low-level moisture and ascend into the lower levels of a severe thunderstorm. This feature represents the primary source of a severe thunderstorm’s “fuel” aside from the primary updraft within the core of the severe storm. This feature can be seen on the left side of the idealized image attached above.
Another major feature which is associated with intense severe thunderstorms is a feature which is referred to as an overshooting top. An overshooting top is a convective feature which is the result of the core of the main updraft within a given severe thunderstorm breaching the capping inversion (i.e., a virtual vertical barrier which changes in height both depending on the season and the environmental set-up) which typically exists close to the tops of most deep convective storms. Thus, the only part which is usually able to breach through said capping inversion is the strongest part of the core updraft which is often observed on various forms of satellite imagery as a “bubbling” section of the top of the severe thunderstorm. The rest of the upper-most part of the severe thunderstorm which is unable to have sufficiently strong vertical motions to break through the capping inversion simply spreads out to the east and/or in all directions and is observed as the anvil of the thunderstorm. The anvil of a severe thunderstorm is what often looks particularly majestic as it can extend for dozens of miles downwind of a given severe thunderstorm depending upon the strength of the prevailing mid/upper-level wind speed/direction.
Another major feature which is associated with intense severe thunderstorms is a feature which is referred to as an overshooting top. An overshooting top is a convective feature which is the result of the core of the main updraft within a given severe thunderstorm breaching the capping inversion (i.e., a virtual vertical barrier which changes in height both depending on the season and the environmental set-up) which typically exists close to the tops of most deep convective storms. Thus, the only part which is usually able to breach through said capping inversion is the strongest part of the core updraft which is often observed on various forms of satellite imagery as a “bubbling” section of the top of the severe thunderstorm. The rest of the upper-most part of the severe thunderstorm which is unable to have sufficiently strong vertical motions to break through the capping inversion simply spreads out to the east and/or in all directions and is observed as the anvil of the thunderstorm. The anvil of a severe thunderstorm is what often looks particularly majestic as it can extend for dozens of miles downwind of a given severe thunderstorm depending upon the strength of the prevailing mid/upper-level wind speed/direction.
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
What causes the different types of frozen precipitation?
One of the first questions which should always be asked when it comes to anticipating any variety of winter weather event is to what extent there is cold air in place. In addition, the other major issue pertains to whether there happens to be warmer air (i.e., above-freezing air) at the surface and/or within some given layer(s) above the surface. If there is warmer air above the surface, this can often lead to either icing events, mixed precipitation events, or even sleet events which can also lead to widespread life-threatening conditions both on the ground and in the air. The main difference is determined by the exact presence as well as depth of the relatively warmer air, if there is any warmer air present near and/or above the surface.
If there is warmer air present through the depth of the low/mid-levels of the atmosphere all the way down to the surface, this leads to the occurrence of an all-rain event. However, when there ends up being a shallow layer (i.e., generally within the lowest 2,500 to 3,000 feet above the surface) of colder air very close to the surface, this can lead to the occurrence of a freezing rain event. This is because rain falling through a somewhat warmer layer above the surface (i.e., often within the last 1,000 to 2,000 feet) before reaching a sub-freezing layer very close to and/or right above the surface of the Earth facilitates a sudden freezing of the falling rain droplets and thus, icing events.
Thus, such situations often lead to what are most commonly recognized as destructive freezing rain events. The other possibility is for the "frozen layer" of the lower part of the atmosphere (i.e., the lowest 4,000 to 5,000 feet) to be substantially deeper than that observed with the freezing rain scenarios which leads to sleet events where rain freezes well above the surface. This leads to ice pellets which can often have quite a high impact since upon reaching a frozen surface, they can adhere to the surface and create a dangerous situation as well. The message to be taken from all of this is that all-out snowstorms are not the only issues to be concerned about when it comes to winter weather events. Moreover, to always pay attention to forecasts from your local National Weather Service office for your area both days prior to and in the hours leading up to the event itself. In addition, be sure to always stay tuned to situational updates from our team at the Global Weather and Climate Center for the latest updates.
Below are some sample soundings that demonstrate how different temperatures can lead to different precipitation types.
Source: NWS Central Region Headquarters
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What is the drought monitor and why is it important?
“It’s a weekly map showing the location and intensity of drought in the United States. Maps come out on Thursday morning, based on data through the preceding Tuesday morning. It’s not a forecast; it’s an assessment of past conditions, although because drought is slow-moving, it’s often safe to assume that an area in drought one week will still be in drought the next week. It’s not based on a statistical model. It’s based on many indicators and observations, not just precipitation.” http://droughtmonitor.unl.edu/AboutUSDM/FAQ.aspx
This is what June 26th’s map looks like:
The Drought Monitor’s website is http://droughtmonitor.unl.edu/