A hot tower is a tropical
cumulonimbus cloud
Cumulonimbus (from Latin ''cumulus'', "heaped" and ''nimbus'', "rainstorm") is a dense, towering vertical cloud, typically forming from water vapor condensing in the lower troposphere that builds upward carried by powerful buoyant air currents. ...
that reaches out of the lowest layer of the atmosphere, the
troposphere
The troposphere is the first and lowest layer of the atmosphere of the Earth, and contains 75% of the total mass of the planetary atmosphere, 99% of the total mass of water vapour and aerosols, and is where most weather phenomena occur. From ...
, and into the
stratosphere
The stratosphere () is the second layer of the atmosphere of the Earth, located above the troposphere and below the mesosphere. The stratosphere is an atmospheric layer composed of stratified temperature layers, with the warm layers of air ...
.
These formations are called "hot" because of the large amount of
latent heat
Latent heat (also known as latent energy or heat of transformation) is energy released or absorbed, by a body or a thermodynamic system, during a constant-temperature process — usually a first-order phase transition.
Latent heat can be understo ...
released as
water vapor
(99.9839 °C)
, -
, Boiling point
,
, -
, specific gas constant
, 461.5 J/( kg·K)
, -
, Heat of vaporization
, 2.27 MJ/kg
, -
, Heat capacity
, 1.864 kJ/(kg·K)
Water vapor, water vapour or aqueous vapor is the gaseous pha ...
condenses into liquid and freezes into ice within the cloud. Hot towers in regions of sufficient
vorticity
In continuum mechanics, vorticity is a pseudovector field that describes the local spinning motion of a continuum near some point (the tendency of something to rotate), as would be seen by an observer located at that point and traveling along wit ...
may acquire rotating updrafts; these are known as ''vortical hot towers'' In some instances, hot towers appear to develop characteristics of a
supercell
A supercell is a thunderstorm characterized by the presence of a mesocyclone: a deep, persistently rotating updraft. Due to this, these storms are sometimes referred to as rotating thunderstorms. Of the four classifications of thunderstorms (s ...
, with deep and persistent rotation present in the updraft.
The role of hot towers in
tropical weather was first formulated by
Joanne Simpson
Joanne Simpson (formerly Joanne Malkus, born Joanne Gerould; March 23, 1923 – March 4, 2010) was the first woman in the United States to receive a Ph.D. in meteorology, which she received in 1949 from the University of Chicago.Atlas D and Lemone ...
in 1958. Hot towers dominated discussions in tropical meteorology in the 1960s and are now considered the main drivers of rising air within
tropical cyclone
A tropical cyclone is a rapidly rotating storm system characterized by a low-pressure center, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain and squalls. Depend ...
s and a major component of the
Hadley circulation
The Hadley cell, named after George Hadley, is a global-scale tropical atmospheric circulation that features air rising near the equator, flowing poleward at a height of 10 to 15 kilometers above the earth's surface, descending in the subtropics, ...
. Although the prevalence of hot towers in scientific literature decreased in the 1970s, hot towers remain an active area of research. The presence of hot towers in
tropical cyclone
A tropical cyclone is a rapidly rotating storm system characterized by a low-pressure center, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain and squalls. Depend ...
s is correlated with an increase in the tropical cyclones's intensities.
Observation
Hot towers were first detected by
radar
Radar is a detection system that uses radio waves to determine the distance (''ranging''), angle, and radial velocity of objects relative to the site. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, w ...
in the 1950s.
Aerial reconnaissance was used to probe hot towers, though planes avoided the most dangerous cores of hot towers due to safety concerns. The launch of the
Tropical Rainfall Measuring Mission
The Tropical Rainfall Measuring Mission (TRMM) was a joint space mission between NASA and JAXA designed to monitor and study tropical rainfall. The term refers to both the mission itself and the satellite that the mission used to collect data. ...
(TRMM) in 1997 provided the resolution and coverage necessary to systematically catalog hot towers and precisely assess their structure globally.
Prior to 1997, the small size and short duration of hot towers limited studies of hot towers to aerial observations as the resolutions of satellite sensors at
microwave
Microwave is a form of electromagnetic radiation with wavelengths ranging from about one meter to one millimeter corresponding to frequencies between 300 MHz and 300 GHz respectively. Different sources define different frequency ran ...
and
infrared
Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...
wavelengths were too coarse to properly resolve details within hot towers.
Structure
The term ''hot tower'' has been applied to both rapidly rising parcels of air and the tall
cumulonimbus cloud
Cumulonimbus (from Latin ''cumulus'', "heaped" and ''nimbus'', "rainstorm") is a dense, towering vertical cloud, typically forming from water vapor condensing in the lower troposphere that builds upward carried by powerful buoyant air currents. ...
s that accompany them.
The regions of rising air are horizontally small and span about across. Their greatest extent is in the vertical, reaching altitudes as high as and exhibiting high
reflectivity
The reflectance of the surface of a material is its effectiveness in reflecting radiant energy. It is the fraction of incident electromagnetic power that is reflected at the boundary. Reflectance is a component of the response of the electronic ...
. Hot towers are effectively undilute; as they ascend, the surrounding air does not mix with the rising parcels of air. As a result, the
equivalent potential temperature Equivalent potential temperature, commonly referred to as theta-e \left( \theta_e \right), is a quantity that is conserved during changes to an air parcel's pressure (that is, during vertical motions in the atmosphere), even if water vapor condense ...
within a hot tower remains nearly constant throughout their entire vertical extent. This allows for efficient transport of heat from the lower
troposphere
The troposphere is the first and lowest layer of the atmosphere of the Earth, and contains 75% of the total mass of the planetary atmosphere, 99% of the total mass of water vapour and aerosols, and is where most weather phenomena occur. From ...
to the
stratosphere
The stratosphere () is the second layer of the atmosphere of the Earth, located above the troposphere and below the mesosphere. The stratosphere is an atmospheric layer composed of stratified temperature layers, with the warm layers of air ...
. Hot towers forming within areas of rotation may feature rotating
updraft
In meteorology, an updraft is a small-scale current of rising air, often within a cloud.
Overview
Localized regions of warm or cool air will exhibit vertical movement. A mass of warm air will typically be less dense than the surrounding region, ...
s; these are known as ''vortical hot towers'' and are associated with localized regions of anomalous vertical
vorticity
In continuum mechanics, vorticity is a pseudovector field that describes the local spinning motion of a continuum near some point (the tendency of something to rotate), as would be seen by an observer located at that point and traveling along wit ...
.
Conceptual development
![DaisyAug2819580530UTCNavyRecon](https://upload.wikimedia.org/wikipedia/commons/e/ed/DaisyAug2819580530UTCNavyRecon.png)
Before the 1950s, the mechanism driving atmospheric
Hadley cell
The Hadley cell, named after George Hadley, is a global-scale tropical atmospheric circulation that features air rising near the equator, flowing poleward at a height of 10 to 15 kilometers above the earth's surface, descending in the subtropics, ...
s—an air circulation that transports tropical heat and moisture poleward—was poorly understood.
It was initially believed that the Hadley cell was fueled by the broad, diffuse, and gradual rise of warm and moist air near the equator. However calculations of
Earth's energy budget
Earth's energy budget accounts for the balance between the energy that Earth receives from the Sun and the energy the Earth loses back into outer space. Smaller energy sources, such as Earth's internal heat, are taken into consideration, but ma ...
using data from
World War II
World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposin ...
showed that the mid-troposphere was an energy deficit region, indicating that the maintenance of the Hadley cell could not be explained by the broad ascent of air. The role of the tropical regions in the global climate system and the development of tropical disturbances were also poorly understood. The 1950s marked a pivotal decade that saw the advancement of
tropical meteorology
A tropical cyclone is a rapidly rotating storm system characterized by a low-pressure center, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain and squalls. Depend ...
, including the creation of the U.S.
National Hurricane Research Project
The National Hurricane Research Project (NHRP) was initiated in 1955 by the United States Weather Bureau in response to the devastating 1954 hurricane season, which saw hurricanes ''Carol'', ''Edna'', and ''Hazel'' bring destruction and floodin ...
in 1956. In 1958,
Herbert Riehl
Herbert Riehl (March 30, 1915 – June 1, 1997) was a German-born American meteorologist who is widely regarded as the father of tropical meteorology. He is well known for his work with Joanne Simpson on the importance of hot towers, and their crit ...
and
Joanne Simpson
Joanne Simpson (formerly Joanne Malkus, born Joanne Gerould; March 23, 1923 – March 4, 2010) was the first woman in the United States to receive a Ph.D. in meteorology, which she received in 1949 from the University of Chicago.Atlas D and Lemone ...
proposed that the release of latent heat caused by condensation within hot towers supplied the energy necessary to maintain Hadley cells and the
trade winds
The trade winds or easterlies are the permanent east-to-west prevailing winds that flow in the Earth's equatorial region. The trade winds blow mainly from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisph ...
; their hypothesis was initially based on aerial observations made by Simpson during her time at
Woods Hole Oceanographic Institution
The Woods Hole Oceanographic Institution (WHOI, acronym pronounced ) is a private, nonprofit research and higher education facility dedicated to the study of marine science and engineering.
Established in 1930 in Woods Hole, Massachusetts, it i ...
.
This mechanism required the existence of undilute cumulonimbus clouds that did not
entrain the surrounding air, allowing for the efficient transfer of heat from the ocean surface into the upper troposphere. The existence of 1,500–2,500 of these clouds was required if they were to support the Hadley circulation. The researchers also argued that hot towers helped maintain the warmth present at the center of
tropical cyclone
A tropical cyclone is a rapidly rotating storm system characterized by a low-pressure center, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain and squalls. Depend ...
s and that the ascent of moist air within tropical cyclones was concentrated around the hot towers.
In their original 1958 paper outlining the role of hot towers, Riehl and Simpson described these clouds as "narrow warm towers", but began terming the idea as the "hot tower hypothesis" by 1960.
For the next two decades, hot towers dominated scientific discussion concerning the interaction between
cumulus cloud
Cumulus clouds are clouds which have flat bases and are often described as "puffy", "cotton-like" or "fluffy" in appearance. Their name derives from the Latin ''cumulo-'', meaning ''heap'' or ''pile''. Cumulus clouds are low-level clouds, gener ...
s and their larger-scale tropical environments.
![Hottower](https://upload.wikimedia.org/wikipedia/commons/d/d0/Hottower.jpg)
Aerial observations of
Hurricane Daisy in
1958
Events
January
* January 1 – The European Economic Community (EEC) comes into being.
* January 3 – The West Indies Federation is formed.
* January 4
** Edmund Hillary's Commonwealth Trans-Antarctic Expedition completes the third ...
suggested that
convection
Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the convec ...
within tropical cyclones was limited to a few areas of cumulonimbus clouds, dispelling the idea that rising air was distributed throughout the entire cyclone's envelope and lending support for the hot tower hypothesis. In the case of Hurricane Daisy, the convecting cumulonimbus clouds represented only about four percent of the total region of
precipitation
In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail. ...
associated with the hurricane. A 1961 analysis by Riehl and Simpson using the NHRP data from Hurricane Daisy concluded that hot towers were the principal mechanism by which tropical cyclones move warm air into the upper troposphere. The newfound importance of hot towers in tropical cyclones motivated the development of
parametrization—the representation of small-scale phenomena and interactions, i.e. individual cumulus clouds—in early
weather models
Numerical weather prediction (NWP) uses mathematical models of the atmosphere and oceans to predict the weather based on current weather conditions. Though first attempted in the 1920s, it was not until the advent of computer simulation in th ...
. The hot tower hypothesis also inspired the development of
convective instability of the second kind
In meteorology, convective instability or stability of an air mass refers to its ability to resist vertical motion. A ''stable'' atmosphere makes vertical movement difficult, and small vertical disturbances dampen out and disappear. In an ''unst ...
(CISK): a conceptual model that emphasized the
feedback
Feedback occurs when outputs of a system are routed back as inputs as part of a chain of cause-and-effect that forms a circuit or loop. The system can then be said to ''feed back'' into itself. The notion of cause-and-effect has to be handled ...
s between the latent heat released by individual cumuli and the convergence associated with tropical cyclones. By the 1970s, many of the ideas and predictions put forth by the hot tower hypothesis had been validated by empirical observations. Critics of the hot tower hypothesis contended it was implausible that a cumulonimbus cloud could be free of entrainment.
This facet of the hypothesis remained untested until dropsondes released into hot towers as part of the Convection and Moisture Experiment in 1998 provided the first direct measurements of the thermodynamic structure of hot towers. The data showed that the
equivalent potential temperature Equivalent potential temperature, commonly referred to as theta-e \left( \theta_e \right), is a quantity that is conserved during changes to an air parcel's pressure (that is, during vertical motions in the atmosphere), even if water vapor condense ...
within hot towers was virtually constant across their entire vertical extent, confirming the lack of entrainment. Other field observations have suggested that some tropical updrafts are diluted by their surrounding environments at altitudes lower than , though strong latent heat generated by
ice within the cloud was sufficient to provide the requisite input energy for the Hadley circulation. Scientific research of hot towers experienced a resurgence in the 2000s with a renewed focus on their role in
tropical cyclogenesis
Tropical cyclogenesis is the development and strengthening of a tropical cyclone in the atmosphere. The mechanisms through which tropical cyclogenesis occurs are distinctly different from those through which temperate cyclogenesis occurs. Tropi ...
and tropical cyclone development.
Effect on tropical cyclones
Vortical hot towers aid in the formation of tropical cyclones by producing many small-scale positive anomalies of
potential vorticity
In fluid mechanics, potential vorticity (PV) is a quantity which is proportional to the dot product of vorticity and stratification. This quantity, following a parcel of air or water, can only be changed by diabatic or frictional processes. It i ...
, which eventually coalesce to strengthen the broader storm. The high vorticity present in the hot towers traps the latent heat released by those clouds, while the merger of the hot towers aggregates this enhanced warmth. These processes are the major part of the initial formation of a tropical cyclone's warm core—the anomalous warmth at the center of such a system—and the increased
angular momentum
In physics, angular momentum (rarely, moment of momentum or rotational momentum) is the rotational analog of linear momentum. It is an important physical quantity because it is a conserved quantity—the total angular momentum of a closed syst ...
of the winds encircling the developing cyclone.
In 2007, the
National Aeronautics and Space Administration
The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research.
NASA was established in 1958, succeeding th ...
(NASA) hypothesized that the
wind shear
Wind shear (or windshear), sometimes referred to as wind gradient, is a difference in wind speed and/or direction over a relatively short distance in the atmosphere. Atmospheric wind shear is normally described as either vertical or horizontal ...
between the eye and the eyewall could enhance updraft through the center of a cyclone and generate convection.
Hot towers may appear when a cyclone is about to intensify, possibly
rapidly. A particularly tall hot tower rose above
Hurricane Bonnie in August 1998, as the storm intensified before striking
North Carolina
North Carolina () is a state in the Southeastern region of the United States. The state is the 28th largest and 9th-most populous of the United States. It is bordered by Virginia to the north, the Atlantic Ocean to the east, Georgia and So ...
.
See also
*
List of cloud types
The list of cloud types groups all genera as ''high'' (cirro-, cirrus), ''middle'' (alto-), ''multi-level'' (nimbo-, cumulo-, cumulus), and ''low'' (strato-, stratus). These groupings are determined by the altitude level or levels in the troposphe ...
*
Pileus
*
Overshooting top
An overshooting top (or penetrating top) is a dome-like protrusion shooting out of the top of the anvil of a thunderstorm and into the lower stratosphere. When an overshooting top is present for 10 minutes or longer, it is a strong indication that ...
*
Rapid intensification
In meteorology, rapid intensification is a situation where a tropical cyclone intensifies dramatically in a short period of time. The United States National Hurricane Center defines rapid intensification as an increase in the maximum sustained w ...
References
Bibliography
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External links
Hurricane Multimedia Gallerynbsp;– a hurricane multimedia page.
{{DEFAULTSORT:Hot tower
Weather hazards
Tropical cyclone meteorology
Articles containing video clips
Cloud and fog physics