The central dense overcast, or CDO, of a

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. Depen ...

or strong

subtropical cyclone A subtropical cyclone is a weather system that has some characteristics of both tropical and an extratropical cyclone. As early as the 1950s, meteorologists were uncertain whether they should be characterized as tropical or extratropical cyclon ...

is the large central area of

thunderstorm A thunderstorm, also known as an electrical storm or a lightning storm, is a storm characterized by the presence of lightning and its acoustic effect on the Earth's atmosphere, known as thunder. Relatively weak thunderstorms are some ...

s surrounding its circulation center, caused by the formation of its

eyewall The eye is a region of mostly calm weather at the center of tropical cyclones. The eye of a storm is a roughly circular area, typically in diameter. It is surrounded by the ''eyewall'', a ring of towering thunderstorms where the most severe weat ...

. It can be round, angular, oval, or irregular in shape. This feature shows up in tropical cyclones of tropical storm or hurricane strength. How far the center is embedded within the CDO, and the temperature difference between the cloud tops within the CDO and the cyclone's eye, can help determine a tropical cyclone's intensity with the

Dvorak technique The Dvorak technique (developed between 1969 and 1984 by Vernon Dvorak) is a widely used system to estimate tropical cyclone intensity (which includes tropical depression, tropical storm, and hurricane/typhoon/intense tropical cyclone intensitie ...

. Locating the center within the CDO can be a problem with strong tropical storms and minimal hurricanes as its location can be obscured by the CDO's high cloud canopy. This center location problem can be resolved through the use of microwave satellite imagery. After a cyclone strengthens to around hurricane intensity, an eye appears at the center of the CDO, defining its center of

low pressure In meteorology, a low-pressure area, low area or low is a region where the atmospheric pressure is lower than that of surrounding locations. Low-pressure areas are commonly associated with inclement weather (such as cloudy, windy, with possible ...

and its cyclonic wind field. Tropical cyclones with changing intensity have more lightning within their CDO than

steady state In systems theory, a system or a process is in a steady state if the variables (called state variables) which define the behavior of the system or the process are unchanging in time. In continuous time, this means that for those properties ''p' ...

storms. Tracking cloud features within the CDO using frequently updated satellite imagery can also be used to determine a cyclone's intensity. The highest

maximum sustained wind The maximum sustained wind associated with a tropical cyclone is a common indicator of the intensity of the storm. Within a mature tropical cyclone, it is found within the eyewall at a distance defined as the radius of maximum wind, or RMW. Unl ...

s within a tropical cyclone, as well as its heaviest

rainfall Rain is water droplets that have condensed from atmospheric water vapor and then fall under gravity. Rain is a major component of the water cycle and is responsible for depositing most of the fresh water on the Earth. It provides water f ...

, are usually located under the coldest cloud tops in the CDO.

Characteristics

It is a large region of thunderstorms surrounding the center of stronger tropical and subtropical cyclones which shows up brightly (with cold cloud tops) on satellite imagery. The CDO forms due to the development of an eyewall within a tropical cyclone. Its shape can be round, oval, angular, or irregular. Its development can be preceded by a narrow, dense, C-shaped convective band. Early in its development, the CDO is often angular or oval in shape, which rounds out, increases in size, and appears more smooth as a tropical cyclone intensifies. Rounder CDO shapes occur in environments with low levels of vertical

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 horizont ...

The strongest winds within

s tend to be located under the deepest

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 conve ...

within the CDO, which is seen on satellite imagery as the coldest cloud tops. The

radius of maximum wind The radius of maximum wind (RMW) is the distance between the center of a cyclone and its band of strongest winds. It is a parameter in atmospheric dynamics and tropical cyclone forecasting. The highest rainfall rates occur near the RMW of tropic ...

is usually collocated with the coldest cloud tops within the CDO, which is also the area where a tropical cyclone's

reaches its maximum intensity. For mature tropical cyclones that are steady state, the CDO contains nearly no

lightning Lightning is a naturally occurring electrostatic discharge during which two electrically charged regions, both in the atmosphere or with one on the ground, temporarily neutralize themselves, causing the instantaneous release of an avera ...

activity, though lightning is more common within weaker tropical cyclones and for systems fluctuating in intensity.

Eye

The eye is a region of mostly calm

weather Weather is the state of the atmosphere, describing for example the degree to which it is hot or cold, wet or dry, calm or stormy, clear or cloud cover, cloudy. On Earth, most weather phenomena occur in the lowest layer of the planet's atmos ...

at the center of the CDO of strong

s. The eye of a

storm A storm is any disturbed state of the natural environment or the atmosphere of an astronomical body. It may be marked by significant disruptions to normal conditions such as strong wind, tornadoes, hail, thunder and lightning (a thunderstorm), ...

is a roughly circular area, typically in

diameter In geometry, a diameter of a circle is any straight line segment that passes through the center of the circle and whose endpoints lie on the circle. It can also be defined as the longest chord of the circle. Both definitions are also valid fo ...

. It is surrounded by the eyewall, a ring of towering

s surrounding its center of circulation. The cyclone's lowest

barometric pressure Atmospheric pressure, also known as barometric pressure (after the barometer), is the pressure within the atmosphere of Earth. The standard atmosphere (symbol: atm) is a unit of pressure defined as , which is equivalent to 1013.25 millibars, 7 ...

occurs in the eye, and can be as much as 15% lower than the

atmospheric pressure Atmospheric pressure, also known as barometric pressure (after the barometer), is the pressure within the atmosphere of Earth. The standard atmosphere (symbol: atm) is a unit of pressure defined as , which is equivalent to 1013.25 millibars, ...

outside the storm. In weaker tropical cyclones, the eye is less well-defined or nonexistent, and can be covered by cloudiness caused by

cirrus cloud Cirrus ( cloud classification symbol: Ci) is a genus of high cloud made of ice crystals. Cirrus clouds typically appear delicate and wispy with white strands. Cirrus are usually formed when warm, dry air rises, causing water vapor deposition on ...

outflow from the surrounding central dense overcast.

Use as a tropical cyclone strength indicator

Within the Dvorak satellite strength estimate for tropical cyclones, there are several visual patterns that a cyclone may take on which define the upper and lower bounds on its intensity. The central dense overcast (CDO) pattern is one of those patterns. The central dense overcast utilizes the size of the CDO. The CDO pattern intensities start at T2.5, equivalent to minimal tropical storm intensity, . The shape of the central dense overcast is also considered. The farther the center is tucked into the CDO, the stronger it is deemed. Banding features can be utilized to objectively determine the tropical cyclone's center, using a ten degree

logarithmic spiral A logarithmic spiral, equiangular spiral, or growth spiral is a self-similar spiral curve that often appears in nature. The first to describe a logarithmic spiral was Albrecht Dürer (1525) who called it an "eternal line" ("ewige Linie"). More ...

. Using the 85–92 GHz channels of polar-orbiting microwave satellite imagery can definitively locate the center within the CDO. Tropical cyclones with maximum sustained winds between and can have their center of circulations obscured by cloudiness within visible and infrared satellite imagery, which makes diagnosis of their intensity a challenge. Winds within tropical cyclones can also be estimated by tracking features within the CDO using rapid scan geostationary satellite imagery, whose pictures are taken minutes apart rather than every half-hour.

References

{{Cyclones Tropical cyclone meteorology Weather hazards