What is a ‘hurricane’?. Where do they form? Why? Why do they move east to west rather than west to east? Why do they have names?!? And how do they pick the names? What is the ‘eye”? The ‘eye wall”? How do hurricanes compare to other storms? Typhoons? mid-latitude cyclones?
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“IVAN IS FORECAST TO SLOWLY DEEPEN OVER THE NEXT 36 HOURS REACHING 130 KT IN 48 HOURS “
DEEPEN? What does this mean? How does it relate to hurricane intensity?
“ESTIMATED MINIMUM CENTRAL PRESSURE 951 MB MAX SUSTAINED WINDS 110 KT WITH GUSTS TO 130 KT. “
What is an “MB”? A “KT”? What is the significance of these values?
“NOW THAT FRANCES HAS MOVED OVER THE WARM WATERS OF THE NORTHEAST GULF OF MEXICO...THUNDERSTORMS ACTIVITY HAS STEADILY INCREASED NEAR THE TIGHT CIRCULATION CENTER DURING THE PAST 6 HOURS. THE LAST RECON REPORT AT AROUND 05Z INDICATED A RAGGED EYE WAS TRYING TO FORM…
NOW THAT THE CENTER OF FRANCES IS BACK OVER WARM WATER...SOME RE-STRENGTHENING IS POSSIBLE. WATER TEMPERATURES ARE NEAR 83-84F AND UPPER-LEVEL OUTFLOW IS STILL QUITE IMPRESSIVE.”
06 Sept. 04 Visible Satellite Image
A SMALL EYE IS EVIDENT ON THE VISIBLE IMAGERY. …. AND SOMEWHAT RAGGED-LOOKING BANDING FEATURES
06 Sept 04 IR Satellite Image
The Dvorak Technique Explained
The Dvorak technique is a method using enhanced Infrared and/or visible satellite imagery to quantitatively estimate the intensity of a tropical system. Cloud patterns in satellite imagery normally show an indication of cyclogenesis before the storm reaches tropical storm intensity. Indications of continued development and/or weakening can also be found in the cloud features. Using these features, the pattern formed by the clouds of a tropical cyclone, expected systematic development, and a series of rules, an intensity analysis and forecast can be made. This information is then standardized into an intensity code
Note the difference in size between the forecast cones… note the degradation of certainty over time, and the higher certainty of the track for hurricane ‘Ivan’
The Earth in Space
Another way of looking at June 22…
Daylight pattern on 22 September… near the equinox
What would December 22 look like?
What significance do the tropics of Cancer and Capricorn have?
Note the influence of clouds… which interrupts the latitudinal patterns
Electromagnetic Radiation (EMR) can be described as waves.
Note the generation of a magnetic field at right angles to an electrical field and both are perpendicular to the direction of EMR propagation
The wavelength () of EMR is directly related to the amount of NRG in the radiation.
Short () (e.g. gamma rays) have a higher NRG content
Long () (e.g. thermal or microwave (Radio) have a lower NRG content
As the velocity of the propagation of EMR (the speed of light) is believed to be a constant (c)
The number of waves to pass a single point in a given period (the frequency, v) is much higher for shorter radiation
Blackbody radiance curves…
Wein’s Displacement Law
When the sun is at very low angles, virtually all the shorter EMR is scattered, allowing only the longer, lower NRG waves (reds) to pass through.
Atmospheric scattering: (skylight or haze)
Rayleigh scatter (molecular scatter) primary factor in upper atmosphere (why the sky is blue)
Mie scatter (non-molecular scattering, water/ice/salt/smoke) primarly in lower atmosphere
Clouds and Ice have an Albedo approaching 100%
Global albedo is not constant… but the overall average is 35%
NRG is transferred by:
Conduction (illustrated here)
Convection (description follows)
Some EMR passes through the atmosphere with no interference (an ‘atmospheric window’
Some EMR is absorbed (or blocked) by components in the atmosphere
The most significant absorbers of EMR in the atmosphere are O2 , N2, O3, CO2, H2O
45 degrees N… halfway between equator and pole
57.5 degrees North (same latitude as Moscow and Hudson’s Bay… semi-tropical gardens on the coast of Scotland
Tropical Weather & Waves
Tropical winds typically blow from the east, and when they encounter a slow moving trough of low pressure, called a tropical wave, the winds initially converge and lift to produce showers and thunderstorms.
Structure of a Hurricane
Tropical cyclones are the international name of hurricanes, which typically form from an organized mass of storms formed along a tropical wave.
In this image of Hurricane Elena, the central area of broken clouds is the eye, surrounded by an eye wall cloud and spiral rain bands, with a total diameter nearing 500 kilometers.
Hurricane Wind Profile
The low pressure core of the hurricane is surrounded by several thunderstorms, each with updraft and downdraft cycles.
The wind and moisture cycle is repeated as:
surface moist air converges in a counterclockwise pattern at the eye, rises to create high pressure aloft, condenses, precipitates, dries, diverges outward in a clockwise pattern, sinks, and warms.
3-D Radar Image of Hurricane
Several key features of a hurricane are shown in this radar composite image, including overshooting clouds, the area of strongest echoes (heaviest rain), and the eyewall.
Formation by Organized Convection
One theory explains that hurricane formation requires cold air above an organized mass of thunderstorms, where the release of latent heat warms the upper troposphere, creates high pressure aloft, which pushes air outward and causes a low to deepen at the surface.
Air moving toward this low intensifies the cycle.
Formation by Heat Engine
Another theory of hurricane development proposes that a heat engine cycle, fueled by warm moist input air and the release of heat when it converts to cool dry air.
Differences in the input and output temperatures determine the amount of work on the ocean and winds that is performed.
Hurricane Stages of Development
The initial components of a hurricane may form as a tropical disturbance, grow into a tropical depression when winds exceed 20 knots, become a tropical storm when winds exceed 35 knots, and finally then qualify as a hurricane when winds exceed 64 knots.
Erratic Paths of Hurricanes
Historical charts of hurricane location may reveal erratic, and hard to predict, patterns of movement.
As this figure shows, hurricanes may occasionally double back.
Further, when removed from the ocean and without a moisture source to supply energy, they may still continue an inland journey.
In the North Atlantic, on average 3 storms per year move inland and bring damaging winds and rain.
North Atlantic Hurricanes
Composite infrared imagery of Hurricane Georges reveals the pattern of a seasonal threat for Central and North America coastlines.
Tropical cyclones at the same latitude survive longer in the Atlantic than Pacific Ocean because of warmer Atlantic Ocean waters.
Hurricane Damage & Warning
Hurricanes have their highest wind speeds on the side where storm pushing winds amplify cyclonic, or counterclockwise, rotational winds. In coastal areas, flooding is aggravated by the hurricane low pressure triggering higher tides and Ekman transport piling up water.
Hurricane Watch & Warning
The National Hurricane Center in Florida issues a hurricane watch 24 to 48 hours before a threatening storm arrives, and if it appears that the storm will strike within 24 hours, a hurricane warning is issued.
While some consider the warning area too large, causing unneeded evacuation, such evacuations have saved many lives.
Hurricane Hugo, with peak winds near 174 knots, caused tremendous damage.
Hurricane Saffir-Simpson Winds
In 1989 Hugo caused nearly $7 billion in damages in the U.S., killing 49 in the Caribbean and United States.
Current classification of hurricanes is based on their wind speed, however, and not on human or property damage.
Hurricanes range from category 1 to 5, with winds of 64 to more than 135 knots.
Hurricane Names and Cost
Category 5 Hurricane Andrew (1992) was the costliest US storm, but it ranks as less intense than 1935 and 1969 hurricanes.
Hurricane names are chosen from an alphabetical list of male and female names for the Atlantic and Pacific, some of which are retired if the storm was especially damaging.
Hurricane Andrew Devastation in Homestead, Florida August 24, 1992
Likelihood for Landfall
Between 1900 and 1999, only two category 5 hurricanes have made landfall along the Gulf or Atlantic.
Numerous category 1, and less damaging storms, that do make landfall may not cause much damage, but bring needed rainfall.