You will be required to interpret Vortex Data Messages (VDMs) in this course, so upon completion of this page, you should be able to completely decode and translate a VDM.
In order to fully extract the relevant information from a Vortex Data Message (VDM), you'll need a bit more information than what you just learned. Namely, you'll need to know specifically what information each item contains, along with various codes and units. To help you with translating a VDM, we'll walk through one, item by item, in detail. Make sure to use the links available to navigate easily between each item and its translation.
Before we begin, however, I should point out that the format of VDMs was significantly changed in 2018. The guide for decoding VDMs below is based on the current format, but if you happen to research VDMs for storms that occurred prior to 2018, the format will be different. To help you with any old VDMs you may encounter if you're researching past tropical cyclones, check out the materials I have for you in the Explore Further section below.
The sample VDM that I will decode below was actually the prototype that NHC mocked up when they announced the format change, so it's based on data collected in a real hurricane prior to 2018 (Otto in 2016, to be exact). VDMs are transmitted in an alphabetical manner, and in each report, a letter of the alphabet is followed by information about the center of the tropical circulation. This information includes such items as lat/long of the center, temperatures inside and outside of the eye of the storm, wind information, minimum pressures, etc.
Sample Report: (clicking on each element will take you to the explanation)
URNT12 KNHC 241133 VORTEX DATA MESSAGE AL162016 A. 24/11:12:50Z B. 10.97 deg N 082.77 deg W C. 700 mb 2927 m D. 977 mb E. 210 deg 11 kt F. CLOSED G. C20 H. 90 kt I. 144 deg 5 nm 11:07:00Z J. 253 deg 78 kt K. 158 deg 8 nm 11:07:30Z L. 95 kt M. 314 deg 5 nm 11:17:00Z N. 033 deg 108 kt O. 349 deg 14 nm 11:17:30Z P. 10 C / 3042 m Q. 18 C / 3045 m R. NA / NA S. 12345 / 7 T. 0.02 / 1 nm U. AF301 0616A OTTO OB 13 MAX FL WIND 108 KT 349 / 14 NM 11:17:00Z
Breakdown of the message:
The first line of the message is the code used to identify a vortex message in various meteorological databases, followed by the date and time (Zulu) the message was transmitted. Back to Message
A. DATE AND TIME OF FIX
The time when the center of the storm was located or "fixed". 24/11:12:50Z means the report is from the 24th day of the month, at 11:12:50Z (hours:minutes:seconds of Zulu time). Back to Message
B. LOCATION OF THE VORTEX CENTER ("FIX")
Latitude and Longitude of the vortex fix in decimal degrees. 10.97 deg N 082.77 deg W means 10.97 degrees North latitude, 82.77 degrees West longitude. This information can be used to plot the latest location of the storm center; comparing the current position to previous positions gives the recent movement of the storm. Back to Message
C. MINIMUM HEIGHT AT STANDARD LEVEL
Standard level refers to certain "slices" of the atmosphere used by meteorologists around the world. The exact altitude of each of these slices relates to the pressure. The lower this height is below the "standard" height indicates how low the pressure is inside the hurricane; stronger storms tend to have lower pressures. The number reported is in meters. Hurricane Hunters fly storms at the "surface" (500 to 1500 feet above the water), 925 millibars (2500 feet or 762 meters), 850 mb (4780 ft or 1457 m), or 700 mb (9880 ft or 3011 m).
The aircraft will fly using an autopilot set to follow a constant pressure altitude. For example, when flying a mission at 700 mb, the aircraft's pressure altimeter will read 9,880 feet all day. But as the plane flies into lower pressure, the plane will actually be flying closer to the ground. A radar altimeter bounces radar pulses off the ground and tells the crew how high they actually are, and the meteorologist uses this number to calculate the height of standard surface. In the example above, the 700 millibar height was 2927 meters, which is 84 meters lower than the standard height of 3011 meters. When flying low-level missions (below 1500 feet) this block is reported as NA (Not Applicable). Back to Message
D. MINIMUM SEA-LEVEL PRESSURE
This value, computed from dropsonde or extrapolation, is one of the key pieces of information which indicates the intensity of the storm. "Standard" sea-level pressure is 1013 millibars. Since hurricanes, tropical storms, and tropical depressions are all low-pressure systems, the pressure reported here is almost always lower than standard. The lower the pressure, the more intense the storm. The word "EXTRAP" precedes any pressures extrapolated from aircraft sensor information; if the word "EXTRAP" is not there, it means the pressure was measured directly by a dropsonde released from the aircraft, and is usually more accurate. This lowest pressure is found in the center of the storm, and in this case it was 977 mb. There may be small fluctuations in pressure due to normal, daily pressure rises and falls. Back to Message
E. DROPSONDE CENTER WIND SPEED AND DIRECTION
The wind direction (in degrees) and speed (in knots) at the center of the storm as measured by dropsonde. In this case, winds were from 210 degrees (south-southwest) at 11 knots. In well-developed tropical cyclones, winds at the center will typically be fairly weak compared to the much faster winds found in the eyewall. Back to Message
F. EYE CHARACTER
This is a brief description of what the eye looks like on radar. "CLOSED" means that the eye is completely surrounded by a ring of thunderstorms. "OPEN NE" means there is a break in the eyewall to the northeast, etc. If the eye is not at least 50% surrounded by eyewall clouds, this item and Item G will be reported as "NA" (Not Applicable). Back to Message
G. EYE SHAPE ORIENTATION AND DIAMETER
Eye shapes are coded as follows: C-circular; CO-concentric; E-elliptical and all diameters are transmitted in nautical miles. In this case, "C20" translates to a circular eye with a diameter of 20 nautical miles. Orientation of major axis of an ellipse is transmitted in tens of degrees. Example: E09/15/5 means elliptical eye oriented with major axis through 90 degrees (and also 270 degrees), with length of major axis 15 nm, and length of minor axis 5 nm. CO8-14 means concentric eye with inner eye diameter 8 miles, and outer diameter 14 miles. The "healthiest" hurricanes usually have a small, circular eye. A concentric eye (a ring inside a ring) is a relatively rare phenomenon that may signal a temporary weakening while the storm reorganizes (which we'll explore later in the course). An eye diameter that shrinks (compared to the previous vortex message) may signal intensification: just as a twirling ice skater spins faster as she pulls in her arms, a hurricane may "spin" faster as its eye gets smaller. Eye diameters are usually 10-20 nautical miles, while we sometimes see them as small as 5 nm to as large as 60 nm. Back to Message
H. ESTIMATE OF MAXIMUM SURFACE WIND SPEED OBSERVED ON INBOUND LEG (IN KNOTS)
90 kt means the highest maximum sustained surface wind speed is 90 knots on this particular inbound leg. In the modern era, a Stepped Frequency Microwave Radiometer takes this measurement (I'll discuss how this instrument operates later in this lesson). Back to Message
I. BEARING, RANGE, AND TIME OF THE WIND SPEED OBSERVED IN ITEM H
The "bearing" is the direction (given in degrees) from the center in which the maximum surface wind speed was recorded (similar to compass headings, except these bearings are in reference to "true" instead of "magnetic" north). Due north is 0 degrees, east is 90 degrees, south is 180 degrees, and west is 270 degrees. The bearing in the example is 144 degrees, which means the surface wind speed was recorded southeast of the center. To pinpoint where this was, you also need to know how far away it was: the "range". In this case, the 90 knot wind reported in part H was found 5 nautical miles (about 6 statute miles) southeast of the center at 11:07:00Z (11:07Z exactly). Back to Message
J. MAXIMUM INBOUND FLIGHT-LEVEL WIND SPEED AND DIRECTION
The highest wind speed in knots (and its direction) observed on the last leg inbound to the storm. These winds are at flight level, and were measured directly by the aircraft's instruments. In the example, the peak wind was 253 degrees, 78 knots, which means the wind was blowing from a direction of 253 deg (west-southwest) at a speed of 78 kts (about 90 miles per hour). Back to Message
K. BEARING, RANGE, AND TIME OF THE WIND OBSERVED IN ITEM J
Same method as reporting bearing, range, and time for the surface winds (see Item I, above). In this example, the 78 knot flight-level wind speed reported in Item J was found 158 degrees (south-southeast) of the center, and 8 nautical miles from the center at 11:07:30Z (in this case, that's 30 seconds after the maximum surface wind speed was observed). Usually the strongest winds are found in the "eyewall" surrounding the eye (if there is an eye), and this gives an idea of how large the center (or eye) of the storm is. Back to Message
L. ESTIMATE OF MAXIMUM SURFACE WIND SPEED OBSERVED WHILE FLYING OUTBOUND (IN KNOTS)
95 kt means the highest maximum sustained surface wind speed estimated while flying outbound from the storm center is 95 knots. Estimates are made in the same fashion as those in Item H. Back to Message
M. BEARING, RANGE, AND TIME OF THE WIND SPEED OBSERVED IN ITEM L
Same method as reporting bearing, range, and time for previous wind observations. In this example, the 95 knot estimated surface wind occurred 314 degrees (northwest) of the center, and 5 nautical miles from the center at 11:17:00Z (exactly 1117Z). Back to Message
N. MAXIMUM OUTBOUND FLIGHT-LEVEL WIND SPEED AND DIRECTIONThe highest wind speed in knots (and its direction) observed while flying outbound from the storm's center. These winds are at flight level, and were measured directly by the aircraft's instruments. In the example, the peak wind was 33 degrees at 108 knots, which means the wind was blowing from a direction of 33 degrees (northeast) at a speed of 108 kts (about 124 miles per hour). Back to Message
O. BEARING, RANGE, AND TIME OF THE WIND OBSERVED IN ITEM N
Same method as reporting bearing, range, and time for previous wind observations. In this example, the 108-knot flight-level wind occurred 349 degrees (north-northwest) of the center, and 14 nautical miles from the center at 11:17:30Z (that's 30 seconds after the maximum surface wind speed was observed while flying outbound). Back to Message
P. MAXIMUM FLIGHT-LEVEL TEMPERATURE / PRESSURE ALTITUDE OUTSIDE THE EYE
This gives an idea of the general temperature surrounding the eye. "Standard" temperature at 700 mb (where we fly most hurricanes) is about -5 degrees Celsius, but in the tropics, it's usually 10 to 15 degrees warmer than "standard". What you especially want to look for is how it compares to the temperature inside the eye, in Item Q. The example shows a temperature of 10 degrees Celsius (50 degrees Fahrenheit) at an altitude of 3042 meters (9,980 feet). The altitude is included because the airplane bumps up and down due to turbulence and other factors, and minor changes in the temperature may be due to changes in altitude. Back to Message
Q. MAXIMUM FLIGHT-LEVEL TEMPERATURE / PRESSURE ALTITUDE INSIDE THE EYE
This is yet another indicator of how "healthy" the storm is. One of the unusual features of a hurricane is that it is warmer inside the eye than outside. What you want to look for here is how much warmer it is than the temperature reported outside the eye in Item "P." A developing storm may be only slightly warmer inside the center, while a strong hurricane may be 10 degrees warmer (or more). In this example, the eye temperature of 18 degrees Celsius (64 degrees Fahrenheit) is eight degrees Celsius higher than the temperatures immediately outside the eye. Be sure to look at the remarks in Item "U" to see if there was an even warmer temperature found inside the eye (but more than 5 miles from the fix position). The aircraft was at a pressure altitude of 3045 meters (9,990 feet). Back to Message
R. DEW POINT TEMPERATURE / SEA SURFACE TEMPERATURE INSIDE THE EYE
If available, the dew point measured at the center of the storm (in degrees Celsius) will be reported here; however, a dew point observation was unavailable in this case, so it was reported as "NA" (not applicable). The second part of Item R is no longer used, as the aircraft do not carry the infrared sensors needed to measure sea surface temperature. Back to Message
S. FIX DETERMINED BY / FIX LEVEL
The first string of numbers indicates what the meteorologist used to find the center of the storm, using numbers 1 through 5, as follows: 1-Penetration, 2-Radar, 3-Wind, 4-Pressure, 5-Temperature. After the solidus ("/"), you'll find one or two numbers which show at what level(s) the center was found, as follows: 0-surface, 1-1500 ft, 8-850 mb, 7-700 mb, 5-500 mb, 4-400 mb, 3-300 mb, 2-200 mb, 9-925 mb.
Example: 12345/7 means the fix was determined by all five means: penetration, radar, winds, pressure, and temperature. The fix was made at 700 mb (approx 10,000 feet). If a calm spot was seen on the surface of the water, the fix level could have been "07" to indicate the surface and the 700 mb center were found within 5 nm of each other. Back to Message
T. NAVIGATION FIX ACCURACY / METEOROLOGICAL ACCURACY
These numbers give an estimate of how accurate the position is, in nautical miles. "Navigation accuracy" is a gauge of how well the navigation equipment is operating (within 0.02 nautical miles, in this case). The "Meteorological Accuracy" depends on how well the storm center can be defined by the meteorological data: if there is a sudden, sharp wind shift, and the temperature peak and pressure drop all coincide, the meteorological accuracy will be a small number. A weaker storm will probably have a larger meteorological accuracy. In this case, the meteorological accuracy was one nautical mile. Back to Message
U. REMARKS SECTION
Always starts with the Mission ID (a unique identifier for each mission): AFXXX AABBC NAME OB DD
Agency: Either AF (Air Force Reserve Hurricane Hunters) or NOAA (National Oceanic and Atmospheric Agency) XXX: Tail number of the aircraft AA: Number of missions flown on this storm system BB: Depression number (or "XX" if it's not a depression or greater) C: Ocean basin. "A"=Atlantic, "C"=Central Pacific, "E"=Eastern Pacific NAME: Storm name, or words CYCLONE (for depression) or INVEST. OB: "Observation." DD: Observation number.
Example: AF301 0616A OTTO OB 13 means Air Force Reserve aircraft number 301 is flying the 6th mission on Hurricane Otto, which is the 16th tropical cyclone of the season in the Atlantic/Gulf/Caribbean, and is making the 13th observation of the storm.
The flight meteorologist may add details of anything he or she feels are interesting to note. There are some standard remarks: "MAX FL WIND 108 KT 349 / 14 NM 11:17:00Z" reminds the public about the location and time of the maximum flight-level wind found in the storm overall (in this case, it's the outbound wind described in Items N and O). Another standard remark is given anytime a temperature peak is seen more than 5 nm from the center location. The flight meteorologist may also further describe characteristics of the eye (such as "STADIUM EFFECT" if the clouds form a solid wall all around the eye, and stretch up and outward to reveal a circle of clear sky above, similar to a football stadium that's 50,000 feet tall), among other things. Back to Message
For History Buffs
As I mentioned above, the format of the VDM underwent significant changes in 2018 to include more information about the maximum outbound flight-level winds, as well as to better organize the data. So, if you happen to be researching VDMs about a storm that occurred prior to 2018, you'll encounter a different format than the one described above. For reference, here's a guide for decoding the pre-2018 format of VDMs, which you may find useful in the event that you want to research historic storms.
I also want to mention an interesting bit of history about the items in the VDM that give an estimate of the maximum sustained surface winds in the storm (Items H and L). In the "good old days", the flight meteorologist applied what could be considered an aviator's version of the Beaufort Wind Scale. Instead of observing canvas sails in the wind (as Sir Francis Beaufort did), the flight meteorologist estimated wind speeds by the "look" of the sea. Indeed, the appearance of white caps, foam, sea spray, patches of green foam, or streaks in ocean foam offers clues that allow an experienced flight meteorologist to gauge the speed (and direction) of surface winds. For an example of green streaks that provided clues to flight meteorologists, check out the photo on the right. A major shortcoming of this approach was that sometimes the weather officer just couldn't see the sea surface (obscured by heavy rain, clouds, darkness, etc.). In this case, an "NA" ("Not Applicable") would appear in the VDM. Furthermore, even when the weather officer could see the ocean surface, its appearance could vary based on the altitude of the flight.
However, beginning in 2008, an instrument called the Stepped Frequency Microwave Radiometer began measuring the maximum sustained wind speed that now appears in Items H and L of the Vortex Data Message (unless the instrument breaks down). We'll talk more about the Stepped Frequency Microwave Radiometer, which allows Hurricane Hunters to estimate surface wind speeds even when the sea surface is obscured, later in this lesson.