Category 5 Hurricanes...Wind...Pressure, and then some

[Anonymous]

I think over the past few years we have learned a few new things about "SUPER" Category 5 Hurricanes.

1. They may not be as rare as we think...

Isabel, Ivan, Katrina, Rita
And nearly reaching Cat 5...Charley, Dennis, Emily.

2. 185 mph may be the strongest we tend to see....

Camille has been said to have winds "of 200 mph" and landfall of "190 mph"... however, if there has been one thing proven by four of the last most intense storms (GILBERT, MITCH, KATRINA, AND RITA) it is that wind speeds eventually level off.

For example:::

Hurricane Gilbert - 185 mph
Due to the extreme SST's and low shear. Also, Gilbert had an extra tight eye and eyewall...creating an extreme low pressure of 888 mb. 888 mb may be more in the 190-195 mph range, and winds eventually had no choice but to increase to atleast 185 mph.

Hurricane Mitch - 180 mph
Pretty much the same reason as Gilbert...although with a 906 mb pressure...one could make the claim that if Mitch's 906 mb was 180 mph...then Rita's 897 mb would HAVE TO BE atleast 180 mph, rather than 175 mph. However...there is always differences with the wind/pressure relationship. Mitch was just able to go 5 mph more than Rita was.

Hurricane Katrina - 175 mph (MAYBE ONLY 165 mph)
There is doubt on whether or not Katrina's winds peaked at 175 mph...or 165 mph. They may have peaked at 170 mph instead of 175 mph. Either way, Katrina was a true Category 5 hurricane. I think one reason it's 902 mb pressure did not make winds of 180 mph like Mitch's 906 mb did, was due to it's soon to be peaking out, then weakening...and it's large eye. The shape and size of the eye may have alot to do with the minimum pressure and wind readings. For example...the 1935 storm was extremely small (ABOUT THE SIZE OF HURRICANE CHARLEY 2004) and quickly deepened at the rate of Hurricane Rita (A CATEGORY 1 TO A CATEGORY 5 in 24 HOURS). This deepening caused a very low pressure of 892 mb...and subsequent winds of 185 mph at Craig Key, FL.

Hurricane Rita - 175 mph
Even if Katrina was not 175 mph...Rita sure was. However, a pressure of 897 mb would typically give you a wind speed of 185-190 mph (EXAMPLE...HURRICANE ALLEN 1980) However, I think this was another of example, like Katrina...of a storm that needed to peak...and was about to begin a weakening trend. Plus, hurricanes are crazy wonders of nature...who knows why Rita did not gain more wind speed.

My thoughts...

I think one thing that must be taken into consideration is...

Is the hurricane in an area of higher or lower pressures?

For example, Hurricane Opal in 1995 (150 mph CAT 4) peaked at 916 mb. Although that is EXTREMELY rapid deepening, the pressure of your typical 150 mph Category 4 hurricane is about 928-933 mb. However, notice Hurricane Opal when it became a minimal hurricane:::


REPEATING THE 10 AM CDT POSITION...21.0 N... 92.5 W. MOVEMENT
TOWARD...WEST NEAR 3 MPH. MAXIMUM SUSTAINED WINDS... 75 MPH.
MINIMUM CENTRAL PRESSURE... 973 MB.

973 mb typically supports a minimal Cat 2...not a minimal Cat 1. So, Opal was in an area of lower pressures. Opal's 973 mb should have been more like 985 mb ith 75 mph. So, Opal was about 15 mb lower than average. With Opal peaking out at 916 mb....add on 15 mb, you get 931 mb...the typical pressure for a 150 mph hurricane

With Hurricane Charley of 2004, Charley was in an area of higher pressures...for example:::

REPEATING THE 5 PM EDT POSITION...21.2 N... 81.9 W. MOVEMENT
TOWARD...NORTH-NORTHWEST NEAR 18 MPH. MAXIMUM SUSTAINED
WINDS...105 MPH. MINIMUM CENTRAL PRESSURE... 980 MB.

980 mb typically equates to about 85 mph or so, not 105 mph. 965-970 mb is typically what you get with 105 mph. So, Charley was about 10-15 mb above average for pressure. His landfall of 150 mph is typical with a 928-931 mb hurricane...not 941 mb. So, subtract 10-15 mb from Charley's 941 mb...you get 926-931 mb...more typical of a 150-155 mph hurricane.

I've typed too much, I know....have a good night.

[f5]

CAT 5 hurricanes are beautiful until you see the destruction the next day

[JtSmarts]

Great post Mike, it goes to show that you really cannot compare certain hurricanes directly without looking at other factors.

[JtSmarts]

Double post.

[Tampa Bay Hurricane]

Great Info

[O Town]

Yes great info. Makes sense to me. Everyone always is talking all about the pressure, but it goes to show as you stated it depends on where the hurricane is in its so called life cycle, if it has already peaked, or about to peak. You can't always go by pressure alone for sure, as you just proved. Great post.

[senorpepr]

Floydbuster mentions pressure of a storm in comparison to the background pressure. However, I want to point out pure pressure gradient which is something a few folks are having trouble understanding.

Okay… to first understand pressure gradient, we need to understand pressure. Pressure is the amount of atmospheric push on Earth. On a weather map we depict pressure by using isobars – lines of equal pressure. The standard weather map places each isobar 4mb apart using a base of 1000mb. Therefore, every location that has a pressure of 1000mb will be located on the same line.



In the above graphic, there are curved lines – those are isobars. They are labeled with the equal pressure that line represents. The line that cuts through the Gulf of Mexico, labeled 1008, means that along that line, pressure is 1008mb. North of that line (closer toward high pressure) the pressure rises. South and west of that line (closer toward low pressure) the pressure drops.

Now… pressure gradient is how close or far apart each isobar is. Notice how the isobars across the Rocky Mountains are closer together? That means the pressure gradient is higher, therefore the winds are higher. Notice how the isobars across the Southeast are further apart? That means the pressure gradient is lower, therefore the winds are lower.

(Of course, this map is time sensitive. If you view this post a day or later from now, this map won’t quite match up, but bear with me.)

Now… I’m going to help demonstrate pressure gradient by using a few examples.

In this case, I’m using a blanket background pressure of 1005mb to designate the extent of the tropical cyclone based on its tropical storm force winds. In other words, each storm listed here, size-wise, will be based on how far its tropical storm force winds extended, in a circular pattern. The pressure at the “edge” of each cyclone will be 1005mb.

Code: Select all

Storm   Radius  Wind  Pressure  PresGradient
Name    (mi)    (mph) (mb)      (mb/mi)
---------------------------------------------
ALPHA   080     100   974       0.379
BRAVO   080     075   985       0.250

CHARLEY 120     085   981       0.200
DELTA   080     085   989       0.200

ECHO    145     155   948       0.393
FOXTROT 205     145   928       0.376

KATRINA 230     165   902       0.448
RITA    185     175   897       0.584


Okay, in the above chart, Storm Alpha and Bravo are of the same size, but Alpha has a lower pressure. In Alpha, for every mile, the pressure drops 0.379mb whereas in Bravo, it drops 0.25mb per mi. Therefore, Alpha has the stronger pressure gradient and winds.

Storm Charley and Delta have the same wind speeds and pressure gradients, but Charley is a much bigger storm than Delta. Because Charley is bigger and his pressure gradient is the same as Delta’s, his central pressure must be lower.

Storm Echo and Foxtrot show how a lower pressure doesn’t always equal a stronger storm. Echo is small (145mi) and has a fairly high pressure (948mb). Think of Echo like Hurricane Charley of last year that struck Florida. Foxtrot is bigger (205mi) and has a lower pressure (928mb). Think of Foxtrot like Ivan last year at landfall. Echo’s pressure is higher than Foxtrot’s, but since Echo is smaller, his pressure gradient is higher. Because of this, Echo is windier than Foxtrot.

Finally, here’s an example of two powerhouse hurricanes this season. I’ve listed Katrina and Rita at their strongest (in terms of pressure).

[Quick side note: NHC, at advisory time, believes that Katrina was never 175 mph and adjusted their flight-level conversion. That is why the pressure dropped, but the winds decreased.]

Katrina was a big hurricane. Her tropical storm force winds extend out 230mi from the center. Now, assume that the “background” pressure is 1005mb. With a central pressure of 902mb, here pressure gradient was 0.448mb/mi.

Rita was much smaller (185mi) as compared to Katrina. The pressure was a shade lower at 897mb. Because Rita was a smaller storm, her pressure gradient was much higher: 0.584mb/mi.

Look at it this way: Charley was 150mph at Cayo Costa, FL, per the NHC Best Track. Katrina was 140mph at Buras, LA, per the NHC Advisory. Charley’s pressure at landfall was 941mb per the NHC Best Track. Katrina’s pressure at landfall was 918mb per the NHC Advisory. Charley’s pressure gradient was much higher than Katrina’s. This allows Charley to be windier at landfall, but have a much higher pressure than Katrina.

Hopefully this better explains pressure gradient. If you are still unsure of it, let me know and I’ll try to explain it further.

[LarryWx]

senorpepr,
Great post. JB seems to always be fixated on the strongest winds of a storm being solely determined by lowest pressure with little or no consideration of either ambient pressure or size of storm. I feel he tries to simplify the lowest pressure-highest wind relationship way too much. Maybe you should send him your write-up.

[MGC]

What makes the wind blow? Several things. Difference it pressure and difference in temperature. We are all familiar with the sea breeze. Wind in a hurricane are driven by the pressure gradient. The pressure gradient in any particular hurricane is different. As senorpepr pointed out the differences between Charley and Katrina. Charley, a much smaller hurricane than Katrina had a much tighter pressure gradient, that is a greater pressure change over a given distance. Intense hurricanes with small eyes typically have tighter pressure gradients than a large hurricane. A good analogy is the funnel one drops a coin in. The slope of the funnel at the outside where the coin enters is not steep and the coin rolls slowly. This represents a loose pressure gradient. As the coin continues its path towards the center the slope of the funnel deepens and the coin speeds up. This funnel shape is very simular to a barograph of a station that has had a hurricane pass over it. It is so simple, the slope of the gradient is proportional to the speed of the parcle of air transversing that given distance (I think). One of you math majors help me out...............MGC

[terstorm1012]

Can I nominate this as a sticky? Very informative.