Hanna made landfall near Calabash NC, about 30 miles further down the coast from where I am. Winds remained fairly strong through the entire night with a peak gust of 72MPH just after 5AM when the center of the storm was located just to our west. That figure was a testament to the accuracy of the NWS intensity at 70MPH...just under hurricane strength. Winds became more variable during that time with alternating strong gusts and near calm.
Power was on and off through the night and I could see the power flashes with the stronger gusts. No real damage here though other than some beach erosion related damage at the south facing shorelines. We've seen more than a half dozen hurricanes in 10 years here so there aren't many weak trees or structures around anymore...just a lot of leaves and dead limbs. Some tough trees indigenous to this area..they took a beating for about 8-10 hours with winds gusting to 50-60 and above. Seems worse at sea level with all the "stuff" in the way as opposed to the summit, where I was lucky enough to experience wind speeds in the 70's in summer during my volunteer stay.
Only 2+ inches of rain. The heaviest rain bands stayed off to the west side (not typical) as dry air was pulled into the circulation on the east side. Inland areas got 5+ inches. Most of our 2" came during the earlier intense rainbands that spiraled in well in advance of the strongest winds....
Glad Ike isn't our problem.
Last edited by timechime; 09-07-2008 at 10:35 PM.
You're right. 70mph at the ocean is stronger than 70mph at 6,000 feet. There are more air molecules pushing on you at lower elevations. Its not faster, it just has more energy behind it. The potential of being hit by debris is also a significant factor as well.
Would you be able to stand in a river of water moving at 10mph? But you could stand in a river of air moving at 10mph. Besides some obvious physical differences the main factor here is density. Water being more dense than air. Air at sea level is denser than air at 6000 feet. 1000mb is 25% more dense than 800mb (at equal temperature and humidity).
You are correct. The 72 MPH observation was at the coast. My guess was sustained was at 60 or so. I live less than 1 mile from the ocean. Once you get only a short distance inland, the relatively constant wind at the ocean becomes disrupted and far gustier.Technically when the NHC says a storm is 70mph that means the sustained winds, nut just the gusts. But keep in mind that hurricanes are ocean storms and their winds rarely translate to the land.
When we have storms here, I will check the Frying Pan Shoals buoy data. FPS is located about 15 miles south of Cape Fear...way out in the open water. Wind speeds measured at the Shoals are typically a good percentage above land based observations.
So...if there's 20% less air at 6,000 Ft, would it stand to reason that wind speed would need to be 20% stronger to be equivalent to sea level? I say 20% stronger and 20% higher in speed since wind forces are not directly proportional to speed, but rather work exponentially.
As for Andrew...Just for arguments sake and weather banter, I was aware that well after the storm, it was officially upgraded to a Cat 5 due to the damage patterns and (limited) wind data. The peak measured gust was 168 before the equipment failed. Andrew was also in a deepening phase at landfall. Perhaps the lag time between pressure drop and increase in wind speed right at landfall created the impression that the winds actually translated from ocean to land. Had it gone through that phase earlier, winds may have ramped up to 175+ sustained before it came inland....Maybe.
I have a cool Excel worksheet that allows you to compare the force of the wind at different speeds,elevations,temps and pressure. Send me a pm with your email if you are interested. It was created a few years ago by an MWO intern.
I think the fact that Andrew was strengthening was a major factor. Sort of like Katrina at landfall in FL, people were very surprised that was a cat 1. It also helped that it was the NHC that measured those peak winds in Andrew, they have some good weather equipment on hand. Its a shame their gear got destroyed, I wonder what they missed.
Good mixing to the surface also helped Andrew. In sharp contrast to CT and Hanna last weekend. We had cells moving over my house, 1000 feet off the ground, at 70kts. Windspeed at my house was calm. Contrast that with a distant TD Ernesto in 2006 that brought 70mph winds to my area from a 35mph tropical depression.
I (think I) know that in a hurricane or tropical system, winds get "mixed down" in the rainbands due to the downward flow created by the falling precipitation. Higher winds at higher altitudes are pulled down to the surface. This could partially account for the very strong winds that typically come with the heavy rains in a tropical system. I'm sure there are other processes that act to bring the winds down to the surface. If I'm mistaken about the process...please correct me.
It is often surprising to see clouds streaking past at great velocity just above the treetops during a tropical storm or hurricane while the air is relatively calm (between gusts) at the surface. Anything that acts to bring that high velocity air downward is said to be "mixing down" the winds.
Last edited by timechime; 09-08-2008 at 11:39 PM.