All along the East coast is being hit hard by wind and rain driven indirectly by Hurricane Joaquin. In my area of North Carolina, it has been raining solid for several weeks. Monday morning of last week our 10th floor office was flooding (leaky roof). Weather agencies are advising people along the NC Coast to be ready to get to high ground quickly and areas of the Outer Banks have been evacuated. Unfortunately, South Carolina has already gotten a mouthful of our potential flooding issues with 3 flood-related fatalities, causing President Obama to declare a state of emergency for SC.
As much damage has been caused by flooding along the East coast, we are only getting a light brush from Joaquin. Bermuda got a smack in the face from this category 4 (almost category 5) hurricane. Joaquin hit the Bermuda islands with 80+ mph winds – strong enough to easily topple trees and demolish buildings – and several feet of storm surge (up to 6ft in some areas). After hitting Bermuda, Joaquin dropped down to a category 1 and is swinging over to visit Europe (click here to track Joaquin).
So what causes a hurricane? And what makes them change strength?
Hurricanes, or tropical cyclones in the jargon, are caused by the movement of warm, moist air. As warm air rises, a “gap” is left near the surface and cooler air rushes in to fill the gap. The cooler air is then warmed by the ocean and rises too. Hold on just a minute, the air is warmed by the ocean? Yep, yes it is. Through the same process that causes water in a pot to boil, the ocean transfers heat to the air near it’s surface. Hurricanes need hot air as the oomph to get started, and that is why hurricanes only form over warm oceans.
Back to hot air… as the warm, moist air rises, it cools and forms clouds. Remember, that air rushes in to replace the rising warm air. This cycle causes the mass of warm air and clouds to begin to spin. More and more air is warmed, rises, and forms clouds, and the storm spins faster and faster, causing an eye to form in the middle. Hurricanes are classified according to how fast they are spinning – aka wind speed. The faster the winds spin, the higher the category. Joaquin was almost a category 5 hurricane, which is the highest category = the fastest and strongest winds.
So can you guess what causes hurricanes to change in strength?
Hot air! The moist bit needs to be in there as well, but without a supply of hot air for energy a hurricane loses strength. Since land isn’t a very good supplier of warm, moist air, hurricanes tend to loose strength when they make landfall. It is not an immediate loss of strength, rather a gradual weakening and the longer a hurricane is over land, the weaker it gets.
If a hurricane happens to pass over a sweet spot of really warm ocean (which means lots of warm air to gobble), it can rapidly gain strength. This is what happened with ‘tropical storm Joaquin’… it began it’s young life as a tropical storm, mozied over near Cuba and picked up lots of energy from it’s tropical vacation (hello Category 4-almost-5), turned around and smashed across Bermuda, got a bit tired (Category 1) and is on it’s way to nap in Europe.
For another excellent explanation of how hurricanes form, check out NASA’s How do hurricanes form?