Why do tornadoes happen in hurricanes?

Tornadoes spawned by hurricanes typically occur in the right front quadrant where wind shear differences due to friction from land is greatest.

When hurricanes make landfall, they can spawn tornadoes.

The friction over land is much stronger than friction over water, where the hurricanes form. Frictional force quickly weakens the farther you get from the ground.

When a hurricane makes landfall, the winds near the ground slow down, while the upper-level winds keep their momentum. This change in the wind speed — and sometimes direction — with height is called a “wind shear.” This can lead to a column of air rotating that can generate a weak tornado.

The tornadoes spawned by hurricanes typically occur in the right front quadrant of the storm and usually within 12 hours after landfall. The tornadoes are very often embedded in rain bands. Unfortunately, meteorologists cannot accurately predict if a hurricane will produce tornadoes.

On average, Florida had 60 tornadoes a year during the period 1989 to 2019. They are mostly associated with hurricanes. Compare this to Wisconsin, which averages 24; Minnesota, averaging 40; and Michigan, which averages 15 tornadoes a year.

The tornado’s strength is determined by the damage the tornado does, which is an estimate of the wind speed of its rotating winds. All tornadoes are assigned a single number from the Enhanced Fujita scale according to the most intense damage caused by the storm.

This scale is based on the research of professor Ted Fujita and uses a set of 28 indicators, such as damage to barns, schools and trees. The degree of damage is used to determine the EF scale of every tornado. The weakest tornado is EF0, with wind speeds of 65-85 mph that will peel the surface off some roofs, cause some damage to gutters or siding and break off tree branches.

Hurricanes by themselves cause natural disasters, so even weak tornadoes are a problem.

Steve Ackerman and Jonathan Martin, professors in the UW-Madison Department of Atmospheric and Oceanic sciences, are guests on WHA radio (970 AM) at 11:45 a.m. the last Monday of each month.

Category: Meteorology, Phenomena, Severe Weather, Tropical

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Are we near the end of hurricane season?

Hurricane Ida left residents of Lafitte, La., dealing with massive flooding Aug. 30. (Photo credit: David J. Phillip, Associated Press)

It has been a particularly impactful hurricane season in the Atlantic thus far. As of Sunday, there have been 12 named storms — Larry being the current storm of interest.

Hurricane Ida was a very impactful storm, and tens of thousands remain without power in the metro New Orleans area. The so-called remnants of Ida also walloped the northeastern U.S. on Wednesday and Thursday, resulting in dozens of deaths and widespread flooding in many states not usually so affected.

The climatological peak of the Atlantic hurricane season is Sept. 12, so we are only halfway into the season thus far.

On Sept. 5, 1935, the devastating Labor Day hurricane swept along the west coast of the Florida peninsula after rumbling through the upper Florida Keys. The central minimum sea-level pressure of that storm got down to 892 hPa, the third-lowest ever recorded in the Atlantic basin. Since standard sea-level pressure is 1,012 hPa, nearly 12% of the air that usually occupies the column of atmosphere from the surface to outer space on that day had been exported to some other place. The extremely low surface pressures coupled with the relatively small radius of the quasi-circular storm, led to a huge horizontal pressure gradient which, in turn, drove the extraordinary winds of the storm — reaching a one-minute average of 185 mph on Sept. 2.

In contrast, Ida had winds of nearly 150 mph just as it struck the Louisiana coast Aug. 29. Only 10 hours later, the winds had weakened to 105 mph as the storm rapidly decayed over land.

Given the greatly increased population density of the region, were another storm as ferocious as the Labor Day hurricane of 1935 to strike the U.S. again, the damage would be even worse than what we saw with Maria in 2017 or Ida just last week.

Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at 11:45 a.m. the last Monday of each month. Send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.

Category: Climate, Seasons, Tropical

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Is hurricane forecasting improving?

GOES-16 “Clean” Infrared Window (10.35 µm) and “Red” Visible (0.64 µm) images (Image credit: CIMSS Satellite Blog)

There are two important components of hurricane forecasting: the hurricane track (where it is going) and hurricane intensity (how and if its winds are increasing).

Hurricane forecasts are becoming more accurate and are extending further out in time. Accurate forecasts provide needed information to make sound decisions and effective risk communication. In addition to improved hurricane forecasts, technological advances, such as smart phone apps, are making the information more accessible and can alert those in harm’s way.

Track forecast error is defined as the great-circle distance between a hurricane’s forecast position and the actual position at the forecast verification time. As one might expect, the errors get larger the further out the forecast.

The 24- and 72-hour track forecast errors over the past 30 years have improved, a trend where the errors have decreased by 70 to 75%. Consider the 2020 hurricane season as an example. This was an extremely active year, having 30 named storms. The 2020 mean track errors were 41 miles at 24-hour forecast, while during the period 2005-2010, the 24-hour track forecast error was 58 miles. Track forecast error reductions of about 60% have occurred over the past 15 to 20 years for the 96- and 120-hour forecast periods.

The improvement in hurricane track forecasts is due to several factors. We now have better and more satellite observations, new observations from drones carrying weather instruments, and more aircraft with better instruments observing hurricanes. Super computers that are faster improve forecast models by allowing more energy and dynamic processes to be incorporated more explicitly into the forecast. Due to improved observations, we can include better descriptions of the initial state of the atmosphere into the models, which leads to more accurate predictions of a storm’s behavior.

Forecasting the intensity of a hurricane has only gradually improved in the last two decades, so work remains to be done in that arena. While we still lack the ability to accurately forecast hurricane intensity, our understanding of how hurricanes evolve has grown substantially.

Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at 11:45 a.m. the last Monday of each month. Send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.

Category: Meteorology, Severe Weather, Tropical

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How often does New England deal with hurricanes?

New England dealt with Tropical Storm Henri over the past weekend — nearly the first hurricane to make landfall in New England in 30 years.

This satellite image taken Friday at 11:40 a.m. Eastern shows Tropical Storm Henri in the Atlantic Ocean. (Image credit: NOAA)

As it turns out, that long interval between landfalling hurricanes in that region is unusually long.

It is not terribly uncommon for Atlantic hurricanes to affect the New England states or even the Maritime Provinces of Canada. The reason they are not even more common that far north is because hurricanes are fueled by evaporation of water vapor off the ocean and its subsequent conversion to liquid water (in the form of torrential rains) that releases enormous amounts of so-called latent heat energy to the atmosphere.

This evaporation is greatly enhanced over a very warm sea surface. In fact, sea-surface temperatures of 26 degrees Celsius (about 79 degrees Fahrenheit) are necessary for the formation of nearly all tropical cyclones, some of which mature into hurricanes.

The shelf waters of the Atlantic Ocean, north of Cape Hatteras, North Carolina, are well below that threshold temperature so very few tropical storms continue to strengthen as they surge northward along the Atlantic coast.

The Gulf Stream current, which roars northward along the east coast of North America, usually turns out to sea at about the latitude of the Carolinas. However, the Gulf Stream occasionally has high-amplitude meanders in it (not unlike the waves in our troposphere) that can temporarily force its warm waters much farther north than normal.

Some of the more famous landfalling hurricanes in New England history may well have been associated with such meanders. Luckily, Tropical Storm Henri will most likely serve only as a helpful reminder to a new generation of New Englanders that they are not immune from these destructive storms.

Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at 11:45 a.m. the last Monday of each month. Send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.

Category: History, Meteorology, Severe Weather, Tropical

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What holds clouds up, and why are some fluffy on top but flat on the bottom?

Cumulus clouds often appear flat on the bottom revealing the lifted condensation level (LCL) the height in the atmosphere where rising air changes phase from water vapor to tiny liquid water droplets. (Photo credit: Steven A. Ackerman)

One of our readers awoke to some beautiful clouds in the summer sky recently, and those two excellent questions popped into her mind.

Clouds are composed of tiny liquid water droplets (whose diameters are about the width of a human hair) and tiny shards of ice in a variety of shapes.

Whether a cloud is mostly liquid water droplets or ice particles depends, as you might guess, on the temperature of the air in the cloud.

Tiny cloud liquid water droplets can remain in the liquid state to temperatures as low as about -10 degrees Celsius (14 degrees Fahrenheit), and when they do they are known as supercooled liquid water droplets.

These droplets feel the downward force of gravity just like a baseball or a watermelon would. But because the droplets are so small, and therefore have small masses, the gravitational force can easily be balanced by an upward friction force resulting from the interaction of the droplets with the air molecules around them. The droplets remain suspended, and that’s what holds clouds up in the air.

When these droplets grow, they gain mass and eventually the gravitational force overwhelms the friction force and the now-larger droplets fall to the surface.

The fluffy appearance of the tops of some clouds are evidence of convection, when buoyant air parcels within the cloud literally bubble to the top. As the air rises, it encounters environments with lower and lower pressure and cools by expansion.

This cooling increases the relative humidity of the air. Once that relative humidity gets to 100%, condensation of the invisible water vapor begins to produce liquid water droplets. The bottom of clouds often appears flat because the first level at which rising air parcels begin to condense is usually rather uniform over a given region. This level is known as the lifted condensation level — that is, the level at which lifted air parcels first begin to experience condensation.

Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at 11:45 a.m. the last Monday of each month. Send them your questions at stevea @ssec.wisc.edu or jemarti1 @wisc.edu.

Category: Uncategorized

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