What is the ozone hole?

Posted on September 27, 2021 by WeatherGuys Editor
Total ozone over the Antarctic on September 25, 2021. The purple and blue colors identify the ozone hole (where there is the least ozone) and yellows and reds indicate the most ozone. (Image credit: Ozone Hole Watch, NASA Goddard)

Ozone is a colorless gas made up of three oxygen atoms (chemically denoted as O3). It occurs naturally in small amounts in the upper atmosphere (the stratosphere), about 18 miles above the surface.

Ozone in the stratosphere is a result of a balance between sunlight that creates ozone and chemical reactions that destroy it. Ozone is created when oxygen (O2), is split apart by ultraviolet energy emitted by the sun into single oxygen atoms. The single oxygen atoms can rejoin to make O2, or they can join with O2 molecules to make ozone.

The winter atmosphere above Antarctica is very cold. These extremely cold temperatures are a good environment for forming polar stratospheric clouds, or PSC. PSCs begin to form during June, which is wintertime at the South Pole. Chemicals on the surface of the particles composing PSCs result in chemical reactions that remove atmospheric chlorine.

When the Sun returns to the Antarctic stratosphere in the spring (our fall), sunlight splits the chlorine molecules into highly reactive chlorine atoms that rapidly deplete ozone. The depletion is so rapid that it has been termed a “hole in the ozone layer.”

The amount of ozone in the atmosphere is routinely measured from satellites. The ozone hole appears high over the continent of Antarctica as very low values of ozone in the stratosphere. Typically, the Antarctic ozone hole has its largest area in early September and lowest values in late September to early October. This year’s hole is shaping up to be larger than average in area, but well within expectations.

Sept. 16 is the International Day for the Preservation of the Ozone Layer. That day celebrates the 1987 anniversary of the Montreal Protocol on Substances that Deplete the Ozone Layer. The Montreal Protocol led to a ban on a group of chemicals called halocarbons that were blamed for exacerbating the annual ozone hole.

While the ozone layer is beginning to recover, it is likely to take until the 2060s for the ozone-depleting substances used in refrigerants and spray cans to completely disappear from the atmosphere.

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, Meteorology, Phenomena

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When does autumn begin?

Posted on September 20, 2021 by WeatherGuys Editor

The changes are subtle but apparent to the keen observer at this time of year — the first changes of color on trees, the earlier sunset and the rapid temperature drop that follows.

Summer is ending and fall is taking its place. This fact is formalized at 2:20 p.m. Wednesday when the autumnal equinox occurs.

This is, of course, the time when the Earth enjoys equal amounts of day and night for the first time in six months. Every day after Wednesday here in the Northern Hemisphere will have a longer night than day with the difference reaching its maximum just before Christmas on the day of the winter solstice at 10:59 a.m. on Dec. 21.

In fact, at the North Pole, the sun will set on Tuesday night and not rise again until late March. The extinction of daylight at the North Pole and its shortening everywhere else in the hemisphere has consequences that are inescapable — the air gets colder with each passing day at high latitude and then begins to creep slowly southward throughout the fall.

Autumn 2019 from space! Credit:CIMSS

Here in Wisconsin, we are less and less susceptible to the invasion of warm and humid tropical air masses. With less water vapor in the air, even pleasant sunny days are crisper and nighttime cooling occurs more rapidly after sunset because water vapor is a potent greenhouse gas. The fraction of our precipitation that comes from more energetic and well-organized storms will also increase, taking over from summertime’s thunderstorms.

It is a beautiful and exciting time of year, and the cosmological stage is officially set on Wednesday.

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, Meteorology, Seasons

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Why do tornadoes happen in hurricanes?

Posted on September 13, 2021 by WeatherGuys Editor
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?

Posted on September 6, 2021 by WeatherGuys Editor
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?

Posted on August 30, 2021 by WeatherGuys Editor
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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