How can there be frost on the ground when my thermometer reads 36 degrees?

Posted on November 27, 2023 by WeatherGuys Editor
Temperature inversions near the ground enable phenomena like frost and fog on cool fall mornings. Photo credit: John Lalande

A reader asked us about an observation she made at her home last week. She reports that her thermometer read 36 degrees Fahrenheit, but there was clearly frost on the front lawn.

This set of circumstances does not mean that her thermometer is faulty and in need of replacement. Instead, it reflects a nearly daily reality that goes undetected for most of the year until the cold season. It turns out that the air does not radiate heat away nearly as well as the solid ground beneath it. As a consequence of this difference, given 13 hours of nighttime with clear skies, the ground radiates a lot more energy away (and cools rapidly) while the air above struggles to cool as efficiently. Over those many hours, this difference results in a big difference between the ground temperature and the air temperature even as little as 5 or 6 feet above the ground.

On almost every clear, windless morning throughout the year you could measure such a temperature structure, known as an inversion, in the lowest 6 or so feet of the atmosphere. In the late fall, the colder ground may be below 32 degrees and permit frost to form while the air just above it, at the thermometer level, could be several degrees warmer than that. Hence, it is not at all unusual for a set of seemingly contradictory observations, like those reported by our reader, to occur at this time of year.

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, Phenomena

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What is the National Climate Assessment?

Posted on November 20, 2023 by WeatherGuys Editor

The U.S. National Climate Assessment is mandated by the Global Change Research Act of 1990. The assessment is conducted about every four years and is an authoritative scientific analysis of climate change risks, impacts and responses in the U.S.

The Fifth National Climate Assessment was released on November 14th.

The nation this month completed the Fifth National Climate Assessment, or NCA5. The National Oceanic and Atmospheric Administration is the administrative agency for NCA5 and certifies that the report meets Information Quality Act and Evidence Act standards. The assessment is an extensive process that includes internal and external review from federal agencies, the general public and external peer review by a panel of experts.

There is unequivocal evidence that our planet is warming at an unprecedented rate. Earth’s average surface temperature has risen almost 2 degrees Fahrenheit since the late 19th century. Human activity is the principal cause. The warming affects agriculture, forests and water quality with impacts for weather events and the way we live. The NCA5 documents the ways in which the U.S. is experiencing the results of climate change and assesses those risks, challenges and opportunities.

The average winter temperatures of the Great Lakes region have warmed by as much as 5 degrees over the past half-century. The ice coverage on the Great Lakes, as well as smaller lakes, is shrinking. The annual maximum ice cover of the Great Lakes is, on average, 22% lower than it was 50 years ago. The decline in ice coverage and thickness has made ice fishing difficult and dangerous.

The warming trend also is reflected in more freeze-free days each year. The impact is an extended growing season. Plants have more time to grow and release potential allergy-inducing pollen. For example, ragweed pollen production typically peaks in September and lasts through October. Warmer fall temperatures can extend the ragweed growing season with health consequences for allergy sufferers.

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, History, Seasons, Severe Weather

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What is a gale wind?

Posted on November 13, 2023 by WeatherGuys Editor

A gale is a strong, sustained wind with wind speeds between 39 mph and 54 mph. The word is typically used as a descriptor for maritime weather.

The National Weather Service issues gale warnings when winds of this strength are expected for maritime settings. Gale warnings allow mariners to take precautionary actions to ensure their safety or to seek safe shelter. A gale warning flag is solid red and in the shape of an isosceles triangle. The equivalent warning for land is a wind advisory. The next level of warning for maritime winds that NWS issues is a storm warning for winds of 55-72 mph at sea.

Gale winds are common in November on the Great Lakes. Last week saw the anniversaries of some strong November gales in the Great Lakes region. The most famous of these include the White Hurricane (Nov. 7-10, 1913), the Armistice Day Blizzard (Nov. 11, 1940), the Edmund Fitzgerald storm (Nov. 9-10, 1975), and the Nov. 10-11, 1998, storm.

The 1913 storm sank 19 ships and killed more than 250 people. Most of the damage occurred in Lake Huron.

The Armistice Day Blizzard dropped 16.7 inches of snow in Minneapolis/St. Paul. The cyclone intensified rapidly and was accompanied by a very intense surface cold front that quickly dropped the temperatures as much as 50 degrees in parts of the Midwest. This rapid drop in temperature caught many people by surprise, and more than 150 people perished.

The Edmund Fitzgerald storm achieved grisly fame through its association with the sinking of the ore freighter and the loss of its 29 crew members.

Gale winds that sunk the SS Edmund Fitzgerald.
Credit: NOAA

That storm also was accompanied by extremely strong winds and rapid intensification over the mid-continent.

That event was memorialized by Gordon Lightfoot’s ballad “The Wreck of the Edmund Fitzgerald.”

The November 10-11 storm from 1998 saw a six-hour period during which its minimum sea-level pressure dropped 15 millibars.

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

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What role did Wisconsin play in establishing the National Weather Service?

Posted on November 6, 2023 by WeatherGuys Editor
Portrait of Increase Lapham (Photo credit: Wisconsin Historical Society archives)

Wednesday was the 153rd anniversary of the first day of operation of what has become the National Weather Service. On Nov. 1, 1870, the first organized set of observations around the country were taken under the auspices of the Army Signal Service.

On Feb. 9 of that same year, President Ulysses S. Grant, fresh from his own experiences during the Civil War, enthusiastically signed the service into existence. Its purpose was “to provide for taking meteorological observations at the military stations in the interior of the continent and at other points in the States and Territories … and for giving notice on the northern (Great) Lakes and on the seacoast by magnetic telegraph and marine signals, of the approach and force of storms.”

Within a week of its first day of operation, the first official weather forecast from a United States government agency was made by Professor Increase Lapham, through the Chicago office. It was a successful forecast of strong winds and significant waves on the Great Lakes, and its issuance may well have saved lives and property, exactly as intended.

Lapham, of course, was perhaps the most famous professor during the early years of the University of Wisconsin-Madison. It was he, in fact, who petitioned U.S. Rep. Halbert Paine, of Milwaukee, himself a Civil War veteran, in the immediate post-war era regarding establishment of a national weather service.

Thus, Wisconsin, and in particular UW-Madison, played a prominent role in the establishment of our National Weather Service.

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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Are hurricanes intensifying more quickly?

Posted on October 31, 2023 by WeatherGuys Editor

Hurricane intensities are classified using the Saffir–Simpson scale, which rates hurricanes on a scale of 1 to 5 based on the damage their winds would cause upon landfall. Major hurricanes are those classified as Category 3 and higher on this scale.

Category 3 hurricanes have one minute of sustained winds between 111 mph and 130 mph. The one-minute sustained winds in a Category 5 hurricane are greater than 155 mph.

Forecasting hurricane intensity is a difficult task, and forecasting how rapidly they might intensify is particularly difficult. Rapid intensification is when a tropical cyclone strengthens dramatically in a short period of time. The National Hurricane Center (NHC) defines rapid intensification as an increase in the maximum sustained winds of a tropical cyclone by at least 35 mph in a 24-hour period.

Intensification of a hurricane requires the right environmental conditions. One is water temperature. If water in the ocean beneath the hurricane is warm enough, it releases large amounts of energy as it evaporates, creating a dip in air pressure that generates powerful winds.

Otis, a storm that struck the southern coast of Mexico, was initially forecast to be a weak tropical storm (one-minute maximum sustained winds between 39 and 73 mph) at peak intensity. Instead, Otis underwent rapid intensification, reaching peak winds of 165 mph when it made landfall near Acapulco, Mexico, at 1:25 a.m. Wednesday. Otis strengthened from a tropical storm to a Category 5 hurricane in only 12 hours. This rapid intensification occurred over a patch of ocean with sea surface temperatures approaching 88 degrees. It came ashore as the strongest storm on record to hit Mexico’s Pacific coast.

Hurricane Otis prior to landfall. Click for an animation. Credit: CIMSS

While forecasts of intensification have improved, challenges remain. A recent paper in Nature found that rapidly intensifying storms that are within 240 miles of coastlines are now significantly more common than they were 40 years ago.

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, Phenomena, Tropical

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