Why is the wind often calmer at night than during the day?

Posted on December 11, 2023 by WeatherGuys Editor

The cycle of daytime heating and night time cooling explains why, under most circumstances, calm winds are near the surface at night.

The wind usually increases with height above Earth’s surface. The wind several thousand feet above the ground is almost always stronger than that experienced near the ground. Friction causes the wind close to the ground to move more slowly. Friction decelerates the wind in the same way a rough road surface slows down a bicycle.

When the sun sets, the ground loses the solar energy from the sun and continues to lose energy via emission of longwave radiation. This cooling ground conducts heat away from the air near the ground, causing that air layer to cool down faster than the layers higher in the atmosphere. This creates a stable area with cool air near the ground and warmer air above. As the word suggests, “stable” means it is difficult to move the air layer, keeping the fast-moving air above from mixing down to the surface. This is called a decoupling: This layer is no longer influenced by what is occurring above it.

Calm conditions due to decoupling are evident in this sunset photo of Lake Mendota taken by Jonathan Gero in 2010.

On many calm nights, there is still wind blowing far overhead. When the sun is up, it warms the surface of Earth, which in turn warms the atmosphere above it. The warm air rises and the displaced air is replaced by the air above. These warm thermals mix up the air, bringing the faster moving air from above down near the surface. As the daytime heating goes on, more air from above is mixed down and the wind speed picks up. This results in turbulence and mixing of the air near the ground.

Of course, if there is a low-pressure area or fronts in the region, the winds will likely blow day or night.

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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Does fall give any hints about the intensity of the coming winter?

Posted on December 4, 2023 by WeatherGuys Editor
Average temperatures were above normal and precipitation was below normal during the September through November timeframe this year in Madison. (Image credit: NWS-Sullivan)

Each winter we keep track of the areal extent of air colder than 23 degrees Fahrenheit at the 850 mb pressure level (about 1 mile above sea-level) around the entire Northern Hemisphere. This measure allows us to characterize the intensity of the winter season with respect to the lower tropospheric temperature.

Over the past 75 seasons there has been a systematic decrease in the December-January-February, or DJF, average areal extent of about 4.6%, and this is an unequivocal sign of global warming, measured in the winter season.

Another interesting aspect of this observational record arises from examining the buildup of the cold air leading into the winter. So, we have also kept track of the areal extent of such air from Sept. 1 to Nov. 30, SON, over the same 75 seasons. This year’s entry into that time series turns out to have been the sixth-warmest of all time — that is, it had the sixth-smallest areal average for SON.

A warm SON by this measure does not always mean a warm DJF in the following season. In fact, if one ranks the 75 SON seasons from warmest to coldest and then ranks the DJF seasons from warmest to coldest, there are years in which a warm SON is followed by a cold DJF and vice versa. Such years would be characterized by what could be termed a large “rank shift” — that is the ranking in one list is very different from the ranking in the other for the same season.

The largest rank shift in which the winter was colder than the fall occurred in 2002-03, when the fall was the 18th-warmest but the following winter was only the 58th-warmest, a rank shift of 39 places. Close on its heels but in the other direction was 2011-12, in which the fall was cold (70th-warmest) and was followed by the 17th-warmest DJF of all time — a rank shift of 53 places.

Current research is investigating whether or not there are systematic signs of a circulation change across the hemisphere that might give an early indication of whether or not a given year may experience such a dramatic change from fall to winter.

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

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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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