What is a mesonet and what are they used for?

Posted on March 2, 2026 by WeatherGuys Editor

The term “mesonet” is a blending of the words “mesoscale” and “network.”

The weather station delivers near real-time data on precipitation, air and soil temperature, humidity and wind speed. (Photo credit: https://wisconet.wisc.edu/about/instrumentation)

In meteorology, “mesoscale” refers to weather events that range in size from about one mile to about 150 miles. Mesoscale events last from several minutes to several hours. Thunderstorms, snow squalls and wind gusts are examples of mesoscale events. A mesonet refers to a network of collectively owned and operated automated weather stations that are installed close enough to each other and report data frequently enough to observe mesoscale meteorological phenomena.

Typically, each observation station in a mesonet is independent, operated by battery or solar panels. Each station records temperature, humidity, wind speed and direction and atmospheric pressure. Some mesonets also measure solar energy, soil temperature and soil moisture. Observations are transmitted to a base station for distribution and long-term storage.

A mesonet can provide realtime 24/7 weather information on a local to regional basis and observe rapidly changing weather conditions that may be significantly different from the conditions over a larger area.

Mesonet data improves weather prediction and is especially beneficial for short-range mesoscale forecasting and tracking hazardous weather. In addition to meteorologists, mesonet data is used for decision-making by firefighters, transportation departments, farmers and agricultural entities — anyone who needs fine-scale weather information. Mesonets also provide data to a wide variety of researchers, scientists and teachers.

The Wisconsin Environmental Mesonet, or Wisconet, is a valuable state resource. This network monitors weather and soil conditions across Wisconsin (wisconet. wisc.edu). There are 78 sites that provide measurements across various ecosystems that reflect Wisconsin’s unique geography. The web interface makes data access easy. Wisconet data is publicly available and may be used commercially under the Creative Commons Attribution license.

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

Category: Meteorology, Seasons

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What did the Endangerment Finding suggest about global warming?

Posted on February 23, 2026 by WeatherGuys Editor

On Feb. 12 President Donald Trump’s Environmental Protection Agency revoked the so-called “endangerment finding,” issued in 2009, asserting that “CO2 and other greenhouse gases endanger public health and welfare.” This scientific finding served as the basis for limits on tailpipe emissions and power plant rules enacted under President Barack Obama and committed the United States to a national effort to arrest the kind of pollution that contributes directly to global warming.

Rescinding the Endangerment Finding weakens regulating greenhouse gases, potentially leading to increased emissions and reduced protections for public health and the environment. (Image credit: US Environmental Protection Agency)

Based on an extensive body of scientific evidence, in the EPA in 2009 singled out six greenhouse gases for the danger they pose to public health and welfare.

It is well known that Trump considers global warming a “hoax” — but it is equally well known that his opinion is ridiculous and utterly without merit. In fact, this very winter, the one in which we suffered from a robust cold spell in late January, is now just days away from becoming the warmest Northern Hemisphere winter since at least 1948-49.

This declaration derives from analysis of observations, not idle speculation or guesswork. The areal extent of air as cold as or colder than -5 degrees centigrade at about one mile above the surface is a routinely available observation, and this year’s Dec. 1-Feb. 28 period is poised to be the smallest integrated value of that measure seen in modern times. This particularly “warm” year is the 79th entry in a time series that has steadily decreased over nearly eight decades and robustly testifies to the reality of global warming.

Scientifically, the dangers posed by these gases still exist, and now those emissions are not regulated. “Revoking” a scientific conclusion borne of an impartial analysis of evidence is nothing more than asserting an alternate reality — a behavior that appears to have become so common in the current administration as to have become a malignant standard of practice. Nothing good ever comes from ignoring 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 noon the last Monday of each month. Send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.

Category: Climate, History, Weather Dangers

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Why don’t trees freeze and burst in the winter like cold pipes?

Posted on February 16, 2026 by WeatherGuys Editor

When liquid water freezes, it expands as the water molecules spread out to form a solid crystalline lattice. If the freezing water is contained in a closed vessel, such as a metal pipe, it can press hard on the sides as it expands, causing the container to burst.

Frost cracks are longitudinal openings that can run the entire length of the trunk, often extend deep into the wood, and permanently damage a tree. In wintertime, the tree may even look like it’s splitting in half! Then warmer weather comes, and the crack seems to close, repeating this process annually. Over time, frost cracks may even develop a raised area where callus tissue develops in an attempt to close over the wound, only to reopen again next winter. (Photo credit: Wisconsin Dept of Natural Resources)

Trees contain water that is transported throughout the tree by the phloem and xylem, which we can think of as pipes.

Trees are cold hardy because of many factors. In preparation for winter, many trees reduce the amount of water held in their tissue. They usually enter a dormant state to survive cold winters. Some trees, such as maple trees, produce more sugar. When this sugar dissolves in the water, it lowers the freezing point of the fluid. Unlike plumbing pipes, tree tissue is somewhat flexible; as water in the channels freezes and expands, the tree’s tissue can stretch somewhat.

Trees can suffer damage during periods of cold winter nights followed by warm sunny days. The bark alternately contracts with freezing temperatures and expands on warm days. The alternating freezing and thawing temperatures can result in a vertical crack called a “frost crack.” This often happens during late winter or early spring. The formation of a frost crack can be accompanied by a loud noise, like a gunshot sound. But the trees don’t “explode” in the in subzero weather.

Dangerous winter weather for trees are storms that lead to heavy snow and ice accumulating on the trees. The weight of frozen precipitation can put enormous stress on tree limbs, causing them to break and fall.

Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at noon 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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Will the recent the groundhog predictions come true?

Posted on February 9, 2026 by WeatherGuys Editor

Punxsutawney Phil saw his shadow, which means there will be six more weeks of winter. On the other hand, Jimmy the Groundhog in downtown Sun Prairie did not see his shadow at sunrise during the 78th annual Groundhog Day Prognostication, which per tradition means we are due for an early spring. Which to believe?

Groundhog Day stats (Image credit: NWS LaCrosse)

We don’t put stock in either.

Humans have long needed weather forecasts. Farmers and sailors particularly needed to know if storms were approaching. Over time, various folklore forecasts, often in the form of short rhymes, were devised and passed down through the generations. The forecast made on Groundhog Day is an example of a folklore weather forecast. Although memorable, the folklore forecasts are of uneven quality — some good, others bad.

The roots of Groundhog Day go back to the sixth century. Feb. 2 is 40 days after Christmas and is known as Candlemas. On this day, candles that are used for the rest of the year are blessed. This is also about the midpoint of winter, in astronomical not meteorological terms. If the day is bright and clear, the groundhog “sees” his shadow and we have more winter.

Of course, after Feb. 2, there are only 47 days left of astronomical winter — which ends on or about March 21, so the forecast accuracy should be more than luck. The predictions made by Punxsutawney Phil with this folk forecast are correct only about 40% of the time, and Jimmy the Groundhog is about 60% accurate — both vastly inferior to what is delivered by modern science.

Of course, the weather conditions on a single day at a single location tell us very little about the weather for the rest of the winter season. But right or wrong, they are fun community celebrations, as well as an entertaining movie.

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

Category: History, Seasons

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Why does the morning low temperature sometimes occur after sunrise?

Posted on February 2, 2026 by WeatherGuys Editor

Those who keep a particularly vigilant eye on the weather might have noticed that on certain mornings, in all seasons, the lowest temperature is often recorded just after sunrise.

Sunrise in winter. (Photo credit: Steve Ackerman)

One of us had a morning paper route as a boy and was puzzled for years about this seemingly counterintuitive phenomenon. How could the temperature continue to fall for the few minutes after sunrise on a cold winter morning?

The answer lies in the fact that Earth and the sun radiate different kinds of energy and one needs to consider the budget of this energy to make sense of this recurring observation. But first a bit about measuring temperature. Atmospheric temperature is measured using thermometers placed in shaded enclosures at a height of about 5 to 6 feet above the ground.

On a clear, calm winter night, Earth’s surface radiates infrared energy upward towards space. With the sun already down, there is no shortwave solar radiation (and only a very little infrared energy from the overlying atmosphere) directed downward toward the surface. Consequently, with each passing second, the surface emits more energy than it receives and the surface temperature drops.

This continues all through the night with the accumulated loss of infrared energy from the surface accounting for the continued decrease in the surface temperature. When the sun finally rises above the horizon and spreads the first faint rays of solar energy across the surface, there is finally some incoming radiation. However, for several minutes the meager amount of incoming radiation is not sufficient to counter the amount of infrared energy still being emitted from the surface. As a result, the surface temperature continues to drop even in the face of the newly risen sun.

Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at noon 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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