How does frost form?

Posted on November 1, 2021 by WeatherGuys Editor
Trees emit radiation toward the ground insulating small areas which is why frost did not form under the tree in this field. (Photo credit: Tim Wagner)

Frost on objects is just water vapor in the air that has deposited itself as ice onto a surface. Frost forms on objects close to the ground, such as blades of grass.

At night, a blade of grass loses energy by emitting radiation (a non-lethal kind) while it gains energy by absorbing the energy emitted from surrounding objects. Under clear nighttime skies, objects near the ground emit more radiation than they receive from the sky, and so a blade of grass cools as its energy losses are greater than its energy gains. If the temperature of a grass blade gets cold enough and there is sufficient water vapor in the environment, frost will form on the grass.

Overnight cooling of the air near the ground causes morning frost on grass and car windshields. Frost will form on a surface only where the temperature is at or below freezing. The observed air temperature may be higher than 32 degrees, since those air temperature observations are taken at about 4 feet above the ground, where it can be warmer than the ground.

You may notice that frost forms in an open field but not under a tree. Trees emit more radiation toward the ground than does the clear sky. Energy losses at the ground under the tree are therefore less than those of the grass in the open field. The grass in the open field cools faster and reaches the frost point before the grass blades under the tree.

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, Seasons

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What is an atmospheric river?

Posted on October 25, 2021 by WeatherGuys Editor

The extreme and persistent drought that has plagued parts of California for several years will be at least slightly remedied by the torrential rains that fell over the weekend over much of the central and northern part of the state.

These rains were associated with a phenomenon called an “atmospheric river.” Atmospheric rivers are organized flows of deep, moist air from the subtropics and tropics that bring many locations in California a large portion of their annual precipitation.

Moisture in the atmosphere measured by satellite microwave instruments depicting the strong atmospheric river impacting the Pacific Northwest on October 24, 2021. Credit: CIMSS

These rivers are not really a distinct feature of the atmosphere, rather they are organized and pushed poleward by the circulations around extratropical cyclones that are strong enough to tap the substantial moisture endemic to the subtropics and tropics.

This weekend’s event is directly tied to the most intense extratropical cyclone to ever visit the waters off the Pacific Northwest. A cyclone with a central pressure in the 944 mb range was just offshore of Washington state. For perspective, the average sea-level pressure is about 1012 mb. Additional perspective on the strength of this storm arises from the fact that category 3 or 4 hurricanes are often characterized by such low central pressures.

The atmospheric river associated with this storm was really a deep flow of moist air ahead of the cold front associated with this monster cyclone. Though it is not truly a distinct meteorological animal, categorizing such moist flows as atmospheric rivers is a useful way to gauge the likely impact of these features on precipitation prospects both in the Central Valley — an enormously important agricultural region of our country — and in the Sierra Nevada, where winter snows are like money in the bank for spring agriculture in Valley.

Steve Ackerman and Jonathan Martin, professors in the UW-Madison Department of Atmospheric and Oceanic sciences, write weekly weather articles in the Wisconsin State Journal and radio guests on WHA (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, Severe Weather

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How is the Artic Sea ice situation?

Posted on October 18, 2021 by WeatherGuys Editor
This NASA Blue Marble image shows Arctic sea ice on September 16, 2021, when sea ice reached its minimum extent for the year. Sea ice extent for September 16 averaged 4.72 million square kilometers (1.82 million square miles)—the twelfth lowest in the satellite record. (Image credit: National Snow and Ice Data Center/ NASA Earth Observatory) 

The sea ice cover in the Arctic Ocean is one of the key components of our climate system.

The brightness of the sea ice reflects more solar energy to space than open water. Global warming is amplified in the Arctic as the ice cover decreases. This is referred to as the ice-albedo feedback.

As the polar regions warm, the amount of sea ice decreases, which allows more solar energy to be absorbed by the Arctic Ocean, which increases the warming, leading to more loss of sea ice. In the winter, the sun is below the Arctic Circle and so the sea ice can grow back.

Observing sea ice coverage from satellites started in 1978. Those satellite-based observations have measured rapid changes in ice coverage, and that coverage has been declining. The overall, downward trend in the minimum extent from 1979 to 2021 is 13% per decade relative to the 1981 to 2010 average. The loss of sea ice is about 31,100 square miles per year, equivalent to losing the size of the state of South Carolina.

The Arctic Sea ice coverage typically reaches its smallest amount in mid-September. This is in response to the setting sun and falling temperatures. Then, the ice extent begins to increase and does throughout the winter. This year, based on satellite observations, the minimum occurred on Sept. 16 of 1.82 million square miles. The 2021 minimum is the 12th lowest in the satellite record. The last 15 years are the lowest 15 sea ice extents in the satellite record.

Multiyear ice is sea ice that exists for more than one year. Multiyear ice extent is one of the lowest on record.

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 did two meteorologists receive the Nobel Prize?

Posted on October 11, 2021 by WeatherGuys Editor
Princeton University professor Syukuro (Suki) Manabe is generally considered the father of climate modelling, a sentiment echoed by the Royal Swedish Academy of Sciences, which said he “laid the foundation for the development of current climate models.” (Photo credit: Denise Applewhite, Princeton University)

It was with great excitement that we learned last week that the Nobel Prize in Physics was shared among three scientists who had each made “groundbreaking contributions to our understanding of complex systems.” Two of the three so awarded were meteorologists, professor Syukuro (Suki) Manabe of Princeton University and professor Klaus Hasselmann of the Max Planck Institute for Meteorology.

Together, they were cited for their work in “the physical modeling of Earth’s climate, quantifying variability and reliably predicting global warming.”

Manabe led pioneering work in the 1960s that demonstrated, through numerical modeling, how increases in the carbon dioxide fraction in the atmosphere results in a global temperature rise. Though his original model was simple compared to the sophisticated tools available today, his sense of what is influential and what is not allowed that model to make remarkably accurate predictions of conditions 50 years later.

Klaus Hasselmann (Photo credit: Max Planck Institute for Meteorology)

He is generally considered the father of climate modelling, a sentiment echoed by the Royal Swedish Academy of Sciences, which proclaimed that he “laid the foundation for the development of current climate models.”

Hasselmann’s work in the 1970s produced the first numerical model that adequately linked climate and weather processes together, thus carrying Manabe’s pioneering work into an even more physically accurate realm.

Together, these giants have set a course for our science in which important advances require simultaneous consideration of climate science and weather systems science.

Awarding the Nobel in Physics for such work is, as Thors-Hans Hansson, chair of the Nobel committee said, a clear indication that “the modelling of climate is solidly based in physical theory.”

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

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What impact did Sputnik have on weather monitoring and predictions?

Posted on October 5, 2021 by WeatherGuys Editor
Sputnik, the first artificial satellite, was launched into space on October 4, 1957, by the Union of Soviet Socialist Republics (USSR). This event pressed the United States to move forward with its satellite program. (Image credit: NASA)

Sixty-four years ago the Soviet Union launched Sputnik, the first artificial satellite. This was not welcome news in the United States as it confirmed that the Soviets were well ahead of us in the development of rocket technology. In fact, this launch greatly accelerated the “space race” that eventually culminated in the U.S. moon landing in July 1969.

The ancillary benefits of that decade of rapid development are still with us today. Much of the remarkable advances in computer technology were made during the space race, as were many component technologies that currently power popular devices such as cell phones and digital music devices.

Quite separate from those advances, the advantage of having satellites in space for the purposes of improving understanding of our atmosphere, monitoring its climate and predicting its weather cannot be overstated. The applications of satellite technology to weather prediction have blosso0med in the last 40 or so years.

Since about 70% of the earth’s surface is covered by oceans, observations of the atmosphere over such areas by conventional observing systems is nearly impossible. Satellites provide an enormous amount of observational data from these regions that is fed into the ceaseless forecast operations around the world.

Satellite observations of tropical weather systems has been extremely important in the rapid increase in hurricane forecast accuracy that as occurred over the past few decades.

Finally, a good number of the scientists currently working on atmospheric problems were attracted to science generally as a result of the Apollo program in the 1960s. Thus, the anniversary today is a particularly compelling example of the power of answering a national call to action in the face of looming threat. Perhaps we can muster the same fortitude as we stare down the climate 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. Send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.

Category: Climate, History

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