What is this upside-down icicle?

Posted on February 10, 2025 by WeatherGuys Editor
Photograph of an ice spike taken the morning of February 4, 2025, on frozen Lake Kegonsa (Photo credit: Daniel Dettmers)

The accompanying photo was taken by Daniel Dettmers in the morning of February 4 on frozen Lake Kegonsa. The high on the previous day was 37°F. This caused puddles of water to sit on the ice of Madison’s regional lakes during the day. Tuesday morning’s low temperature was below 20°F with calm winds.  These are just the right conditions to form what are called ‘ice spikes,’ as shown in the photograph.

When water freezes, it expands and becomes less dense. Ice floats on water. But if the lake ice is thick, when puddles form on a warm day, they sit on the ice surface.  With the cold nighttime temperatures, the surface of the puddle freezes, trapping liquid water below. As the puddle freezes, it can leave a small hole in the surface of the ice.   

The puddle water is sitting on ice, and with the cold nighttime conditions, the water below the surface freezes from the outside inward. The expansion of the water as it freezes, slowly pushes some of the puddle water up through the hole. This water freezes around the edge of the hole forming a hollow tube. As the puddle continues to freeze, the hollow spike grows in length as water is pushed up into it. Eventually, the whole thing freezes and a solid spike of ice is left. The energy required to push the puddle water up into the spike comes from the expansion of the water while it is freezing.

While light winds can help in the formation of spikes, ice spikes don’t form under windy conditions.

Other names for ice spikes include “ice candles” and “ice towers.” They have been observed in bird baths and outdoor pet water bowls as well.

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, Uncategorized

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Is it getting windier in Wisconsin?

Posted on February 3, 2025 by WeatherGuys Editor

Wind speed and direction are variables that change over space and time, and conditions can change considerably from month to month, as well as from year to year. Scientifically assessing any long-term changes in weather elements requires a long-term data set of accurate measurements.  Temperature data goes back hundreds of years, and even thousands of years, using ancillary data such as from tree rings.

Annual mean wind speed for Madison, WI , showing the changes from 1948 to 2024 (Image credit: Ed Hopkins, Wisconsin State Climatology Office)

Wind is a more difficult parameter to study and analyze. The observations of wind speed and direction need to be made at the same height above the surface. The type of surface also impacts the measurement, as the wind sensor should not be too close to trees or buildings.  Finally, large annual fluctuations make long-term trends difficult to detect.

Some research has indicated that between 1978 to 2010, the average global wind speeds decreased by approximately 2.3 percent per decade. A 2019 study found that after 2010, global average wind speeds had increased from 7 miles per hour to 7.4 miles per hour.

To address this question with respect to our state, we turned to the Wisconsin State Climate Office and send thanks to assistant state climatologist Dr. Ed Hopkins for his assistance. His careful analysis used wind observations from 1948 to 2023/24 for Madison, Milwaukee, La Crosse, Eau Claire, and Green Bay. The trend over the entire period showed a decreasing average wind speed at all the stations.  The observations at La Crosse also had a decreasing trend but also had more variation over the period.  A large portion of these decreasing trends in wind speed is exhibited in the period prior to the installation of Automated Surface Observing Systems (ASOS), which occurred in the mid-1990s. After that installation, the trend in decreasing wind was much smaller or showed nearly no trend in those weather stations.

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, Meteorology

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Is there such a thing as “thundersnow”?

Posted on January 27, 2025 by WeatherGuys Editor
Annotated view of massive winter storm system pummeling eastern United States on Jan. 23. Imaged by the day/night band of the Visible Infrared Imaging Radiometer Suite (VIIRS) on the Suomi NPP satellite at 2:15 a.m. EST. (Photo credit: NOAA/NASA)

A reader recently asked us if thunder ever occurs with snowfall.  It turns out that such “thundersnow” does, in fact, occur occasionally in very intense winter storms.  Clouds and precipitation develop when the air is forced to rise to higher heights where the pressure is always lower.  The rising air expands into its lower pressure environment and the expansion results in a cooling of the air.  This cooling raises the relative humidity of the air and sometimes brings it to saturation, at which point invisible water vapor condenses into liquid water or goes straight to the solid ice phase.  During winter, the dynamical forces that create ascending air are very strong and well organized on large scales.  However, the stability of the stratification is stronger, partly because the air is generally much drier, which discourages thunderstorm development.  During summertime the large-scale is less organized but there is more abundant water vapor, weaker stratification and stronger individual updrafts of air that form intense thunderstorms.  Thundersnow is not very common because it requires moist, poorly stratified air (more characteristic of the warm season) and strong large-scale dynamics (more characteristic of the cold season) to occur simultaneously. 

            Rare though it is, when it occurs, thundersnow is captivating and provides the viewer with an unforgettable meteorological spectacle.  The most impressive thundersnow  we have ever seen occurred on the afternoon of January 26, 1996 when 8”of snow fell in just under 3 hours in Madison with vivid lightning and crashing thunder.

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: Phenomena, Severe Weather

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What causes the Santa Ana winds?

Posted on January 21, 2025 by WeatherGuys Editor

Santa Ana winds are dry, warm, and gusty winds that blow from the interior of southern California toward the coast and offshore. They are a type of downslope wind, which is a wind directed down a slope produced by processes larger in scale than the slope.

Santa Ana winds can occur when the pressure gradient caused by a high-pressure region over the Rockies, in combination with friction, forces air from the mountainous West down the San Gabriel Mountains in southern California.

1-minute GOES-18 Shortwave Infrared (3.9 µm) images (left) and Red Visible (0.64 µm) + Fire Mask derived product (right), with 15-minute METAR surface reports plotted in yellow, from 1801 UTC on 7th January to 0000 UTC on 8th January; Interstate highways are plotted in red. (Image credit: CIMSS Satellite Blog)

Atmospheric pressure always increases as one gets closer to Earth’s surface. An adiabatic process is one in which no heat energy is gained or lost by the system in question – in this case, a descending parcel of air. So, as an air parcel descends, it is compressed, which results in an adiabatic warming. The parcel warms at a rate of 10° C per kilometer, or about 29°F per mile of descent. This adiabatic warming also results in a lower relative humidity.

Santa Ana winds cause the temperature to increase and the relative humidity to plummet because of adiabatic warming.  The wind speed also increases as the air squeezes through mountain passes and canyons, like a slow-moving river that suddenly narrows and turns into rapids. The strong winds become warmer as they descend as well as drier in terms of relative humidity. Santa Ana winds can turn large geographic areas into bone-dry tinderboxes. As in recent news, infernos from wind–related fires can rage throughout the affected area. 

There are other types of downslope winds around the world that similarly bring hot, dry conditions to regions downwind of mountains and deserts. Some of these are the berg wind of South Africa, the leveche of Spain, and the sirocco of the Mediterranean Sea.  

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, Severe Weather

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Beyond the headlines, what else has been happening in the weather lately?

Posted on January 13, 2025 by WeatherGuys Editor

There has certainly been a lot of interesting and, in many cases, devastating weather around the country in the past couple of weeks. The heavy snow in parts of the country that don’t often see it along with the California wildfires have caught the attention of lots of us in the first days of the new year.

Wisconsin’s 2024 average temperature departure from the 1991-2020 Normals in degrees Fahrenheit. (Image credit: Midwest Regional Climate Center, Purdue University)

But in the background is a rather remarkable one-week stretch that occurred in the last week of December.

As you have heard a few times before, each day we calculate the areal extent of air colder than minus 5 degrees Celsius (23 degrees Fehrenheit) at 850 mb (about 1 mile above sea level) over the entire Northern Hemisphere.

This calculation can be used to assess the severity of the winter in a given year compared with other years stretching back to 1948. Each day from Dec. 24 through Dec. 30 of this season set the all-time minimum areal extent for that calendar day, meaning that the hemisphere was warmer over each day in that last week of December than it had been on those same days in any of the previous 77 years.

This is both remarkable and sobering as it indicates that, despite the local weather being quite wintry in some locations that don’t often see such extremity, the entire Northern Hemisphere is still clearly demonstrating the fact that the planet is warming.

What this means for the rest of the winter, both over the hemisphere and more locally in southern Wisconsin, is hard to say. However, the lack of snowfall to date in Madison will surely moderate the extreme cold that appears to be looming for midweek next 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 noon 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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