When does the winter cold end?

Posted on February 20, 2023 by WeatherGuys Editor

It has now been more than two weeks since our last below-zero morning here in Madison — Feb. 3 was minus 8 degrees. Thus, a natural question might be this: Are we done with bitter cold for the winter?

If we use an overnight low temperature at or below zero as the metric, then 45 of the 83 winters since 1939-40 have seen a cold spell after Feb. 20 — a surprising 54.2% of the time. In fact, in 1982, the last zero degree (or colder) night was not until April 7, the latest date in the last 83 years.

On the other hand, 12 of the past 83 years have seen the last below-zero morning of the year before Feb. 3, with the earliest ever such date being Jan. 8 in 2017.

None of this really has any predictive value for the rest of this year. In fact, it should be noted that in the past 20 years, the latest below-zero morning occurred on March 10, in 2003.

Recent research at UW-Madison has documented a systematic warming of the lower troposphere during Northern Hemisphere winter over the past 74 seasons. Elements of that analysis have suggested that the hemisphere begins its warmup much earlier than the spring equinox, which occurs around March 22. In fact, the hemispheric warmup really accelerates in the first few days of March.

NOAA’s Temperature Outlook for March 2023

Though it is true that this hemispheric signal does not always translate to a similar warmup at any given location, it is comforting to know that the entire hemisphere is trying to shake off winter as early as the next couple of weeks. So, climatologically, we really do not have that much further to go.

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: History, Seasons

Comments Off on When does the winter cold end?

What is a stratospheric sudden warming?

Posted on February 13, 2023 by WeatherGuys Editor

The region of the atmosphere closest to Earth, where the temperature decreases as you go up, is called the troposphere. The top ranges from about 6 miles over the cold poles to 10 miles over the tropics.

Earth’s Atmosphere

Above the tropopause lies the stratosphere, where temperature increases with altitude. Temperatures increase because ozone molecules in the stratospheric ozone layer absorb solar energy within the stratosphere. Air flow in the stratosphere is much less turbulent than in the troposphere. For this reason, jet aircraft pilots like to cruise at stratospheric altitudes, where the flight is less bumpy. In polar regions, the bottom edge of the stratosphere starts about five miles above the ground and extends upwards to around 30 miles.

The polar vortex is a band of strong winds, high up in the atmosphere, that spins counterclockwise around the North Pole. At the southern edge of the vortex is the polar jet stream, which separates warm air to its south from increasingly colder air to its north.


A sudden stratospheric warming, or SSW, occurs in the winter, when the polar stratosphere warms and the winds that normally flow from west to east around the pole weaken dramatically and even reverse direction. This phenomenon occurs about six times per decade and leads to a breakdown of the polar vortex. SSWs can cause the polar night jet to weaken, which allows cold air near the polar cap to expand into the middle latitudes.

SSW leads to cold air descending and warming rapidly, depicted by magenta line on the graph showing the sudden jump in temperature. This can lead to a displacement of the polar vortex, with cold air descending into the mid-latitudes. Credit: NOAA’s CPC

Weather models are signaling a sudden stratospheric warming event in the coming week. This could bring more wintry weather across the Northern Hemisphere. The main uncertainty with these events is that we don’t know exactly where this cold air will end up across the Northern Hemisphere.

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

Comments Off on What is a stratospheric sudden warming?

How did January measure up?

Posted on February 6, 2023 by WeatherGuys Editor

What a strange month January 2023 was in Madison.

The cloudiness was remarkably persistent and, though we don’t have a measurement of that variable for the month at hand, anecdotal assertions we have heard suggest that Madison got only about 8% of its possible January sunshine this past month.

Thanks to a major snow event the last weekend of the month, in which 7.3 inches of snow fell on Saturday and Sunday combined, we ended up at just about a normal amount for the month (13.9 inches was 0.2 inches above the average).

January 2023 climate statistics for Madison and Milwaukee complied by the NWS

Of all the strange aspects to the last month, however, the strangest might just be the temperature. For almost the first four weeks of the month (that is, through Jan. 27) we averaged 10.53 degrees above normal — way out of line.

In fact, had we managed to remain at that pace we would have ended up with the third-warmest January ever recorded in Madison, behind January 2006 (plus 11 degrees) and 1933 (plus 10.7 degrees).

As it was, the snowstorm on Jan. 28 ushered in a fairly significant cold spell during which we recorded our coldest morning of the season at minus 11 on Jan. 31. We ended up at 7.3 degrees above normal for the month, which was still the eighth-warmest January ever in Madison. Milwaukee, at 7.9 degrees above its normal, recorded its fourth-warmest January ever.

Of course, none of this has much to say about what February might be like, but it is true that, climatologically at least, we have already been through our coldest week of the 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: Meteorology, Seasons

Comments Off on How did January measure up?

What is intermittent snow?

Posted on January 30, 2023 by WeatherGuys Editor
Dane County snowplows clear the Beltline during a 2019 snowfall. (Photo credit: John Hart, State Journal archives)

A weather forecast of snow indicates a steady fall of snow. The forecast may be modified by terms such as “light,” “moderate” or “heavy” to indicate the intensity of the snowfall.

The intensity is measured in terms of visibility during the snowfall:

  • Light snow — visibility of 1 kilometer (1,100 yards) or greater.
  • Moderate snow — visibility between 0.5 kilometer (550 yards) and 1 kilometer (1,100 yards).
  • Heavy snow — visibility of less than 0.5 kilometer (550 yards).

A forecast for intermittent snowfall often refers to light snow falling for short durations. No accumulation, or a light dusting, is expected. The National Weather Service defines snow flurries as intermittent light snow. Snow flurries are short durations of snowfall and produce no measurable precipitation, only trace amounts. Snow flurries tend to come from stratiform clouds.

The term “snow shower” refers to a short period of light to moderate snowfall and is characterized by a sudden beginning and ending of the snowfall. Some accumulation is possible. The atmosphere is susceptible to the development of snow showers when the difference between the surface temperature and the temperature some distance above the ground is large.

Snow squalls are brief, intense snow showers accompanied by strong, gusty winds. Accumulation of snow on the ground may be significant during snow squalls.

Blizzard conditions include winds greater than 35 mph with heavy snow or blowing snow. Visibility is poor, less than a quarter of a mile, and these conditions are expected for a period of three hours or more.

Snow is difficult to measure because it can be moved by the wind. Newly fallen snow is reported in inches to the nearest tenth of an inch. The measurement is taken as soon as the snowfall has stopped. Meteorologists use a snowboard — a flat wooden surface -– and measure in a location where drifting does not usually occur.

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, Seasons

Comments Off on What is intermittent snow?

What is the status of sea ice around Antarctica?

Posted on January 23, 2023 by WeatherGuys Editor
Sea ice around Antarctica reached the lowest extent ever recorded in December 2022 when NASA’s Aqua satellite captured this image of the Antarctic Peninsula and adjacent sea ice.

The Southern Ocean around Antarctica freezes to form sea ice every year. Scientists use observations from satellites to monitor the sea ice extent throughout the year.

Sea ice melt in Antarctica is a seasonal occurrence that usually begins in September (the end of Southern Hemisphere winter) and continues into February (end of Southern Hemisphere summer). As the pole warms during summer, the sea ice around the continent breaks up and melts.

Sea ice extent increases and decreases with the seasons. The decline in the amount of sea ice from winter to summer is very steep in the Antarctic. The seasonal variation is much larger than the interannual difference. Since the year 2013, Antarctic sea ice has exhibited its highest and lowest extents on record, with the highest-ever winter maximum in September 2014, and possibly this year for the lowest-ever summer minimum. The extent of sea ice around Antarctica at the end of December 2022 was the lowest in 45 years of satellite records.

Overall, the long-term annual trend in the amount of Antarctic sea ice is nearly constant. Sea ice at the Southern Hemisphere surrounds a large continent and has more room to expand in the winter than in the Arctic. Because of this geography, that ice also stretches into warmer latitudes, leading to more melting in summer.

Less Antarctic sea ice survives the summer than in the Arctic. On average, in the Southern Ocean only about 15 percent of winter ice cover survives the summer, while about 40 percent of the Arctic Ocean’s winter ice cover remains at the summer minimum. Since so little Antarctic ice persists through the summer, most of the Antarctica’s sea ice is only one winter old and is relatively thin, often about 3 feet or less.

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

Comments Off on What is the status of sea ice around Antarctica?