The Weather Guys

How does factory exhaust contribute to snowfall?

Posted on December 16, 2024 by WeatherGuys Editor

An article in the Wisconsin State Journal reported that on November 26, 2024, a factory near Menomonie, Wisconsin contributed to a narrow band of snow that extended for nearly 100 miles. This can occur only with certain atmospheric conditions.

Factory exhaust caused a narrow snow band, as seen on the preliminary, non-operational GOES-19 Advanced Baseline Imager. (Image credit: Tim Schmit, NOAA)

A cloud deck that contained supercooled water droplets was present over the area. Supercooled water droplets are liquid water that are at temperatures below freezing. This frequently occurs in cloud decks.

Relative humidity and dew point are common ways of reporting the amount of water vapor in the atmosphere. Another way is vapor pressure. Gas molecules exert a pressure when they collide with objects. The atmosphere is a mixture of gas molecules and each type contributes its part to the total atmospheric pressure. The pressure that water molecules exert is called the vapor pressure. When the relative humidity is close to 100%, the vapor pressure is close to the saturation vapor pressure.

The bonding forces of the molecules in ice are much stronger than those in water. As a result, the saturation vapor pressure over ice is much lower than that over liquid water when at the same temperature. If the air is saturated with respect to water droplets, it is supersaturated with respect to the ice crystals. Water vapor molecules will deposit onto the crystals, lowering the relative humidity of the air. In response, water molecules evaporate from the water droplets, supplying more water molecules to the air that then deposit onto the crystals. Emissions from the factory likely contained some particles that collided with the supercooled cloud droplets, and then the droplets converted to ice. This increased the ratio of ice to liquid and the ice crystals grew as the water droplets evaporated. At some point, the ice crystals were large enough to fall out of the cloud as snowfall.

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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Does the warmest autumn on record mean anything about the coming winter?

Posted on December 9, 2024 by WeatherGuys Editor

In the past several years we have occasionally mentioned our tracking of the areal extent of air colder than minus 5 degrees centigrade at about 1 mile above the surface of the Earth. This measurement has proven to be a very valuable addition to the collection of metrics of global warming. We have particularly commented on the wintertime (December, January and February) average of this extent measured over the entire Northern Hemisphere, noting that since 1948 the wintertime average extent has systematically decreased.

Wisconsin autumn temperature anomalies (Image credit: NOAA/NWS/LaCrosse)

We have also been tracking this variable throughout the autumn over all these years and can report that this fall (Sept. through Nov. 30) recorded the smallest average areal extent of this cold air since at least 1948. That means we have just experienced the warmest Northern Hemisphere autumn in at least the past 77 years.

Naturally, one wonders if this warm start will carry on throughout the coming winter. This is not a simple question as it turns out. If one were to rank the 77 autumns since 1948 in a list from warmest to coldest, one would find that the warm autumn does not necessarily lead to warm winter. In fact, only 27 of the previous 76 seasons have seen rank changes of less than 10 places between autumn and winter. So, autumn does not really provide a reasonable forecast for winter.

A fairly recent example of this disconnect comes from September 2011 to February 2012. The autumn portion of that period ranks as the sixth-warmest autumn in the time series but the following winter was the 60th-warmest winter — that is a huge change in ranking.

So, we can’t make a reliable hemispheric prediction about winter based on the recent record-setting autumn.

Category: Climate, Seasons

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Is climate change uniform across the globe?

Posted on December 2, 2024 by WeatherGuys Editor

As the map below shows, most land areas have warmed faster than most ocean areas, and the Arctic is warming faster than most other regions. Recent warming is much faster than the longer-term average, with some locations warming by 1 degree Fahrenheit or more per decade. Differences are most dramatic in the Arctic, where the loss of reflective ice and snow amplifies the rate of warming. (Image credit: NOAA Climate.gov, based on data provided by NOAA National Centers for Environmental Information)

Temperature is a fundamental indicator of a climate. Annual and seasonal temperatures patterns have a defining role in the types of animals and plants that reside in an ecosystem. Rapid changes in temperature can disrupt a wide range of natural processes. This is one reason we monitor temperature changes as a metric for global change. The National Oceanic and Atmospheric Administration’s National Centers for Environmental Information maintain a collection of climate data online at: www.ncei.noaa.gov

Concentrations of heat-trapping greenhouse gases, such as carbon dioxide, are increasing in the Earth’s atmosphere. This increase is due to anthropogenic activity. In response, the average temperatures at the Earth’s surface are increasing and are expected to continue rising. Though global temperature changes can shift the wind patterns and ocean currents, the regional warming is not uniform.

The observed global average surface temperature has risen at an average rate of 0.17°F per decade since 1901. Since 1901, the average surface temperature across our contiguous 48 states has similarly risen at an average rate of 0.17°F per decade. The average temperatures have risen more quickly since the late 1970s: from 0.32 to 0.51°F per decade since 1979. For the contiguous United States, nine of the 10 warmest years on record have occurred since 1998.

The changes across the globe show no uniform patterns in the rate of increase. The temperatures of the Arctic are rising two to four times faster than the global average. The Antarctic Peninsula has also experienced a similarly dramatic warming. Desert areas also warmed at rates exceeding the global average warming rate.

Of course, this regional variability makes the problem of accurately predicting how the associated changes in weather patterns across the globe will change in a warmer world even more difficult to solve. However, understanding such weather variability is absolutely critical to meeting the warmer future with in the least disruptive way.

Category: Climate, History

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How and when do Madison lakes freeze?

Posted on November 25, 2024 by WeatherGuys Editor

The surface of a lake exchanges energy with the air above. Cold air cools the lake surface through energy exchanges with the atmosphere, determined by the weather above. As cool surface water cools, it becomes denser than the warmer water below and so the cooled water sinks. Water from below then rises to the surface where it begins to cool.

The Lake Mendota buoy project is a collaboration between the University of Wisconsin Limnology, Space Science and Engineering Center (SSEC) and Environmental Engineering. The buoy measurements provide researchers valuable information to better understand the biological process governing the health of the lake and the impact of human activity on water quality. The buoy is located approximately 1.5 km North East of Picnic Point.

What is unique about the H₂O water molecule is that as liquid water cools, its density increases until about 39°F (4°C). At that point, the colder water becomes less dense, stays at the surface, and continues to cool. Once the surface water cools to approximately 32°F, the water molecules crystallize into interlocking lattice-like patterns and ice is formed. For a lake surface to freeze, the entire lake needs to be at a temperature of 39°F; only then as the surface cools will the temperature of the liquid water at the surface remain less dense than the water below and thus float and begin to form ice. Shallower lakes usually freeze before deeper lakes since shallower lakes contain less water that needs to be cooled.

When a lake will freeze is not determined solely by cold autumn weather but also depends on the lake’s temperature throughout the year. If the lake warms more than average during the summer, it could take longer to cool because the entire lake must reach a temperature of 39°F. If you want to monitor Lake Mendota’s temperature (and other factors), data is provided to the public by the Lake Mendota Buoy (or David Buoy): https://metobs.ssec.wisc.edu/mendota/buoy, a service of UW-Madison.

A lake’s freezing date is the first date that most of the lake surface is estimated to be frozen. The Lake Mendota, Lake Monona, and Lake Wingra median freeze dates are December 20, December 15, and November 29, respectively.

Category: Meteorology, Seasons

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What is climate change?

Posted on November 19, 2024 by WeatherGuys Editor

Increased concentrations of GHGs from anthropogenic sources have increased the absorption of infrared radiation, enhancing the natural greenhouse effect. (Image credit: Center for Sustainable Systems (CSS), University of Michigan)

Climate can be defined as the collective state of the atmosphere for a given place over a specified interval of time. There are three parts to this definition: location, because climate can be defined for a globe, a continent, a region or a city; time, because climate must be defined over a specified period; and the collective state of the atmosphere, which includes averages and extremes of variables such as temperature, precipitation, pressure and winds.

Climate change is a long-term change in the average weather patterns that have come to define Earth’s local, regional and global climates.

Climate change can result from natural events, such as volcanic eruptions, asteroid impacts or changes in our sun’s energy output.

Climate change can also be caused by human activities. The building of cities is a well-documented example of inadvertent modification of a climate by human activities. The urban heat island effect refers to the increased temperatures of urban areas compared with a city’s rural surroundings. Several factors contribute to the relative warmth of cities, such as heat from industrial activity and the thermal properties of buildings and roads.

Since the 1800s, human activities have been the main driver of observed climate change, primarily due to the burning of fossil fuels such as coal, oil and gas. Burning fossil fuels generates greenhouse gases, which are transparent to solar radiation but absorb large amounts of terrestrial infrared radiation that results in warming the atmosphere.

Over the past two centuries, the global average surface temperature has increased noticeably. Currently, Earth is about 2.11 degrees Fahrenheit warmer than the late 19th-century preindustrial average. The 10 most recent years are the warmest on record. There is no debate about the cause of this warming trend; it has resulted from human activities, principally through emissions of greenhouse gases.