What is snow sparkle?

Posted on February 17, 2020 by WeatherGuys Editor
The sun shines through the trees in the UW-Arboretum. Light reflecting off ice crystals that have settled on the top of snow can produce snow sparkle. (Photo credit: Michelle Stocker, Capital Times archives)

Saturday was a bright, sunny day, and if you were walking by an undisturbed field of snow, the snow may have appeared to sparkle.

Snow sparkle is caused by light reflecting off ice crystals that have settled on the top of the snow.

When light hits an object three things happen. The light can be reflected, in which case it bounces off in a new direction; it can be absorbed, in which case the object is heated; and it can be transmitted, in which case light passes through the object. The law of reflection states that when light is reflected from a smooth surface, the angle of reflection is equal to the angle of incidence and the two rays lie in the same plane.

Snow is made of ice crystals, and as a crystal gently falls on a surface, it may lie relatively flat. Some crystals are smooth and can act like mirrors and reflect light. When conditions are right, rays of light hit individual ice crystals that are on the uppermost layer of snow and reflect the light upward, at the angle of reflection. The reflected light will be bright, a small image of the sun.

For the light ray to hit your eye and become visible, the crystal has to be lying at the correct angle on the surface so the angle of reflection sends the beam your way. Of the thousands of ice crystals lying on the surface, you see only those that align correctly. If you move, you encounter different crystals that also are at the correct orientation.

Dry conditions can increase the likelihood of seeing snow sparkles because ice crystals often stay separate in drier conditions. Warmer days decrease the likelihood of seeing snow sparkles because, as the crystals may melt, they merge together and may not lie as a flat reflective surface.

Category: Phenomena, Seasons

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What is geoengineering?

Posted on February 10, 2020 by WeatherGuys Editor

Geoengineering literally means “Earth-engineering.” It is a term that describes how people could intervene in Earth’s functions to slow down or reverse the effects of climate change.

Current discussions of geoengineering focus on two broad categories to reduce global warming.

The first general idea is to cool the planet by reducing the amount of solar energy it absorbs. This could be done by increasing the amount of solar energy reflected back to space.

One approach is to build space reflectors, which would block a small portion of sunlight by reflecting the energy away from Earth.

Some geoengineering ideas replicate effects of volcanic eruptions, which often cause cooling because ash and aerosols reflect solar energy back into space.
(Photo credit: Pixabay)

Another proposed technique is to inject aerosols into the stratosphere. This approach attempts to replicate the effect of explosive volcanic eruptions, such as Mount Pinatubo in 1991. That eruption spewed tiny aerosols into the stratosphere that scattered sunlight back into space, which over the next 15 months decreased the average global temperature by about 1 degree.

The second category of ideas seeks to remove carbon dioxide from the atmosphere and thus reduce its accumulation in Earth’s atmosphere. Carbon geoengineering proposals posit that carbon can be removed from the atmosphere on a massive, global scale using a combination of biological and mechanical methods. Global-scale tree planting is one example. Another is to build large machines that directly remove atmospheric carbon dioxide and store it elsewhere.

Geoengineering comes with risks and significant uncertainties. Intervening in a complex system can give unexpected results. While there may be global benefits, local impacts could vary widely and may not benefit the region. For example, shifts in precipitation patterns leading to local droughts. There is also the question of economic cost as some of the proposed techniques are costly.

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: Climate

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Is water everywhere?

Posted on February 3, 2020 by WeatherGuys Editor
Water Vapor (right) and True Color (left) satellite images revealing water on Earth.

Evidence of the presence of water in our atmosphere is ubiquitous.

Water occurs in the Earth’s atmosphere in all three of its phases — solid (snow and ice), liquid (rain and dew) and gas (invisible water vapor).

In the next weeks, as we begin to emerge from winter and enter spring, we may begin to see more dew on the ground and on the windshields of cars in the morning. The air nearly always holds some amount of water vapor. Dew is liquid water that condenses overnight onto objects when the air that contains the water vapor cools to a sufficiently low temperature.

One of the important and microscopic characteristics of the condensation process is that water vapor will not condense into liquid water very easily unless it condenses onto a foreign object, such as the tiny hairlike structures on grasses or dust and pollen particles on windshields. In fact, on particularly dewy mornings, if you wait for the dew to evaporate you may find yellow stains on your windshield that are left as the liquid water evaporates leaving the pollen particles on which it originally condensed.

The formation of raindrops requires a similar collection of foreign objects upon which water vapor can condense. Such objects are known as cloud condensation nuclei, and a great number of naturally occurring substances can serve this role, including dust particles, smoke particles, salt particles, pollen grains, particulate matter from smokestacks and naturally occurring aerosol particles.

Without these cloud condensation nuclei, the formation of cloud liquid water droplets, and eventually precipitation-size particles — which are 1 million times more voluminous — would be considerably more difficult in our atmosphere. In that case, rain and snow would be rare occurrences, and life on the planet would be put at risk.

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: Climate, Meteorology

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How did 2019 weather align with climate trends?

Posted on January 22, 2020 by WeatherGuys Editor

Government scientists concluded that the globally averaged temperature for 2019 was 1.71 degrees Fahrenheit above the 20th-century average and 2.07 degrees above the 19th-century average. This is the 43rd consecutive year that the global temperature was above the 20th-century average.

This was the second-highest since record keeping began in 1880 and was just 0.07 degrees less than the record value set in 2016. Nine of the 10 warmest years have occurred since 2005, and the five warmest years have occurred since 2015.

Overall, North America’s temperature was 1.62 degrees above the 1910–2000 average, marking the 14th-warmest year. Record high annual temperatures were measured across parts of central Europe, Asia, Australia, New Zealand and southern Africa.

The average annual temperature for the contiguous U.S., which doesn’t include Alaska or Hawaii, was 52.7, or 0.7 degrees above the 20th-century average. This ranked in the warmest third of the record and was the coldest year since 2014. Wisconsin’s annual average temperature ranked 46th in the record.

The contiguous U.S. average annual precipitation was 34.78 inches, 4.84 inches above the long-term average. The year 2019 was the second-wettest year on record. Wisconsin, along with North Dakota, South Dakota, Minnesota and Michigan, had its wettest year on record in 2019.

Also, 2019 saw the continued trend in the decline of Arctic sea ice extent. The average annual sea ice extent in the Arctic was approximately 3.94 million square miles, the second-smallest in the 1979-2019 record. This continues a trend of sea ice loss of about 18,000 square miles per year. That is more than a quarter of the size of Wisconsin.

Over the 41-year satellite record of sea ice coverage in December, the Arctic has lost about 734,000 square miles of ice — 11 times the size of Wisconsin. The extent of sea ice around the Antarctic was also the second-smallest annually averaged value 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.

Category: Climate, Meteorology

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What is a fire cloud?

Posted on January 21, 2020 by WeatherGuys Editor
Massive smoke rises Jan. 2 in East Gippsland, Victoria, as wildfires ravage Australia’s eastern coast. A pyrocumulus cloud forms from rising air that results from intense heating of the surface by phenomena such as wildfires or volcanic eruptions. (Photo credit:
Department of Environment, Land, Water and Planning, Gippsland, Australia)

With the raging fires in Australia, you may have heard news reports of pyrocumulus, or fire clouds.

In Latin, pyro means “fire” and cumulus means “pile up.” Cumulus is a type of cloud that is common in Wisconsin, particularly in summer. Cumulus clouds are those puffy white clouds with tops that have a cauliflower appearance.

Pyrocumulus clouds are grayish or brown in color because of the ashes and smoke of the fire. The tops of these clouds can reach as high as 30,000 feet. It is difficult to locate the bottom of a pyrocumulus cloud as it is often obscure by the ash generated by the fire or the volcanic eruption.

A pyrocumulus cloud forms from rising air that results from intense heating of the surface by phenomena such as wildfires or volcanic eruptions. The fires that generate these clouds can be man-made or natural. A big fire produces strong upward moving air currents that carry water vapor and ash upwards. The water vapor can condense on the ash forming cloud drops. The vigorous upward motions produce these pyrocumulus clouds that look similar to thunderstorm clouds, which also form due to strong upward moving air.

If lots of water vapor is available, the pyrocumulus can develop into a cumulonimbus, or thunderstorm. When a thundercloud forms, it is called pyrocumulonimbus. Like thunderstorms, pyrocumulonumbus can produce lightning because of the strong updrafts. Rain can also fall from these clouds, which could help extinguish the fire generating the cloud. Of course, the lightning might cause another fire.

There have also been reports of fire tornadoes in the recent Australian fires. A fire tornado is a swirl of fire that extends upward from a ground fire. They are also called fire whirls. These vortices can occur over a range of fire sizes, but the largest are associated with wildfires like those in Australia.

There is a wide range in the properties of a fire tornado, but they are usually 30 to 200 feet tall and about 10 feet wide. Generally, they last for only a few minutes. Fire tornadoes were reported during the famous Peshtigo Fire of October 1871.

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, Phenomena, Severe Weather, Weather Dangers

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