Why do we have a new normal in weather?

Image credit: NCEI and NOAA Climate.gov

The National Ocean and Atmosphere Administration’s National Climatic Data Center (or NCDC) calculates the average weather conditions over a 30-year period for more than 7,500 locations in the United States.

A reliable estimate of an average requires at least 30 years. These 30-year averages are referred to as the U.S. Climate Normal. They provide a baseline that allows everyone to compare a location’s current weather to the average weather that location would expect to see — whether a particular day’s temperature is cooler or warmer than normal, or if a particular month is wetter than normal.

Revised normals are computed every decade to account for the most recent 30 years of climate change. Over the past decade, the normals have been based on weather observations from 1981 to 2010. In early May, climate experts at NOAA issued an updated collection based on the weather occurring from 1991 to 2020.

Scientists use the 20th-century average (1901-2000) to examine long-term climate trends. If we compare the 1991-2020 annual temperature normals to the 20th-century average, we see warming occurring across the U.S. No region is cooler in the past 30 years than it was during the 20th century. The nation’s midsection — from the Gulf of Mexico to the Great Lakes — did not warm as much as other parts of the country but saw a 5% to 15% increase in precipitation over the 20th-century average.

The new climate normals for Madison are warmer, wetter and snowier. The annual precipitation for Madison increased almost 3 inches, and the annual snowfall increased almost an inch. The average annual temperature went from 46.5 degrees to 47 degrees. Each season is warmer and wetter 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 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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Can space dust affect our atmosphere?

Space contains tons of dust. When dust particles approach Earth, they can be captured by gravity and enter the atmosphere at very high speeds.

Particles with diameters larger than about 2 millimeters undergo very rapid heating through collisions in our atmosphere. As they heat up, they can produce a short-lived trail of light known as “shooting star.” Most dust particles entering the atmosphere are estimated to be much smaller than this and don’t provide a visible trail.

The dust aids with the formation of “noctilucent” clouds — the highest clouds in the Earth’s atmosphere — by providing a surface on which ice crystals can form. Noctilucent clouds develop during summer in the polar regions.

Noctilucent clouds over the Baltic Sea in June 2019.
Credit: Matthias Süßen via Wikimedia Commons

A micrometeorite is a small dust particle that reaches the Earth’s surface. They are a few tenths to hundredths of a millimeter in size. Antarctica is a good place to look for micrometeorites because of the low accumulation of snow and the virtual absence of ground dust. After a 20-year collection of extraterrestrial particles in Antarctica, a peer-reviewed publication estimates that every year over 5,000 tons of space dust fall on Earth. For reference, the average U.S. car weighs about 2 tons.

While small in size, over time space dust can add up. On geologic time scales, the dust fall could have delivered significant amounts of water to our atmosphere. They also can fall in the ocean, fertilizing marine phytoplankton with iron.

The micrometeorites undergo change as they pass through our atmosphere colliding with molecules. One way to capture this dust before it enters our atmosphere is to fly through comet tails. The NASA mission Rosetta discovered free oxygen in a comet coma, or tail. NASA’s Stardust mission flew through the tail of comet, collected dust and returned the particles to Earth for analysis. That mission found glycine, an amino acid in the coma.

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, Meteorology, Phenomena

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What caused the snow showers of last week?

Last week’s snow showers garnered their fair share of attention.

The showers were intermittent, a snowy period often followed by a break in the clouds through which an interval of sunshine prevailed. On both days this showery activity ended near or just before sundown.

Satellite image displaying cloud-free and cloudy skies over Wisconsin on April 21 with glaciating cloud tops in green where snow showers developed.

The conditions that foster such behavior are interesting but not all that uncommon. The way in which the temperature of the air varies with increasing height above the ground determines the atmospheric stability. It turns out that when the difference between the surface temperature and the temperature some distance above the ground is large, the atmosphere is very susceptible to the development of showers.

Our showery days last week were characterized by air temperatures at about 3 miles above the ground that were near minus 35 degrees Celsius, while the surface temperature was about 5 degrees Celsius. That 40-degree temperature difference rendered the stratification quite unstable — especially to the ascent of saturated air.

Consequently, any initial movement upward of air bubbles near the ground resulted in those air bubbles rising freely to much higher elevation. As air rises it expands into the lower pressure environment and the expansion forces cooling which increases the relative humidity of the air. Eventually such freely rising bubbles of air produce clouds and precipitation.

The scattered nature of the snow showers resulted from the fact that the regions of ascending air just described are surrounded by sinking air on their sides. The sinking forces the air to warm and dry out — thus producing the cloud-free regions between the showers. In such cloud-free regions, the surface temperature can rise in response to sunlight, which initiates another round of ascent and cloud production.

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

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Are April showers likely to be snow?

After a benevolent first week of April, we have really been brought back to reality as temperatures in Madison have been increasingly chillier since the monthly high of 79 was recorded on April 6.

April 14 was particularly chilly, with the high only getting to 40 under persistent clouds. It could be worse, however. Last Friday it snowed over southern New England — with totals as high as 9 inches in southern New Hampshire and widely greater than 2 inches in the northern suburbs of Boston.

Madisonians are not strangers to April snow, as it has struck us quite substantially in the past two Aprils — on the 12th through 14th last year and on the 10th and 27th in 2019.

Spring flowers struggling in Wisconsin’s 2019 April snow.
Credit Momcrieff.

Unusual weather is also occurring right now just east of the Philippines, where Typhoon Surigae roared with sustained winds above 140 mph (Category 4) over the weekend. Such early season typhoons (or hurricanes) are not without precedent, but they are unusual. Luckily, the storm center remained offshore and limited the damage. This storm was accurately predicted nearly two weeks in advance — quite remarkable given the unusual nature of a mid-April typhoon.

Unlike the recent snowy weather in southern New England, the morning of April 19, 1775, at Lexington, Massachusetts, dawned clear with a modest west wind, rising pressure and a temperature of 45.7 at 6 a.m., according to professor John Winthrop, who lived in a house on the Lexington Green. Later that morning, the first shots of the American Revolution were fired, and the more substantial Battle of Concord ensued in the afternoon.

Winthrop, in a wonderfully understated entry, noted in his journal that the “Battle of Concord will put a stop to observing.”

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, Meteorology, Seasons

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Does it rain only on Earth?

Raindrops on Earth are made of water. On Neptune, scientists suspect it rains pure carbon in the form of diamonds. (Photo credit: John Hart, State Journal Archives)

It does precipitate on other planets and moons in our solar system.

On Earth, when particles fall from clouds and reach the surface as precipitation, they do so primarily as rain, snow, freezing rain or sleet.

On the average, a raindrop is between 0.1 to 5 millimeters. Raindrops on Earth are made of water. Sometimes they can pick up pollen or dust suspended in the atmosphere as the rain falls towards the ground.

The rain on other planets has very different chemical compositions. On Venus, it rains sulfuric acid. On Mars it snows dry ice, which is carbon dioxide in a solid state. Saturn’s moon Titan rains methane and on Jupiter, it rains helium and mushy ammonia hailstones. On Neptune, scientists suspect it rains pure carbon in the form of diamonds.

A recent science study simulated the maximum size of liquid droplets that would fall as “rain” under the different planetary conditions. It is a fairly narrow size range, given the large variation in the gravity of the planets and moons involved. Raindrops that are too big break up into smaller ones, while raindrops that are too small evaporate before they hit the ground.

On Earth, the maximum raindrop size is about 7/16 of an inch, a similar size as on Saturn. The maximum raindrop size on Titan is about 1 and 3/16 inches, and on Jupiter the maximum size is about 9/32 of an inch.

While cartoonists typically draw raindrops in a teardrop or pear-shape, raindrops are not shaped in those forms. They are drawn as teardrops to give the image of falling through the atmosphere, which they do.

No matter the planet or moon, as raindrops fall they are flattened and shaped like a hamburger bun by the drag forces of the air they are falling through.

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

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