Where does water exist?

GOES East satellite image showing global water vapor. Values measured are temperature but colors also indicate moisture in the atmosphere where greens and blues reveal high water vapor content and yellows and orange indicates dry air.

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). As we begin to emerge from the recent cool spell and really enter spring/summer, 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-sized 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. Send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.

Category: Climate, Meteorology, Seasons

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How unusual was this cold first half of May?

Tulips bloom around the Wisconsin state Capitol during eight straight chilly May mornings with overnight lows in the 30s.
(Credit: Ruthie Hauge, The Capital Times)

The first two days of this month had high temperatures of 87 and 84, respectively, and daily average temperatures (the average of the daily high and daily low) that were 15 and 19 degrees above normal.

In the next 12 days, the daily average temperature has been just shy of 6 degrees below normal. In fact, during that nearly two week stretch, nine days have had overnight lows in the 30s, including a streak of eight straight days from May 7 through Friday. The morning low on Tuesday was 30 degrees — notably cold and memorable for most of us, but only the 61st coldest morning in the first half of May in Madison’s history, far behind the all-time lowest of 19 degrees recorded on May 1, 1978.

However, the streak of cold mornings we have just endured is much more unusual. In fact, only four eight-day streaks of early May mornings with a low temperature at or below 39 degrees have occurred in Madison history — in 1971, 1976, 1989 and this month.

During this unusual spell of persistent cold nights, we have also been falling behind in precipitation. For the month of May, we are already 0.92 inches below normal and we are only halfway through the month. Coupled with our unusually dry April (and dry winter months of December-March) we are currently running 5.21 inches below normal in precipitation since December 1.

That is a substantial deficit which will require excessive late spring and summer rains to remedy. So, with the near-certain approach of warmer weather, attention to our accrued water deficit will become more urgent.

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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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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