How can the Upper Midwest get such high dew points every year?

Posted on September 3, 2024 by WeatherGuys Editor

The dew point temperature is a measure of the amount of water vapor in the atmosphere and is also a good way to tell how uncomfortable you might feel on a hot day. Hot days and high water vapor contents are a serious health risk for some. Also, high water vapor content is a key fuel for severe thunderstorms.

There are three primary sources of water vapor that address this question: advection of water vapor largely from the Gulf of Mexico, evaporation over the Great Lakes, and transpiration from corn fields. The contribution from the Great Lakes in summer is minimal in comparison to the other factors, except maybe near the shorelines.

The process of evapotranspiration. Corn tends to release more vapor because it is a taller plant and tends to use more water than most crops. (Image credit: USDA)

Southerly winds help bring warmer temperatures to the Upper Midwest but they also transport moisture from the Gulf of Mexico. This leads to higher dew points over Wisconsin and the Midwest. Studies have demonstrated that summertime precipitation in the Midwest is strongly correlated with the strength of this Great Plains low-level jet stream, which transports relatively warm, moist air northward from the Gulf of Mexico.

Transpiration is the release of water vapor from plant leaves. Transpiration has three main steps: the plant’s roots uptake water from the soil, the water moves through plant tissues, and the leaves release water vapor into the air through their stomata. During summer months, maturing cornfields become a huge source of water vapor to the atmosphere. Studies suggest a single acre of corn can transpire 3,000 to 4,000 gallons of water per day. On calm days, an analysis of dew points over the Midwest can identify the location of the Corn Belt. This transpiration by corn is sometimes referred to as “corn sweat.”

Together, these factors can result in summer dew points in the low to middle 70Fs for the Upper Midwest.

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: Climate, Phenomena, Seasons

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Does the U.S. have a new weather satellite?

Posted on August 26, 2024 by WeatherGuys Editor

Weather satellites fly around Earth in two basic orbits: a geostationary Earth orbit (abbreviated as GEO) and a low Earth orbit (abbreviated as LEO). LEO satellites’ orbits often travel over the Earth’s polar regions, flying at an altitude of 530 miles. Geostationary satellites orbit Earth as fast as Earth spins and so hover over a single point above Earth at an altitude of about 22,300 miles. To maintain its position, a GEO satellite must be located over the equator. In contrast, polar satellites go around from pole to pole as the Earth rotates beneath the satellite; each orbit is slightly to the west of the previous one.

Since 1975, Geostationary Operational Environmental Satellites (GOES) have provided continuous imagery and data on atmospheric conditions and solar activity (space weather). They have even aided in search and rescue of people in distress. GOES data products have led to more accurate and timely weather forecasts and better understanding of long-term climate conditions. The National Aeronautics and Space Administration (NASA) builds and launches the GOES, and the National Oceanic and Atmospheric Administration (NOAA) operates them. (Image credit: GOES-R.gov

Satellite observations provide valuable information to meteorologists. The United States usually operates two geostationary satellites called GOES (Geostationary Operational Environmental Satellite). One has a good view of the East Coast; the other is focused on the West Coast. They are a basic element of the U.S. weather monitoring and forecasting enterprise. The first GOES satellite, GOES-1, was launched in October 1975. Upgrades to the instruments are made as needed and the U.S. is currently in the fifth generation of GOES satellites. These are the GOES-R series, first launched in 2016.

The current series of GOES provides advanced imagery and atmospheric measurements, real-time mapping of lightning activity, and monitoring of space weather. This is the Western Hemisphere’s most sophisticated weather-observing and environmental-monitoring system.

The procurement, design, and manufacture of GOES satellites is overseen by NASA, while NOAA is the official provider of both GOES terrestrial and space weather data. The most recent GOES satellite was launched June 25, 2024 and is currently undergoing post-launch check out. NOAA expects to see the first images from the new GOES-19 in September.

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: History

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Is our rainy summer finally drying out a bit?

Posted on August 19, 2024 by WeatherGuys Editor

Readers may recall we reported on the unusually wet nature of our late spring/ early summer about a month ago. At that time, Madison had recorded the wettest May 1-July 15 period since at least 1939 while averaging more than 2.5 degrees Fahrenheit above normal for that same period.

Another month has ticked away in this summer, and it has been a bit different than the ones that came before it. Through Aug. 15, Madison has recorded 1.73 inches of precipitation, which is 0.35 below normal. Ten of the first 15 days of the month had no precipitation at all and, from Aug. 7 to Aug. 13 we had a streak of seven straight days with no rain at all. That is now the longest dry streak since May 1.

One last update for the current month is that we have averaged 0.8 degrees Fahrenheit below normal for the first 15 days.

So, both the coolness and the dryness have been very different thus far in the new month.

Despite this recent lack of precipitation recently, we will still have the wettest May 1 to Aug. 15 ever recorded in Madison. To top it off, if we were to receive more than 4.63 inches of rain between now and September 1, we would surpass the all time record for wettest May 1 through September 1, 29.09 inches, set in 2018. That would require an average of nearly 0.29 inches each day until then — not likely since this time of the year averages less than half that amount.

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: Uncategorized

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What is Tornado Alley?

Posted on August 12, 2024 by WeatherGuys Editor

A tornado is a powerful column of winds that rotate around a center of low pressure. The winds inside a tornado spiral inward and upward, often exceeding speeds of 300 mph. Tornadoes form in atmospheres that have extremely unstable moist air, large amounts of vertical wind shear and weather systems, such as fronts or thunderstorms, that force air upward.

The yellow areas, darker with increasing frequency, show large tornado outbreaks from the 1950s through 1980s. The blue striped areas show the corresponding frequency of outbreaks from 1989 through 2019. (Image credit: Daniel P. Huffman in Scientific American)
The yellow areas, darker with increasing frequency, show large tornado outbreaks from the 1950s through 1980s. The blue striped areas show the corresponding frequency of outbreaks from 1989 through 2019. (Image credit: Daniel P. Huffman in Scientific American)

The continental United States provides these three ingredients in abundance.

A plot of tornado tracks in the contiguous U.S. from 1990 through 2011 reveals a relatively high frequency in the central Great Plains. Texas to Kansas stands out as a region of high tornado occurrence. Tornado Alley traditionally refers to this region known for frequent tornadoes. It is a colloquial term, and there are no explicit boundaries to Tornado Alley.

A recent study found the corridor where many tornadoes in the U.S. occur has changed in recent decades. There appears to be an eastward shift of tornado activity, with a corridor that encompasses the states of Arkansas, Louisiana, Tennessee, Alabama and Georgia.

The published study analyzed tornado tracks that spanned two 35-year periods, one from 1951 to 1985 and the second from 1986 to 2010. In the first 35-year span tornado formations peaked in northern Texas, Oklahoma and Kansas — the area traditionally known as Tornado Alley. From 1986 to 2020 the most active tornado corridor shifted eastward, peaking in Mississippi, Louisiana and Alabama.

Within the United States, tornadoes can occur in nearly every state and in every month of the year. When considering tornado activity, meteorologists focus on the specific conditions conducive to their formation: warm, moist, unstable air, and changes in wind speed and direction with height, rather than a fixed geographical area.

The scientific study also demonstrated that there are more winter tornadoes than in past decades. Wisconsin had its first tornado in February this year.

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: Severe Weather, Weather Dangers

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Do thunderstorms cause milk to spoil?

Posted on August 5, 2024 by WeatherGuys Editor

No, but it was once widely thought that summertime thunderstorms could cause fresh milk to curdle.

This thinking was based on observations that raw milk would spoil during lightning and thunderstorms. But it was neither the thunderstorms nor lightning that caused the spoiling; rather it was the atmospheric conditions. The conditions that are optimum for thunderstorm development are the same as those that would cause milk to spoil.

Milk curdles because of bacteria, not because of thunderstorms. Microbes in milk consume the milk’s natural sugars and produce lactic acid as a waste product. This coagulates the milk’s proteins, resulting in lumps in the milk and making the milk taste sour. Bacteria thrive in warm, humid conditions, so hot and humid days were particularly troublesome to early dairy farmers. These same atmospheric conditions are also favorable for summertime thunderstorms.

Thunderstorms require warm moist air that rises, so the environmental conditions that support thunderstorm development are like those in which milk can spoil. This similarity led to the observed correlation of milk spoiling during thunderstorms. But the conclusion that the thunderstorm caused the milk to spoil was incorrect.

An article in the journal Science in 1891 by A.L. Treadwell may be the first to suggest that milk spoils because of bacteria. In 1927, E.H. Farrington published an article that summarized his experiments and noted that “sultry atmosphere usually precedes thunder showers and provides favorable conditions for the growth of milk-souring bacteria.” Part of his career included a professorship at the University of Wisconsin, where the paper was published. “Sultry” is typically used to describe summer stretches of warm days and nights with high dew points.

Pasteurization and refrigeration prevent the milk from spoiling, as they make environments that are too hot or cold for bacteria to thrive. We have not developed the technologies to prevent thunderstorms from developing.

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, Severe Weather

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