What is the longest lightning bolt?

Posted on September 2, 2025 by WeatherGuys Editor

Lightning is a huge electrical discharge, or spark, that results from vigorous motions in thunderstorms.

The GOES-R GLM is the first instrument of its kind in geostationary orbit and provides detailed satellite data that not only depicts current weather conditions, but also helps predict hazardous conditions in the future.
Developed as part of ProbSevere, a statistical model that predicts the probability that a storm will produce severe weather in the near-term, scientist John Cintineo trained a sophisticated machine-learning algorithm to recognize complex patterns in GOES-R Advanced Baseline Imager (ABI) imagery that often precede lightning activity detected by GLM. (Image credit: CIMSS)

Storms are composed of ice crystals and liquid water droplets. Winds inside the storm cause particles to rub against one another, causing electrons to be stripped off, making the particles either negatively or positively charged. The charges get grouped in the cloud, often negatively charged near the bottom of the cloud and positively charged up high. This is an electric field, and because air is a good insulator, the electric fields become incredibly strong. Eventually a lightning bolt happens, and the flow of electrons neutralizes the electric field.

Most lightning flashes reach less than 10 miles. On occasion a lightning bolt can be 60 miles or more in length, and that is referred to as a “megaflash.” They originate in massive convective storm complexes known as mesoscale convective systems. Fewer than 1% of thunderstorms generate megaflash lightning.

The World Meteorological Organization, or WMO, investigates and certifies meteorological records. The record flash occurred on Oct. 22, 2017, was 515 miles in length, running from eastern Texas through Oklahoma, Arkansas and Kansas to near Kansas City, Missouri, in seven seconds. The research team used data from the Geostationary Lightning Mapper sensor aboard a NOAA geostationary satellite to support its measurement.

Lightning can travel from cloud to cloud, within the same cloud, or between the cloud and ground. In-cloud lightning discharges are more common than cloud-to-ground discharges and are not as hazardous.

Cloud-to-ground is the best-known type of lightning, and it poses the greatest risk. On average there are 21 lightning deaths a year in the U.S., down from 55 deaths in 2001 before the start of a national lightning safety campaign: “When lightning roars, go indoors.”

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, Severe Weather

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Has it been unusually rainy so far this summer?

Posted on August 25, 2025 by WeatherGuys Editor

As we head into the last week of August, it is of interest to review aspects of this summer’s precipitation in Madison. 

July precipitation departure from normal, in which much of the north was near or below average. Small pockets of Grant, Green, and Rock counties in southern Wisconsin were four inches wetter than normal. (Image credit: Wisconsin State Climatology Office)

First of all, every one of the three traditional summer months has been wetter than normal with June, July, and August (through Aug 20) coming in at 0.26”, 1.55” and 2.02” above normal for a total surplus of nearly 4”. 

In addition, though we have had a few really wet days – June 23 (2.48”), July 10 (2.01”) and August 10 (2.07”) – another aspect of this summer’s precipitation has been its remarkable regularity.  From June 1 – August 20 we have had only ONE streak of more than 3 consecutive days without precipitation.  That 9-day streak occurred from July 31 – August 8. 

Perhaps equally remarkable is the fact that in June and July we had only 4 streaks of 3 consecutive days without precipitation.  We have compiled statistics on such streaks for June/July going back to 2000.  In that time only 8 June/Julys have had fewer rain-free streaks of 3 or more days than we did this year (2004 wins with only 2 such streaks!).  So, by this measure, this has been a pretty unusual summer with regard to the regularity of rainfall. 

In addition, this summer’s dry streaks have been the shortest in the last quarter century – the average longest streak is 7 days and no streak lasted longer than 3 days this June/July.  (In case you are wondering, the record longest such streak since the turn of the century was 11 in 2021).  So, if you have been thinking this has been an unusual summer weatherwise, there is good justification for that!

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, Seasons

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How does this summer dew point temperature compare with previous years?

Posted on August 15, 2025 by WeatherGuys Editor
Trend in the summertime average dew point temperature for Madison, WI. (Image credit: Dr. Edward Hopkins, WI State Climatology Office)

The dew point temperature is the temperature to which the air must be cooled, at constant pressure, to get dew to form. As the grass and other objects near the ground cool and reach the dew point temperature, some of the water vapor in the atmosphere will condense into liquid water on the objects.  While the dew point temperature is a temperature, it is also a measure of the humidity of the air. The higher the dew point temperature, the greater the amount of water vapor in the air.

To know how close the air is to saturation, we need to know the dew point and the air temperature. The closer the dew point is to the air temperature, the closer the air is to saturation. When the dew point equals the air temperature, the air is saturated, so the dew point temperature cannot be greater than the air temperature.

The dew point temperature can vary considerable from day to day. The WI State Climate Office keeps a record of the observed dew point for Madison and other cities across our state. The average dewpoint measured in Madison (observation period 1948-2023) for the month of July is 61.2F, with the maximum average value during this time of 66F and a minimum average of 56F.  In 2025, the average July dewpoint has is 65.3F, higher than normal and a bit higher than the July 2024 observed value of 62.0F. For June the daily average, average maximum and average minimum dew point values are 56.1F, 60.9F and 49.7F respectively.

The average dew point temperatures for our summer months (June, July and August) shows an increasing trend of 2.3F since 1971. Not too surprising given that as the average temperature increases the amount of water vapor in the atmosphere also tends to increase.

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, Meteorology, Seasons

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Does the Dept. of Energy’s latest climate report ring true?

Posted on August 12, 2025 by WeatherGuys Editor

The Trump Administration continues to double down on its naked denialism of the nature of, and threat posed by, human-induced changes to the global climate. 

In 2024, global surface temperature was 2.32°F (1.29°C) above the 20th-century average. This ranks as the highest global temperature in the period of 1850–2024, beating the next warmest year (2023) by 0.18°F (0.10°C). The 10 warmest years since 1850 have all occurred in the past decade. (Image credit: NOAA/National Centers for Environmental Information)

Late last month the Department of Energy released a 151-page report entitled “A Critical Review of Impacts of Greenhouse Gas Emissions on the U.S. Climate.”  This report was commissioned this spring and work on it was conducted from early April until late May with a May 28 deadline to deliver a draft.  It was authored by 5 scientists who share a deep skepticism regarding the broad scientific consensus that industrialization has played a leading role in altering the chemical composition of our atmosphere in such a way as to encourage global warming, ocean acidification, increased wildfire threat and an increased risk of high impact weather systems in all seasons. 

This is hardly a non-biased or balanced panel and the report refutes prior National Climate Assessments, some written during the first Trump Administration.  In addition, a number of chapters in the document contain multiple references to the 5 authors’ prior work on the issue – a good deal of which is far outside the scientific mainstream.  Both the manner in which it was created and the content itself should suggest to any reasonably skeptical citizen that this report is little more than broadcast of a predetermined position on this important issue dressed up in official sounding scholarly language.  We should expect better from our federal government.

This Trump administration recently dismissed everyone working on the Sixth National Climate Assessment, a congressionally mandated climate report that involves hundreds of scientists and experts and includes careful peer review. The scientific consensus remains that the global warming trend observed since the mid-20th century is due to human enhanced “greenhouse effect”.  

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, History

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How far has summer beaten back the areal extent of cold air?

Posted on August 4, 2025 by WeatherGuys Editor

On more than one occasion in this column we have commented on the areal extent of air colder than 23 degrees Fahrenheit at 850 hPa (about 1 mile above the ground) as a measure of the extremity of winter.

The 850 hPa -5C isotherm from 12z Sunday August 3, 2025.  The shaded area covers 2.582 million square KM.

In the middle of January, about 68 million square kilometers of the Northern Hemisphere are covered by air that cold at that level. Between about July 5 and July 20 that area shrinks to zero, and the complete absence of such air lasts only a very few days. At the beginning of August, we are just past the point in this summer that this area is beginning to increase again. Thus, despite the fact that we have been through a couple of really hot (and humid) weeks to end July, the return of winter has already begun in terms of this measure.

This is largely because the number of daylight hours has already begun to decrease — noticeably here in Madison. At every latitude north of 66.5 degrees, however, the sun was out for 24 hours long on the day of the summer solstice (June 21). Ever since, the spectre of nighttime has been creeping poleward with the daylight decreasing ever so slightly just above the Arctic Circle. The associated night time cooling is greater during a longer night, resulting in the gradual increase in the amount of cold air covering the Northern Hemisphere.

So, if these past couple of weeks have been a bit too much for you, perhaps some solace can be found in the fact that the tide is already, however imperceptibly, beginning to turn.

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, Seasons

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