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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What is a heat dome?

Posted on July 28, 2025 by WeatherGuys Editor

“Heat dome” is a term used explain extreme heat conditions across large geographic regions.

High pressure systems aloft that can create a “cap” that traps heated surface air in place. Heat waves develop as the weather pattern becomes stagnant and the heat builds over numerous days. (Image credit: NOAA/NWS)

The American Meteorological Society maintains a glossary of meteorological terms and defines a heat dome as, “An exceptionally warm air mass at middle latitudes during the warm season that that is associated with a synoptic-scale area of high pressure aloft. This area of high pressure aloft can have a doming effect on the warm air mass below by suppressing rising motion and the development of clouds and precipitation.” This is not the same as a heat wave, which is a spell of 3 or more abnormally hot days.

A heat dome develops when a ridge of high pressure builds over an area and resides there for a week or more. High pressure is associated with very few clouds and lots of sunshine, leading to warm temperatures near the surface. The sinking motion in the high pressure prevents warm air near the surface from rising. This subsidence motion causes further warming of the air by compression. Unless the upper atmospheric pattern changes, the high pressure will continue to exacerbate the hot conditions. The ground also warms and loses moisture, which can lead to drought conditions and the risk of wildfires. The term “ring of fire” is a weather term that is a related to heat dome, as it describes thunderstorms that develop at the boundaries of the heat dome.

Hot and humid conditions during a heat dome can lead to heat-health issues. When our bodies get hot, we cool down by sweating. The sweating does not directly cool our bodies; it is the evaporation of the sweat that cools us. If the air has a high humidity, then the rate of evaporation is reduced, hampering the body’s ability to maintain a constant internal body temperature.

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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How is rainfall intensity changing?

Posted on July 21, 2025 by WeatherGuys Editor

NOAA maintains observations of temperature and precipitation extremes in the U.S. The data indicate that cold extremes in the U.S. have become less frequent. Since the 1930s, there have been many more record-high temperatures compared to record-low temperatures. With warmer temperatures, the amount of precipitable water in the atmosphere also increases.

Extremely heavy rainfall is increasing across the United states. This map shows observed percent changes in total precipitation falling on the heaviest 1% of rainy days from 1958–2021. Numbers in black circles depict percent changes at the regional level. (Image credit: Fifth National Climate Assessment)

Consistent with the increased atmospheric moisture, rainfall and extreme precipitation events have also increased. Almost all areas in the U.S. exhibit increases in heavy precipitation, with the largest increases in the Midwest (45% increase) and Northeast (60% increase). The Northwest has the smallest increase (1%) increase followed by the Western regions (17% increase).

A rain day is defined as a period of 24 hours in which 0.01 inches or more of rain is recorded. The intensity of rainfall can be classified as: “light,” the rate of fall varying between a trace and 0.10 inch per hour; “moderate,” from 0.11 to 0.30 inch per hour; and “heavy,” 0.30 inch per hour.

Recent observations and studies suggest shifts in rainfall intensity. A recent study investigated daily precipitation intensity distribution from 1951–1980 and 1991–2020. They grouped the data for these two 30-year time periods according to 17 climate regions of the United States. Regions in the central and eastern U.S. show a consistent increase in average precipitation and its variability. Changes are mixed in the western U.S. The study also quantified a shift in the number of low precipitation events to the number of higher intensity events.

Changes in the rainfall distribution has implications for water management, agriculture, and potential flood risk. NOAA’s sustained observations and research are critical to observing and understating why extreme events are changing now and how they may change in the future.

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, Meteorology

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