The Weather Guys

Are we seeing more ozone advisories in southcentral Wisconsin?

Posted on June 12, 2023 by WeatherGuys Editor

Ozone (O3) is a molecule formed by three oxygen atoms.

Ozone that resides in the stratosphere absorbs ultraviolet rays of the sun, protecting life on Earth’s surface from these high-energy electromagnetic waves. O3 also can occur near the ground, where it is considered a pollutant, as it is a chemically reactive gas that can cause respiratory problems when breathed.

O3 near the ground is created when oxides of nitrogen chemically react with volatile organic compounds. Typically, ground level O3 has highest concentrations in mid-summer with hot weather, minimal amounts of cloud cover and light winds. In Wisconsin, O3 is highest near Lake Michigan due to transport from the Chicago area as well as chemistry occurring over the lake. But triggers such as the chemicals found in forest fire smoke can result in high concentrations of O3 outside that typical time period.

Because O3 can be a health hazard, there are air quality standards set to protect public health. Public warnings are issued whenever these thresholds are approached or exceeded. The Clean Air Act requires the Environment Protection Agency to periodically review all the National Ambient Air Quality Standards to ensure that they provide adequate health protection. The current standard for O3 is 70 parts per million.

The EPA uses an Air Quality Index (AQI) for reporting air quality so that risks can easily be interpreted. The AQI is divided into six categories corresponding to different levels of health concern. Each category also has a specific color to enable the public to quickly determine air quality in their community: green is good air quality, yellow moderate, orange unhealthy for sensitive groups, and red is unhealthy.

During the past three years, there have been more early-season ozone alerts in April and May, in part due to hotter temperatures earlier in the season, and in part this year due to the transport of wildfire smoke from Canada to Wisconsin.

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, Phenomena, Weather Dangers

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Was May abnormal this year?

Posted on June 5, 2023 by WeatherGuys Editor

The just completed month of May was quite unusual in southern Wisconsin this year.

First, it was a bit cooler than normal, with the average temperature ending up 0.6 degrees below normal. Dryness over central and western Canada brought large wildfires to those areas much earlier than normal this year, and the smoke from those fires gave us a number of orangey sunrises and sunsets this past month, which are more normal in July and August.

Statistics for May 2023 for Madison and Milwaukee, Wisconsin (Image credit: NOAA/NWS/MKX)

The dryness was widespread this May, in fact, as the Dane County Regional Airport totaled only 0.87 inches of rain in May, 3.23 inches below normal for the month. That is the predominant reason lawns around town are looking a bit yellow this early in the season — again, more like what we normally see in July and August.

Despite the relative coolness of the month as a whole, the temperature soared to 88 degrees on May 30, 91 degrees (our first of the season) on May 31, 89 degrees on June 1, 89 on June 2 and 91 on June 3. Were we just a bit warmer on three of those days, we would have set the earliest five-day streak of 90-degree days in Madison’s meteorological history. In fact, no five-day or longer such streak has ever had its origin in May.

Very recent events are the only close competitors, in fact. May 10-13 last year we had four straight days at 90 degrees or above (the earliest ever such streak) and from May 26 to 29, 2018, we also had a four-day streak. It had never happened before and almost happened again this year. Summer is certainly making itself known.

Category: Climate, Meteorology, Seasons

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Are the number of thunderstorms in Wisconsin decreasing?

Posted on May 29, 2023 by WeatherGuys Editor

This question comes from one of our readers, based on casual observation. It is always good to get data and analyze such a generalization to find the best answer. So, we turned to the Wisconsin State Climatology Office.

The National Weather Service records the number of thunderstorm days at several sites across the U.S. A thunderstorm day is when thunder or lightning is detected at least once during the day. Since the mid-1990s, the nation’s primary surface weather observation network is the Automated Surface Observing Systems, or ASOS program, which has essentially replaced human observers.

ASOS is a joint effort of the National Weather Service, the Federal Aviation Administration and the Department of Defense. These automated weather stations routinely measure temperature, humidity, winds, visibility and precipitation. Some ASOS sites also have a single-site lightning sensor. We can detect lightning by observing an optical flash or an electrical field change. If lightning is detected, a thunderstorm is in the area.

Additionally, two different networks in the United States — the National Lightning Detection Network and the Earth Networks Total Lightning Network — also detect lightning flashes. These two systems work by detecting radio waves generated by lightning strokes. Since 2016, a Geostationary Lightning Mapper instrument has flown on two weather satellites that map total lightning activity.

An analysis of the annual number of thunderstorm days at the Dane County Regional Airport in Madison shows no obvious trend, although the average number of thunderstorm days between 1990 and 2020 is a bit smaller than during the 30-year period of 1950-1980. An analysis of the number of thunderstorms in Green Bay and La Crosse does show a gradual decline in thunderstorm days. There are no upward trends in the number of thunderstorm days at these sites.

Category: Climate, Meteorology, Severe Weather

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Why was the sun so red this past week?

Posted on May 22, 2023 by WeatherGuys Editor

Light is a form of electromagnetic energy that does not need matter to propagate. We can characterize this energy by its wavelength, which is the distance along a wave from one crest to another. Our eyes are sensitive to light with wavelengths between approximately 0.4 to 0.7 microns. Blue colors have shorter wavelengths, while red colors have longer ones.

When light interacts with particles suspended in air, it can be scattered or absorbed. Energy that is scattered causes a change in direction of the light path. The amount of light that is being scattered is a function of the size of the particle relative to the wavelength of the light falling on the particle. While all colors are scattered by air molecules, violet and blue are scattered most. The sky looks blue, not violet, because our eyes are more sensitive to blue light.

Wisconsin sunrise on May 22nd. Credit: Amy Larson

This past week, upper-level winds carried smoke from fires in western Canada over the Midwest. Smoke can cause the sky to appear hazy, even if the smoke is high above the ground. When smoke is thick, it can cause brilliant red sunsets and sunrises. Small smoke particles scatter blue light. So, as the sun sets or rises and its rays pass through the smoke plume, all the blue lights are scattered out of the path between the setting sun and your eyes, leaving only the red and orange colors. This results in the sun having a bright red color.

If the winds are right, the smoke can be transported down to the ground. This can cause a reduction in air quality. The small particles that make up the smoke can cause respiratory problems, particularly for children, the elderly, and people with asthma.

Category: Meteorology, Phenomena, Weather Dangers

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Can record high and low temperatures help define regional definitions of seasons?

Posted on May 15, 2023 by WeatherGuys Editor

Though it now seems to be fully in swing, the spring has sure seemed delayed in coming this year in southern Wisconsin. This made us wonder if there might be a more refined, and local, way to think about the calendar-day boundaries of the seasons.

In research undertaken to write a recent column, we catalogued Madison’s record high and low temperature data for each calendar day employing data that went back to 1939. An interesting partition of the full year resulted from this simple analysis.

It turns out that not once over these past 83 years has a daily record low temperature been 32 degrees or lower between June 11 and Sept. 11. Might this be a valid reason to suggest summer in Madison extends from June 11 through Sept. 11?

On the other extreme, at least once over these last 83 years, a record low has been zero or below on every day from Nov. 23 to March 15. Might this be a valid reason to suggest that winter in Madison extends from Nov. 23 to March 15?

Once these notions are entertained, they really start to make intuitive sense. Of course, then “spring” in Madison would extend from March 16 to June 10 (again, roughly consistent with a native’s impression), and “autumn” would extend from Sept. 12 to Nov. 22.

2023 Daily Temperatures for Madison bounded by historical averages.
Credit: Wisconsin State Climatology Office

There may be more sophisticated ways to characterize the seasons locally, but using just two seemingly arbitrary temperature thresholds as we’ve done here returns a fairly satisfactory result.