The Weather Guys

Who or what triggers the decision to sound a tornado siren?

Posted on May 18, 2026 by WeatherGuys Editor

The National Weather Service is responsible for issuing forecasts, watches and warnings for a variety of weather and water hazards. A “warning” is issued when hazardous weather poses an immediate threat to life or property.

Dane County uses a zone-based method for activations, so only sirens directly affected by the areas included in a Tornado Warning or a Severe Thunderstorm Warning with the “Destructive” tag will sound. That warning area is set by the National Weather Service office in Milwaukee/Sullivan. (Image credit: EmergencyManagement@danecounty.gov)

A warning can be issued for a variety of hazardous weather, including tornadoes, thunderstorms and flash floods. These threats can form quickly, last a few minutes and impact a small area. Forecasters must rely heavily on observational data from Doppler radar, satellite, and other ground-based equipment and sensors to inform them during the warning process.

Hazardous weather also occurs on a larger scale and are slower evolving, such as low-pressure systems that produce ice storms. In these cases, forecasters use numerical weather models and statistical analysis to come up with a most likely forecast. The forecaster studies the data to determine the need to issue a warning. Once forecasters have high enough confidence there will be significant impacts, a warning is issued as far in advance as possible. The warning covers areas that are expected to see hazardous weather.

The broad reliance on the NWS is a good thing, as publicly accessible data that is standardized and updated regularly helps ensure accuracy of the forecasts.

Sirens are typically activated by city or county officials, usually a police or fire department or emergency management personnel. Individual counties’ emergency management teams or local municipalities own the sirens. The NWS will indicate where and when a tornado warning is in effect, and the local dispatch centers active the sirens. Each county uses its sirens differently. Some will sound sirens across the entire county. Other will only activate sirens within the warned area.

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, Weather Dangers

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Are we ahead of schedule with the spring greening?

Posted on May 11, 2026 by WeatherGuys Editor

We all just lived through a historic April here in Madison, and broadly across the region.

In Madison and Milwaukee, April 2026 was the wettest April since record keeping began in 1869 and 1871, respectively. Madison received 7.26 inches of rain during the month, more than half of that amount coming on just three days (1.34 inches on April 2, 1.53 inches on April 14 and 0.98 inch on April 17).

Rain fell on 22 of the 30 days of April in Madison, with measurable rain on 18 of those days. On two days we set new daily records for precipitation. The previous record for the month was 7.19 inches, set in 1909. Milwaukee shattered its old monthly record by 2.11 inches, as 9.49 inches of rain fell on the city this April, breaking the prior record set in 2013. As was the case in Madison, it rained on 22 of the 30 days, with 18 of them recording measurable rain — one was a daily record.

The extraordinary total featured six particularly rainy days, with nearly half of the monthly total coming on consecutive days from April 12 to April 17. Both cities also had relatively warm Aprils, as Madison came in 15th and Milwaukee ninth all-time for April warmth, that ddespite a record chilly end to the first week of the month.

The rainy, relatively warm April has put the spring greening well ahead of schedule in the southern part of the state. In fact, the regional spring leaf-out this year is running two to five weeks early, with southern Wisconsin on the high end of that range.

So, if you have been wondering if the trees look a bit ahead of schedule for the first week of May, your suspicion is correct. There really is some potency to those April showers.

Category: Climate, Seasons

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Is global warming impacting bird migration?

Posted on May 4, 2026 by WeatherGuys Editor

Global warming refers to the rise in global temperatures due to the increasing concentrations of greenhouse gases in the atmosphere. One impact is that northern latitudes are experiencing warmer mean annual temperatures and experiencing earlier springs, milder winters and delayed falls.

The American Robin population in WI is arriving ~13 days earlier in 2010 than it did in the year 1990.
FAD – First arrival date; MAD “Mean” or “population: arrival data. (Graph: Jones, G. M., B. Zuckerberg, A. T. Paulios. 2012. The early bird gets earlier: A phenological shift in migration timing of the American Robin (Turdus migratorius) in the State of Wisconsin. The Passenger Pigeon, 74:131-140.)

Bird migration is a natural phenomenon that involves the seasonal movement of birds from one place to another. Their evolutionary adaptation allows species to take advantage of seasonal resources and avoid harsh winter conditions. Because seasonal change is a dependable feature of our planet, migratory bird species have adapted to this seasonality as it coincides with the optimal conditions for feeding, breeding and raising their young.

Bird migrations are closely linked to mean annual temperature, which influences both departure and arrival times at breeding and wintering grounds. This dependence makes many migratory birds vulnerable to global and regional warming. Changes in seasonal conditions affect plant and insect populations, which serve as natural food sources for birds. Studies have shown that, in response to warming temperatures, many bird species are migrating earlier in the spring. For example, American robins, the state bird of Wisconsin, have advanced their spring arrival.

Changes in climate and weather that can arise from global warming include extreme precipitation events and extreme temperatures. Climate change can result in a multifaceted set of pressures on bird populations. A warmer climate system will accelerate Earth’s hydrologic cycle — the cycling of water from invisible vapor to liquid and frozen precipitation. Heavy rainfall and flooding threaten bird populations by destroying nests, drowning chicks and limiting foraging for aerial insectivores.

Global and regional warming are ongoing realities. Addressing these challenges and their impacts on bird migration requires collaborative efforts among researchers, policymakers and conservation organizations to ensure effective conservation and management of bird populations in the face of climate change.

Category: Climate, Phenomena

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What is the tornado scale?

Posted on April 29, 2026 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. We classify the strength of a tornado after trained observers assess the damage it did to the area.

The “Modified” Fujita Scale uses damage caused by a tornado and relates the damage to the fastest 1/4-mile wind at the height of a damaged structure. (Image credit: NOAA/Storm Prediction Center)

All tornadoes are assigned a single number from the Enhanced Fujita Scale (EF) according to the most intense damage caused by the storm. When tornado-related damage is surveyed, it is compared to a list of damage indicators and degrees of damage to help estimate the range of wind speeds the tornado likely produced. The rating is assigned based on a set of 28 damage indicators, such as barns, schools and trees; the degree of damage to each one is used to determine the EF scale of every tornado. The breakdown and example damage of the Enhanced Fujita, or EF, scale is:

EF0 (weak) — 65-85 mph: Peels surface off some roofs; some damage to gutters or siding; branches broken off trees.
EF1 (weak) — 86-110 mph: Roofs severely stripped; mobile homes overturned or badly damaged; loss of exterior doors.
EF2 (strong) — 111-135 mph: Roofs torn off well-constructed houses; foundations of frame homes shifted; mobile homes completely destroyed.
EF3 (strong) — 136-165 mph: severe damage to large buildings such as shopping malls; trains overturned; trees debarked; heavy cars lifted off the ground and thrown.
EF4 (violent) — 166-199 mph: Well-constructed houses and whole frame houses completely leveled.
EF5 (violent) — 200-230 mph: Strong frame houses leveled and swept away; steel-reinforced concrete structures badly damaged.

The National Weather Service is always looking for trained volunteers to provide severe weather reports, including reports of tornadoes. For more information, go to www.weather.gov/skywarn/wi-skywarn.

Steve Ackerman and Jonathan Martin, professors in the UWMadison 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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Why do clouds turn green during some storms?

Posted on April 20, 2026 by WeatherGuys Editor

The visible light spectrum covers colors from violet to red. Each color corresponds to a different wavelength, with green sitting roughly in the middle. Blue and violet have shorter wavelengths. Light rays change direction when they hit particles — a process known as scattering. The sky looks blue because air molecules scatter shorter wavelengths more effectively.

*The sky on Tuesday, April 14th, in Dane County. (Photo credit: Kaitlyn Richardson)*

Clouds are made of water drops and ice crystals that scatter light from the sun in all directions. It is the multitude of drops and crystals that make a cloud look white during the day. Sometimes only a small amount of light escapes out the bottom, and so cloud bottoms often appear grayish.

At sunrise and sunset, clouds can appear orange or red. With the sun lower in the sky, the sunlight passes through a significantly longer atmospheric path, resulting in the removal of blue wavelengths before reaching the cloud. Only the longer wavelengths remain visible, accounting for the beautiful orange and red hues observed during sunsets.

Intense thunderstorms sometimes have a green tint. These storm clouds are packed with massive amounts of water and ice and often occur late in the day. The reduced presence of the short wavelength blue portion of the spectrum results from its being scattered out of the column during its long path through the atmosphere. Liquid water has a slight preference for absorbing the oranges and red colors. In the presence of so much liquid water in a thunderstorm cloud, the sligh absorption of red and orange is just enough to filter out some o those longer wavelengths. The middle visible wavelengths, the greens, are then able to dominate and become visible.

While a green sky often suggests severe weather, it does no guarantee you’ll encounter tornadoes or larger hail. The green colo is a visual indicator that the storm has the potential to be severe.