What causes a thunderstorm?

Posted on June 22, 2026 by WeatherGuys Editor

Thunderstorm development requires three basic ingredients: moisture, unstable air and upward motion.

The shear environment is important in determining the thunderstorm type. Both the vertical speed shear and directional wind shear have varying magnitudes. To simplify, there are two categories: weak and strong.(Image credit: Weather.gov)

Moisture comes from regions like oceans, lakes and vegetation that provide the water vapor necessary for cloud formation and precipitation.

Thunderstorms are triggered by various mechanisms that lift parcels of air to form clouds. Surface heating by the sun is a common lifting mechanism in summertime thunderstorm development. Lifting also occurs along boundaries of air masses with different temperature and moisture properties, such as fronts.

A parcel of air will not rise unless it is forced upward away from the surface and/or is unstable — warmer than its surrounding environment. Unstable air occurs when warm, moist air is near the ground and colder, dry air is above. This vertical temperature structure creates an instability that allows the warm air to rise.

Rising unstable air will rise and keep rising when given a nudge upward. To interpret how the environment affects thunderstorm potential and severity, meteorologists have invented several stability indices that characterize the atmosphere in a single number.

A factor affecting thunderstorm intensity is the change of atmospheric wind speed and direction from the ground up, known as vertical wind shear.

Small amounts of vertical wind shear lead to upright and majestic but shorter-lived thunderstorms. Moderate amounts of vertical wind shear cause thunderstorm clouds to tilt. If the wind changes direction and increases in speed to a large extent, the thunderstorm itself rotates.

Rotating thunderstorms generally cause the worst severe weather, including large hail and violent tornadoes. A simple rule of thumb is that the greater the vertical wind shear, the more severe the thunderstorm.

Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on Wisconsin Public Radio 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

Comments Off on What causes a thunderstorm?

Are daily weather forecasts affected by climate change?

Posted on June 15, 2026 by WeatherGuys Editor

A friend of ours recently asked whether the accuracy of day-to-day forecasts of weather is affected by climate change. This is a very interesting question whose answer helps to further elucidate the difference between climate and weather.

As it turns out, predictions of the coming weather are nearly exclusively dependent on the observed conditions of the atmosphere in the day (or days) prior to the forecast period. These conditions are known formally as initial conditions.

Those initial conditions are only partially sampled by the many observational platforms (routine surface observations, upper air observations and various satellite platforms, for instance) that have been devised over the years. These initial conditions must be as accurate as possible as they represent the current state of the atmosphere. Once they are acquitted, they are fed into sophisticated computer programs known as numerical weather prediction models, and the ensuing calculations give meteorologists guidance in creating their forecasts.

Even though the climate is changing in the background, accurate measurements of these initial conditions is not substantially affected by that change and so the forecast models are unaffected. Consequently, it is not likely that climate change has any discernible effect on the quality of weather forecasts.

The behavior of the climate system, on the other hand, is governed by a different set of conditions, known as boundary conditions. These include fundamental measures such as the length of the day, the amount of energy coming from the sun, and the chemical composition of the atmosphere. Though these conditions do change, they only do so on long time scales — scales not particularly important to day-to-day weather forecasts.

Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on Wisconsin Public Radio at noon the last Monday of each month. Send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.

Category: Climate, Meteorology, Uncategorized

Comments Off on Are daily weather forecasts affected by climate change?

What is the prediction for the 2026 hurricane season?

Posted on June 8, 2026 by WeatherGuys Editor

Tropical cyclones are large, whirling storms that obtain their energy from warm ocean waters.

A summary infographic showing hurricane season probability and numbers of named storms predicted from NOAA’s 2026 Atlantic Hurricane Season Outlook. (Image credit: NOAA/AOML)

Tropical cyclones with maximum sustained surface wind speeds of less than 39 miles per hour are called tropical depressions. Those with maximum sustained winds of 39 mph or higher are called tropical storms. Hurricanes are tropical cyclones that have sustained wind speeds of greater than 74 mph and that originate in the Atlantic Ocean, Caribbean Sea, Gulf of Mexico, or the eastern North Pacific Ocean. A general rule of thumb is that hurricanes will not form unless the water temperature is at least 80 degrees Fahrenheit.

A tropical cyclone is assigned a name if its sustained wind speeds are 39 mph or higher. Storms are named to reduce confusion and improve communication when two or more storms occur at the same time. The World Meteorological Organization adheres to a strict procedure in assigning names.

The Atlantic hurricane season officially starts June 1 and lasts until Nov. 30. An average hurricane season produces 14 named storms, of which seven become hurricanes, including three major hurricanes (wind speeds greater than 111 mph).

A below-normal Atlantic hurricane season is expected this year.

There are two competing events that make the hurricane forecast challenging and interesting this year.

El Niño is expected to develop and intensify during the hurricane season, while ocean temperatures in the Atlantic are expected to be slightly warmer than normal. El Niño conditions tend to support fewer tropical storms and hurricanes, while warmer ocean temperatures support a more active year. The National Oceanic and Atmospheric Administration prediction for 2026 is a likelihood of 8 to 14 named storms. Three to six of those are likely to become hurricanes, including one to three major hurricanes.

Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on Wisconsin Public Radio at noon the last Monday of each month. Send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.

Category: Meteorology, Tropical, Weather Dangers

Comments Off on What is the prediction for the 2026 hurricane season?

How important was the weather forecast on D-Day?

Posted on June 1, 2026 by WeatherGuys Editor
ERA-CLIM, a ECMWF reanalysis, forecasts at 84 h (upper left), 60 h (upper right) and 36 h (lower left) range, all valid at 12 UTC 6 June 1944, and the 13 UTC analysis of surface pressure for 6 June (Image credit: Forecast for Overlord by J. M. Skagg, Gen Eisenhower's chief meteorologist).
ERA-CLIM, a ECMWF reanalysis, forecasts at 84 h (upper left), 60 h (upper right) and 36 h (lower left) range, all valid at 12 UTC 6 June 1944, and the 13 UTC analysis of surface pressure for 6 June (Image credit: Forecast for Overlord by J. M. Skagg, Gen Eisenhower’s chief meteorologist).

D-Day was the name given to the June 6, 1944, invasion of the beaches at Normandy in northern France by troops of the Allied Forces during World War II. The goal was to cross the narrow, often stormy, English Channel into Nazi-occupied France.

The successful D-Day invasion required a combination of environmental factors: initial invasion around sunrise and at at low tide; nearly clear skies; at least 3 miles of visibility; close to a full moon; relatively light winds; non-stormy seas; and good conditions persisting for at least 36 hours and preferably for four days. To make the task of forecasting even more difficult, the Allies needed at least two days advance notice of these conditions —and because the science of meteorology was only just beginning to modernize, at the time this was a nearly hopeless task.

Krick strongly advocated for analog forecasting. The British Royal Air Force team was led by Sverre Petterssen, who was an expert in the science of air masses, cyclones and upper air patterns. His co-leader was C.K.M. Douglas, who made forecasts by interpreting weather maps and pattern recognition. Reginald Sutcliffe, the British scientist who laid the groundwork of modern weather forecasting, also worked on the D-Day forecast.

While the weather forecasters initially suggested June 5, they realized that a one-day postponement would offer better conditions. This prompted Eisenhower to invade on Tuesday, June 6. German meteorologists did not predict the clearing on the morning of the invasion, adding to the Allies’ advantage.

Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on Wisconsin Public radio at noon the last Monday of each month. send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.

Category: History, Severe Weather, Weather Dangers

Comments Off on How important was the weather forecast on D-Day?

What is fire weather?

Posted on May 26, 2026 by WeatherGuys Editor

Wildfires need three essential elements: heat, oxygen and dry fuel. Fire weather refers to a mix of meteorological conditions that make it easy for wildfires to ignite and spread quickly. The main weather factors are low humidity, strong winds, warm temperatures and atmospheric instability.

Dixie Fire in 2021 burned 73,240 acres in Lassen Volcanic National Park. (Photo credit: Steve Ackerman)

When humidity is low, moisture is drawn out of plants, turning them into highly combustible fuel for fires. Extended dry spells or droughts remove the moisture that helps prevent ignition, making the physical environment more vulnerable. Strong winds provide fires with oxygen and carry hot embers ahead of the fire line, starting new fires. Warm temperatures heat dry vegetation, making it more likely to ignite. Unstable atmospheric conditions allow smoke plumes to rise rapidly, intensify fires, and create unpredictable shifting winds.

National Oceanic and Atmospheric Administration meteorologists use specific alerts to warn when dangerous conditions are present. A Fire Weather Watch is issued when conditions may develop in the next 12 to 72 hours, while Red Flag Warnings are issued when critical fire weather conditions are imminent or already occurring.

Thresholds for issuing Red Flag Warnings vary by region due to differences in vegetation, terrain and climate, but they generally include winds of at least 15 mph measured 20 feet above ground level, relative humidity below 25% and 10-hour fuel moisture (a measure of the water content of dead vegetation in the quarter-inch to 1-inch size range) at or below 10%. In rare cases, the Red Flag Warning may designate an area as being in a “Particularly Dangerous Situation,” such as when warning criteria are greatly exceeded.

During Red Flag Warnings, dry vegetation and strong winds make it easy for fires to spread rapidly, posing a serious threat to people and property. Under Red Flag Warnings, even a single spark can trigger a major wildfire, so it is important to avoid using open flames outdoors during these events.

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: Phenomena, Severe Weather, Weather Dangers

Comments Off on What is fire weather?