The Weather Guys

What is considered “heavy rain”?

Posted on September 14, 2026 by WeatherGuys Editor

The National Weather Service defines rainfall rate as the intensity of liquid precipitation falling over a specific, short duration, typically expressed in inches or millimeters of accumulation per hour.

The American Meteorological Society defines heavy rain as a rainfall rate of 0.30 inches of rain per hour. Rainfall rate measures how hard rain falls at any given moment, while accumulation measures the total depth of water accumulated.

Rain gauges catch falling precipitation and record its amount and intensity. There are three major types of rain gauges: the standard rain gauge, the tipping bucket rain gauge and the weighing rain gauge.

The standard rain gauge is a cylinder with a funnel leading into a narrow measuring tube. It collects rainwater and a person manually reads and records the water level in the tube daily.

A tipping bucket rain gauge has a funnel that channels water into one of two small, balanced side-by-side buckets. When a specific amount of rain fills a bucket, it tips over, empties and triggers an electronic pulse to record the amount and timing. This is ideal for automated weather stations and real-time rainfall tracking.

A weighing rain gauge has a collecting bucket that sits on a mechanical scale and continuously weighs the accumulated water and records the total mass over time on a chart or digital logger.

Weather radars measure rain rate by sending out radio waves and calculating how much energy bounces back from raindrops. Colors on a weather radar image show the deduced intensity of precipitation. Green indicates light rain, mist, or drizzle; yellow indicates moderate rain (0.11 to 0.3 inches per hour), orange areas indicate heavy rain, and red are downpours of more than 0.3 inches per hour. Purple or magenta colors indicate extreme rainfall, severe storms or large hail.

Steve Ackerman and Jonathan Martin, professors in the UWMadison 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

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Two Questions about Clouds

Posted on September 8, 2026 by WeatherGuys Editor

One of our readers awoke recently to some beautiful clouds in the summer sky and two excellent questions popped into her mind. First, what holds the clouds up in the air. And then what makes some clouds appear to be fluffy on top but flat on the bottom?

The fluffy appearance of the tops of some clouds are evidence of convection — that is, the air parcels within the cloud are buoyant and literally bubble to the top.
(Photo credit: Alex Buzurnyuk)

Clouds are composed of tiny liquid water droplets, whose diameters are about the width of a human hair, and tiny shards of ice in a variety of shapes, or habits. Whether a cloud is mostly liquid water droplets or ice particles depends, as you might guess, on the ambient temperature of the air in the cloud.

Tiny cloud liquid water droplets can remain in the liquid state to temperatures as low as about 14 degrees Fahrenheit, and when they do they are known as supercooled liquid water droplets. These droplets feel the downward directed force of gravity just like a baseball or a watermelon would. Because the droplets are so small, and therefore have small masses, the gravitational force can easily be balanced by an upward directed friction force resulting from the interaction of the droplets with the air molecules around them and so the droplets remain suspended.

This is what holds clouds up in the air.

When these droplets grow, by a variety of interesting processes, they gain water mass, and eventually the gravitational force overwhelms the friction force and the now larger droplets fall to the surface.

The fluffy appearance of the tops of some clouds is evidence of convection — that is, the air parcels within the cloud are buoyant and literally bubble to the top. As the air rises, it cools by expansion as it encounters environments with lower and lower pressure the higher it goes. This cooling increases the relative humidity of the air, and once the humidity gets to 100%, condensation of the invisible water vapor begins to produce the cloud liquid water droplets.

The bottom of clouds often appears flat because the first level at which rising air parcels begin to condense is usually rather uniform over a given region. This level is known as the lifted condensation level — that is, the level at which lifted air parcels first begin to experience condensation.

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, Phenomena

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Do all hurricanes have an eye?

Posted on August 31, 2026 by WeatherGuys Editor

Seen from space, a hurricane appears as a vast spiral of clouds stretching across hundreds of miles. Near the center of strong hurricanes is the eye, a roughly circular region of comparatively calm weather, light winds, and low pressure.

*Hurricane Lala’s eyewall brushing the Big Island of Hawai’i at 5pm on Sat., Aug 15, 2026. (Image credit: MauiNow.com Facebook post; link to CIMSS Satellite Blog for video)*

The eye is one of the storm’s most recognizable features, but it is not the most dangerous part. That distinction belongs to the eyewall, the ring of towering thunderstorms that surrounds the eye and contains the hurricane’s strongest winds and most intense weather.

The lowest barometric pressure in a hurricane is typically found inside the eye, where pressure can be much lower than in the surrounding storm. Just beyond the calm center, however, conditions change sharply. Across the eyewall, pressure rises quickly and winds accelerate, creating the steep pressure gradient that helps produce a hurricane’s powerful circulation.

The contrast between the eye and eyewall comes from the way air moves through the storm. Warm, moist air spirals inward near the surface and rises rapidly in the eyewall, fueling deep thunderstorms. Inside the eye, by contrast, air sinks gradually from above. As this sinking air descends, it warms and becomes more stable, which helps suppress cloud formation. That is why the eye of a strong hurricane can be relatively warm, calm and nearly cloud-free, aside from occasional low clouds or thin cirrus overhead.

A hurricane does not necessarily have an eye throughout its entire life cycle. Eyes usually form only after a tropical cyclone becomes well organized and sufficiently strong. Even then, the eye can change size, become less distinct, or disappear for a time, especially during eyewall replacement cycles, when a new eyewall forms around the old one. These changes are important because they can signal shifts in the storm’s structure and intensity.

Category: Phenomena, Severe Weather, Tropical

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Is it dangerous to fly through a thunderstorm?

Posted on August 24, 2026 by WeatherGuys Editor

Thunderstorms pose several hazards, whether you are on the ground or in the air, namely lightning, hail, turbulence, icing, downbursts and even tornadoes. Because of these potential hazards, planes don’t fly through thunderstorms. Instead, they fly around them.

Probability Severe storm metrics for a thunderstorm over Atlanta’s Hartsfield-Jackson International Airport (ATL)airport on 6 March 2026. ( Image credit: CIMSS Satellite Blog)

Pilots have onboard weather radar and air traffic control support to steer around thunderstorms rather than fly straight through them. In addition to pilots being trained to avoid thunderstorms, modern engineering has led to commercial aircraft designs that can withstand many of these threats.

Lightning for instance, isn’t much of a danger. On average, lightning strikes a plane once or twice a year without incident. Turbulence is a familiar part of flying for pilots and passengers. Turbulence is uncomfortable but not structurally dangerous to the aircraft. Extreme turbulence can cause injuries to passengers, particularly those not buckled in. In addition to being in the vicinity of a thunderstorm, turbulence can be caused by other atmospheric phenomena.

Icing occurs when super cooled water (liquid water that is already below the freezing point temperature), encounters an object that is below freezing. Ice that accumulates on the wings makes it more difficult to maneuver the plane.

Updrafts and downdrafts are vertical air movements that drive a thunderstorm’s circulation. Updrafts in a thunderstorm are rising currents of warm air that can reach several miles high. Downdrafts are sinking currents of cooler air that spread outward as they hit the ground. For aircraft, strong downdrafts are especially hazardous because they can reduce lift and push the plane downward, making it harder to maintain altitude. In severe cases, a downdraft can contribute to a stall if the aircraft cannot generate enough lift to remain safely airborne.

For passengers, the bigger issue associated with thunderstorm weather is usually experienced on the ground — the disruption caused by air traffic delays that are directly related to the weather.

Category: Severe Weather, Weather Dangers

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Why is wildfire smoke unhealthy?

Posted on August 17, 2026 by WeatherGuys Editor

The smoke from wildfires can linger in the air and often travels long distances carrying unhealthy matter.

*True color image from the GOES West satellite on Aug 1, 2026, showing wildfire smoke over eastern Washington, Idaho and Montana. (Photo credit: CSPP GeoSphere)*

Depending on how hot the fire is and what’s burning — trees, grasslands, or buildings — wildfire smoke can contain toxic chemical substances, including carbon monoxide, carbon dioxide, sulfur dioxide, nitrogen oxides, benzene and metals. The smoke can also change over time and become more toxic.

In addition to noxious gases, the wildfire smoke consists of tiny particles that can be inhaled deep into our lungs. The smoke is lofted upward into the general westerly wind currents, which can spread the smoke hundreds or even thousands of miles from the fire, impacting air quality in areas far removed from the blaze.

Regions downwind of wildfires often experience significant declines in air quality, leading to health alerts and increased risks for vulnerable populations. Wildfire smoke can make anyone sick, but some people are at higher risk. Smoke can be of particular concern to people with asthma and heart disease but can also cause respiratory irritation and shortness of breath to all who breath them.

The Environmental Protection Agency regulates inhalable particles, which are particles smaller than 10 micrometers. EPA monitors particulate matter, or PM, that is smaller than 2.5 microns, or PM2.5, and those smaller than 10 microns, or PM10. The U.S. Air Quality Index is EPA’s tool for communicating about outdoor air quality and health. The AQI includes concentrations of PM2.5, particles so small that they can travel deep into the lungs, leading to serious health issues.

Wildfire smoke is different from that of your campfire. Campfire smoke is mostly from a small fire pit and involves controlled burning of clean wood. Exposure lasts a few hours and quickly dissipates in open air.