The Weather Guys

Why is fog more common in the early morning?

Posted on September 30, 2025 by WeatherGuys Editor

Fog can occur any time of day throughout the year. In southern Wisconsin, fog is most frequent in the morning, between about 6 and 8 a.m.

*Fog at the mouth of the Yahara as it enters Lake Mendota. (Photo credit: Steve Ackerman)*

Fog formation is the result of two primary processes. First, the air is cooled to the dew point, and the relative humidity reaches 100%. When the air temperature is the same as the dew point temperature, condensation of the atmospheric water vapor occurs forming fog droplets. Another method of fog formation requires water to evaporate from the surface into the air, increasing the relative humidity until condensation occurs.

During this time of year, as the nights are getting longer, we can have clear skies and calm winds at night. These conditions help the ground, and the air near the ground, to rapidly cool. As the temperature of the air drops, the relative humidity increases as the dew point approaches the temperature, leading to fog formation. Light winds can help with fog formation as they gently mix moist air near the ground upward. Strong winds mix the air near the ground with the drier, warmer air above, keeping the air near the surface from saturating.

If you take a long, hot shower, you may “fog up” the bathroom. Some of the warm water droplets from the shower evaporate into the cooler bathroom air, moistening it to saturation and forming a fog. Evaporation fogs also form over lakes when colder air moves over warmer water. Evaporation of the warm water into the air causes the relative humidity to increase and a fog may form.

Fogs often dissipate with the morning daylight. This is sometimes referred to as the fog ‘burning off’, but that analogy is not correct. When the sun rises, the air and ground warm up, which increases the air’s capacity to hold water vapor. Since fog is a cloud of liquid water droplets the warming encourages the fog droplets to evaporate.

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

Comments Off

When will we stop seeing 80F this year?

Posted on September 22, 2025 by WeatherGuys Editor

At the time of this writing, Madison is in its 6^th straight day of temperatures at or above 80F and it is likely we will have one more to reach seven in a row.

Daily temperatures for Madison, Wisconsin, from 1869 to the present day, using combined data from the stations at the Madison city weather bureau office (January 1869–December 1946) and Dane County Regional Airport (January 1947–present). This chart shows the observed high and low temperatures compared with the normal highs and lows. The bumpy lines are the observed temperatures; the smooth lines are the normal temperatures. (Image credit: Wisconsin State Climatology Office)

As recently as 2022, we have had 13 of the first 21 days of September reach 80F or higher. In fact, it was just last year that we set the all-time record for longest consecutive streak – initiated within September – of 80F or warmer days at 12 straight, running from September 10 – 21! This also happens to be the longest consecutive streak of such days in September ever in Madison (1978 and 1981 both had 11 straight 80F September days). It may have occurred to you that this mid-September heat is a bit unusual – though it is far from unprecedented.

Interestingly, the earliest day on which Madison has ever recorded its last 80F day of the year was September 2, 1977 (and 2020). The all-time latest 80F day in Madison’s history was, again, last year on October 29.

The average last such day (since 1939) is September 29. Within the 85 seasons (not including this one) since 1939 there have been 61 times when the last 80F day occurred after September 21 (the average date of the autumnal equinox). Thus, summer-like warmth is a common feature of early fall in southern Wisconsin.

So, we hope you have enjoyed the brilliant sunshine, light winds and dry conditions that have set in over us these last few days – we all know it won’t last – though, as the foregoing attests, there may still be time for another visit from summer before it is officially a thing of the past.

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: History, Seasons

Comments Off

What does the CDC have to do with the National Weather Service?

Posted on September 15, 2025 by WeatherGuys Editor

As the flu season ramps up and brings along with it an increased threat of COVID-19 infections, it has been distressing for many to witness the assault on the leadership of the Centers for Disease Control and Prevention, known as the CDC.

Under the leadership of Health and Human Services Secretary, Robert F. Kennedy Jr., an infamous vaccine skeptic, the leadership of the organization has been decimated and confidence in its ability to meet the challenges we regularly face from infectious disease has dramatically decreased. These troubling developments come just weeks after the Department of Energy released a report on the impact of greenhouse gases on the climate of the United States written by five well-known skeptics of the broad scientific consensus on this urgent issue.

At every turn the current administration appears determined to undermine expertise, rather than employ it, too often substituting hard won understanding of how the natural world works with the untested and sometimes flatly erroneous assertions of unqualified, overconfident activists who demonstrate little respect for the extraordinary effort and patience required to develop scientific understanding.

Imagine if our weather forecasts, the importance and ubiquity of which are manifest in some of the most popular apps on our smartphones, were left to be made by crazy uncles or chemtrail conspiracy theorists. Would you have any confidence that such forecasts would be accurate? Wouldn’t that matter on snow days, thunderstorm days, heavy rain and wind days? Wouldn’t it matter to farmers and builders (among others) who often need time-specific forecasts to plan their many tasks?

There is no better organization to turn to for such vital information than our exceptional National Weather Service (NWS) precisely because it is staffed by experienced, scientifically trained meteorologists. The NWS is a clear example of the benefit accrued to societies wise enough to trust in, and invest in, expertise.

Category: Meteorology, Weather Dangers

Comments Off

What is a haboob?

Posted on September 8, 2025 by WeatherGuys Editor

A haboob is dust storm that can be several miles long, several thousand feet high, and can travel hundreds of miles. Haboobs are caused when an intense column of sinking air in a thunderstorm hits the ground and lofts soil particles into the air. These downdrafts, referred to as a microburst, can hit the ground at 50-80 mph and then spread in all directions. The resulting winds stir up dust and dirt from large arid areas which then get blown along in front of the approaching thunderstorm. The lofted particles can reach heights of 5000 ft and can extend up to 100 miles wide.

The name “haboob” is derived from the Arabian word ‘hab’, meaning ‘to blow,’ and was originally the name for a dust storm or sandstorm in the northern part of Sudan.

These storms can strike suddenly, making driving conditions hazardous. The dust quickly reduces visibility and has caused massive pileups. The strong winds can stir up infectious spores and make them easier to inhale, increasing the risk of respiratory infection.

Phoenix, Arizona residents have experienced these storms and were recently engulfed by one on August 25, 2025. Haboobs are associated with convective downdrafts within a storm but also by the condition of the surface – how much vegetation is on the surface and is the surface wet or dry. Climate change suggests the weather factors to cause these storms may become more common. Localized storms in the summer have become more intense since the 1970s while it is raining less often; droughts last longer and some dry areas are expanding. While the odds may become more favorable weatherwise, there is no strong statistical signal that points to a clear trend in the frequency of haboobs in the U.S. Southwest.

Category: Meteorology, Phenomena, Severe Weather

Comments Off

What is the longest lightning bolt?

Posted on September 2, 2025 by WeatherGuys Editor

Lightning is a huge electrical discharge, or spark, that results from vigorous motions in thunderstorms.

The GOES-R GLM is the first instrument of its kind in geostationary orbit and provides detailed satellite data that not only depicts current weather conditions, but also helps predict hazardous conditions in the future.
Developed as part of ProbSevere, a statistical model that predicts the probability that a storm will produce severe weather in the near-term, scientist John Cintineo trained a sophisticated machine-learning algorithm to recognize complex patterns in GOES-R Advanced Baseline Imager (ABI) imagery that often precede lightning activity detected by GLM. *(Image credit: CIMSS)*

Storms are composed of ice crystals and liquid water droplets. Winds inside the storm cause particles to rub against one another, causing electrons to be stripped off, making the particles either negatively or positively charged. The charges get grouped in the cloud, often negatively charged near the bottom of the cloud and positively charged up high. This is an electric field, and because air is a good insulator, the electric fields become incredibly strong. Eventually a lightning bolt happens, and the flow of electrons neutralizes the electric field.

Most lightning flashes reach less than 10 miles. On occasion a lightning bolt can be 60 miles or more in length, and that is referred to as a “megaflash.” They originate in massive convective storm complexes known as mesoscale convective systems. Fewer than 1% of thunderstorms generate megaflash lightning.

The World Meteorological Organization, or WMO, investigates and certifies meteorological records. The record flash occurred on Oct. 22, 2017, was 515 miles in length, running from eastern Texas through Oklahoma, Arkansas and Kansas to near Kansas City, Missouri, in seven seconds. The research team used data from the Geostationary Lightning Mapper sensor aboard a NOAA geostationary satellite to support its measurement.

Lightning can travel from cloud to cloud, within the same cloud, or between the cloud and ground. In-cloud lightning discharges are more common than cloud-to-ground discharges and are not as hazardous.

Cloud-to-ground is the best-known type of lightning, and it poses the greatest risk. On average there are 21 lightning deaths a year in the U.S., down from 55 deaths in 2001 before the start of a national lightning safety campaign: “When lightning roars, go indoors.”