Does lightning travel from the sky downward or ground upward?

Posted on November 3, 2025 by WeatherGuys Editor

Lightning is a huge electrical discharge that results from vigorous motions that occur in thunderstorms.

The average number of cloud-to-ground lightning flashes per kilometer across the lower 48 United States. (Image credit: Ackerman and Knox, Meteorology: Understanding the Atmosphere, Fourth edition)

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.

A typical cloud-to-ground flash begins as a negative charges travel towards the ground in a sequence of spurts. This makes the ground positively charged. As the negative charge approaches the ground, there is an upward stream of positive charged. When the two streams meet an initial flash occurs and a channel forms so that electricity can flow back forth between the cloud and ground. This occurs so quickly that it looks like a single brilliant flash, but high speed photography shows several bolts.  Cloud-to-ground lightning starts from the sky and heads downward, but what we see can travel from the ground upward.

Lighting also has different appearances. Staccato lightning is a cloud-to-ground lightning strike which is a short-duration stroke that often, but not always, appears as a single very bright flash and often has considerable branching.  Forked lightning is a name, not in formal usage, for cloud-to-ground lightning that exhibits branching of its path. Ribbon lightning occurs in thunderstorms with high cross winds and multiple strokes. The wind will blow each successive stroke slightly to one side of the previous stroke, causing a ribbon appearance. Bead lightning is a type of cloud-to-ground lightning which appears to break up into a string of short, bright sections.

Heat lightning is a common name for a lightning flash that appears to produce no discernible thunder because it occurs too far away for the thunder to be heard.

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, Phenomena, Severe Weather

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Are the Arctic and Antarctic warming at the same rate?

Posted on October 27, 2025 by WeatherGuys Editor

Global warming is occurring. We know this through scientific observations and our understanding of atmospheric physics and chemistry. We understand that human activities associated with burning fossil fuels is a primary forcing function. While global warming is occurring, observations indicate that rate of warming varies by region.

Comparison of Arctic vs global surface air temperature anomalies. (Image credit: NOAA Climate, archived)

The Arctic and Antarctic are geographic opposites, and not just because they sit on opposite ends of the globe. The land-sea arrangements are different. Antarctica is a continent surrounded by water; the Arctic is basically a water body surrounded by land.

Observations show that the Arctic is warming faster than Antarctica. The Antarctica continent is very high (about 7,500 feet) and thus much colder to begin with so the snow and ice on the land are not melting as fast as the snow and sea ice in the Arctic.

The ice thickness on Antarctica averages about 1.4 miles. The brightness of this ice sheet reflects solar energy out to space, which limits regional warming in summer.  The Antarctic peninsula, which juts further north into the Southern Ocean, warmed faster than any other terrestrial environment in the Southern Hemisphere during the latter half of the 20th century.

Observations indicate that the Arctic is the fastest warming region on the globe, a phenomenon called Arctic amplification. Summer sea ice in the Arctic has been shrinking more than 10 percent per decade over the past 40 years. As sea ice melts, solar energy is not reflected to space and helps warm the region. The accelerated Arctic warming influences the weather around the Northern Hemisphere by changing the temperature contrast between middle and high latitudes. Such influences are being studied intensively at present.

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: Climate, History

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Does the federal shutdown impact weather forecasting?

Posted on October 20, 2025 by WeatherGuys Editor

Weather forecasts provided by the government will continue during the shutdown. The National Weather Service (NWS) and other National Oceanic and Atmospheric Administration (NOAA) agencies provide essential services; therefore, most NWS federal forecasters are considered emergency employees and are required to work without pay during the shutdown.

The National Digital Forecast Database (NDFD) is a suite of products generated by the National Weather Service (NWS) using data from regional NWS Weather Forecast Offices (WFOs) and the National Centers for Environmental Prediction (NCEP).

While NWS will continue their life-saving essential work, some weather and climate data feeds and public-facing services will have limited or no access to researchers and the public. According to a NWS webpage announcement, “Due to the government shutdown, all public National Weather Service activities, including tours and other outreach activities, have been canceled or postponed until further notice. We apologize for any inconvenience. The NWS will continue to provide critical forecast, watch, and warning information to protect life and property throughout the shutdown.”

The National Hurricane Center (NHC) will also continue to provide hurricane updates and forecasts. There are a few “non-essential” fields in NOAA that are not operating during the shutdown, including some active research.

Thousands of government forecasters monitor the weather 24 hours a day, 7 days a week. They issue forecasts, watches, and warnings that are critical to the protection of life and property. As essential workers they are expected to report as usual during a shutdown, working without pay for the duration of the shutdown.

As of this writing, Republicans and Democrats appear no closer to an agreement to reopen the government. Federal workers will miss their paychecks during the government shutdown. There also are contractors who perform work for the federal government and it is not certain they will be eligible for back pay after the shutdown.

Accurate weather forecasting requires equipment like weather radars, satellites, and observation stations. These will continue operating during the shutdown, although repairs and upgrades may be delayed until a federal budget is restored.

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, Weather Dangers

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How are fall colors impacted by our weather?

Posted on October 13, 2025 by WeatherGuys Editor

The most vibrant fall color will occur on years with ample moisture in the summer and dry, clear weather in the fall. Extreme heat and moderate-to-severe drought may negatively affect the fall foliage season. 

Sunlight coming through the fall leaves. (Photo credit: Steve Ackerman)

In summer and spring, leaves are green because they contain chlorophyll, which reflects green light more than other colors. Other colors are absorbed by chlorophyll for photosynthesis.

As temperatures begin to drop in late September, trees gradually slow down the production of chlorophyll. As the green chlorophyll disappears from the leaves, we begin to see yellow- and orange-colored leaves. These colors have been in the leaves all along; we can’t see them in the summer because of the chlorophyll. Orange colors come from carotene and the yellows from xanthophyll in the leaves. The bright red and purple colors come from anthocyanin pigments, which are made from leftover glucose trapped within the leaves of some trees (e.g. maples).

Brilliant fall colors with bright red and purple colors require conditions in which leaves can make a lot of anthocyanin pigments. The best weather conditions for this are an autumn with bright sunny days and cool, but not frosty, nights. A drab autumn has lots of cloudy days and warm nights. An early frost speeds up the fall of the leaves and brings a quick end to the fall color. Of course, too much strong winds or heavy precipitation would bring the leaves down.

Global warming is making the fall season much warmer on average. This warming trend allows chlorophyll production to continue and can delay the onset of fall colors.

If you are interested in taking a tour of the fall colors in Wisconsin, the Travel Wisconsin website provides a daily report of the fall colors (https://www.travelwisconsin.com/fall-color-report).

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: Climate, Seasons

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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

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