How do we measure global cloud cover?

Because clouds are involved in the global water cycle as well as our planet’s energy gains and losses, they constitute an important component of our atmosphere, weather, and climate.

Determining global cloud cover requires observations that cover the entire planet. Observations from weather satellites provide a modern estimate of the global cloud cover. These satellites are operated by several countries and their data is shared through international agreements. Discrimination between clear and cloudy regions is a crucial first step in most applications of satellite data, such as estimating ocean temperature or assessing the health of land vegetation.

Annual average total cloud amount over the Earth (period 1991 to 1995) derived from the ISCCP data sets (Image credit: International Satellite Cloud Climatology Project, NASA)

Clouds are generally brighter and colder than the underlying surfaces. During daylight most clouds are detected via reflection of visible and near infrared energy coming from our sun.  At visible wavelengths, snow surfaces can be as bright as clouds, making cloud detection a bit more challenging. But at other wavelengths, ones our eyes cannot detect, clouds and snow appear very different.  During day, clouds are often colder than the underlying service and so infrared observations help in cloud detection. At night, contrasts in temperatures between cloud and the surface are sufficient for the detection of most middle and high-altitude clouds. However, there can be challenges with low altitude clouds, where the ground can become colder or at the same temperature as the clouds at night.

Global cloud cover is approximately 65% with the average cloud cover over ocean exceeding the average over land. Cloud cover over land tends to be greater in daytime than at night, while the ocean shows little day–night difference.

Modern satellite observations can also be used to assess additional cloud properties, such as altitude and whether the clouds are composed of ice crystals or liquid water droplets.

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, Meteorology

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What does the National Weather Service do?

The National Weather Service (NWS) is a subdivision of the National Oceanic and Atmospheric Administration (NOAA), which is an agency within the United States Department of Commerce.  It is the nation’s official source of weather information. The NWS’s primary responsibility is providing weather forecasts, warnings, and other weather-related services to the public. The NWS plays a crucial role in safeguarding lives and property as the office collaborates with emergency management agencies, the media, and private industries to ensure that the public receives good weather information in a timely manner. The NWS disseminates weather warnings and advisories through a wide variety of communication channels, such as radio, television, websites, and mobile apps.

Kelli Barendse, meteorologist at the National Weather Service station in La Crosse, monitors an incoming storm on Tuesday, Feb 11. Station employees interpret data and trends to alert the public and local officials of weather conditions. (Photo credit: Saskia Hatvany, River Valley Media Group)

The NWS gathers and analyzes weather data from various sources, including a vast network of satellites, airplanes, radar, surface weather observations, and ocean buoys. These data, along with computer models, enable meteorologists at NWS to generate accurate and up-to-date weather forecasts, severe weather advisories, and warnings across the United States. The NWS includes a network of local weather forecast offices throughout the country. These offices have trained meteorologists who monitor regional weather conditions and generate accurate timely weather forecasts and alerts as needed.

These NWS forecasts and weather alerts are accurate, widely available and free to the public. They impact everyday life. These forecasts also support industries that rely on weather forecasts. Private industries, including TV forecasters, AccuWeather, and the Weather Channel, all use NOAA data and NWS forecasts. NOAA and NWS provide weather and climate data and forecast models that are a national treasure. The weather and climate models produced continuously by the NWS are accurate and robust. It is impossible and inefficient for private industry to provide comprehensive weather data in a reliable way that is also freely accessible to everyone.

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: History, Meteorology

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Why does the United States have a National Weather Service?

While successfully prosecuting the Civil War against the Confederacy, Gen. Ulysses S. Grant* had learned that weather information – even if NOT in the form of a forecast – was extremely valuable for operations.

Portrait of Increase Lapham. Wisconsin’s first published scientist, Dr. Lapham was instrumental in helping to establish the National Weather Service. (Photo credit: Wisconsin Historical Society archives)

Coincidentally, in the years following the war, Dr. Increase Lapham, a Milwaukee scientist, lobbied Milwaukee’s congressman, Gen. Halbert Paine, to push for establishment of a storm warning service for the Great Lakes. 

On February 2, 1870, Halbert introduced a Joint Congressional Resolution requiring the Secretary of War “to provide for taking meteorological observations at the military stations in the interior of the continent, and at other points in the States and Territories…and for giving notice on the northern lakes and on the seacoast, by magnetic telegraph and marine signals, of the approach and force of storms”.

On February 9, 1870 (155 years ago last Sunday) a sympathetic President Ulysses S. Grant* signed the resolution into law and what is now known as the National Weather Service was born. 

Thus, the service began its life within the U.S. Army Signal Service’s Division of Telegrams and Reports for the Benefit of Commerce.  Observations officially began on November 1, 1870.  Exactly a week later, on November 8, Dr. Lapham issued the Service’s first storm warning on the approach of a storm over Lake Michigan. 

On October 1, 1890, at the request of President Benjamin Harrison, Congress passed a law transferring the meteorological responsibilities of the Signal Service to the newly-created U. S. Weather Bureau which was housed in the Department of Agriculture.  The Weather Bureau became the National Weather Service in 1970 with the creation of the National Oceanic and Atmospheric Administration (NOAA). 

The NWS provides value, in terms of forecasts and warnings, that by reasonable estimates account for savings of well over $10B each year to commerce across our vast country.  It is important to be reminded of its exceptional value to us all given current developments in our national politics.

*Interesting footnote: An online version of General Grants memoirs is available at https://www.gutenberg.org/files/4367/4367-h/4367-h.htm 

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: History, Meteorology

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What is this upside-down icicle?

Photograph of an ice spike taken the morning of February 4, 2025, on frozen Lake Kegonsa (Photo credit: Daniel Dettmers)

The accompanying photo was taken by Daniel Dettmers in the morning of February 4 on frozen Lake Kegonsa. The high on the previous day was 37°F. This caused puddles of water to sit on the ice of Madison’s regional lakes during the day. Tuesday morning’s low temperature was below 20°F with calm winds.  These are just the right conditions to form what are called ‘ice spikes,’ as shown in the photograph.

When water freezes, it expands and becomes less dense. Ice floats on water. But if the lake ice is thick, when puddles form on a warm day, they sit on the ice surface.  With the cold nighttime temperatures, the surface of the puddle freezes, trapping liquid water below. As the puddle freezes, it can leave a small hole in the surface of the ice.   

The puddle water is sitting on ice, and with the cold nighttime conditions, the water below the surface freezes from the outside inward. The expansion of the water as it freezes, slowly pushes some of the puddle water up through the hole. This water freezes around the edge of the hole forming a hollow tube. As the puddle continues to freeze, the hollow spike grows in length as water is pushed up into it. Eventually, the whole thing freezes and a solid spike of ice is left. The energy required to push the puddle water up into the spike comes from the expansion of the water while it is freezing.

While light winds can help in the formation of spikes, ice spikes don’t form under windy conditions.

Other names for ice spikes include “ice candles” and “ice towers.” They have been observed in bird baths and outdoor pet water bowls as well.

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, Uncategorized

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Is it getting windier in Wisconsin?

Wind speed and direction are variables that change over space and time, and conditions can change considerably from month to month, as well as from year to year. Scientifically assessing any long-term changes in weather elements requires a long-term data set of accurate measurements.  Temperature data goes back hundreds of years, and even thousands of years, using ancillary data such as from tree rings.

Annual mean wind speed for Madison, WI , showing the changes from 1948 to 2024 (Image credit: Ed Hopkins, Wisconsin State Climatology Office)

Wind is a more difficult parameter to study and analyze. The observations of wind speed and direction need to be made at the same height above the surface. The type of surface also impacts the measurement, as the wind sensor should not be too close to trees or buildings.  Finally, large annual fluctuations make long-term trends difficult to detect.

Some research has indicated that between 1978 to 2010, the average global wind speeds decreased by approximately 2.3 percent per decade. A 2019 study found that after 2010, global average wind speeds had increased from 7 miles per hour to 7.4 miles per hour.

To address this question with respect to our state, we turned to the Wisconsin State Climate Office and send thanks to assistant state climatologist Dr. Ed Hopkins for his assistance. His careful analysis used wind observations from 1948 to 2023/24 for Madison, Milwaukee, La Crosse, Eau Claire, and Green Bay. The trend over the entire period showed a decreasing average wind speed at all the stations.  The observations at La Crosse also had a decreasing trend but also had more variation over the period.  A large portion of these decreasing trends in wind speed is exhibited in the period prior to the installation of Automated Surface Observing Systems (ASOS), which occurred in the mid-1990s. After that installation, the trend in decreasing wind was much smaller or showed nearly no trend in those weather stations.

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: History, Meteorology

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