Why does the severe weather threat increase as spring and summer approach?

As the threat of winter snows recedes across the country, it is replaced by the threat of severe weather (i.e. thunderstorms with hail, damaging winds and tornadoes).

A visualization of the Northern Hemisphere’s polar jet stream swirling weather patterns from west to east across North America. Visualization made with data from NASA’s MERRA dataset. (Image credit: NASA’s Goddard Space Flight Center)

The severe weather season, though broadly spanning March through August across the United States, is actually quite regional. It begins in March in the southern states, moves to the southern Plains during April and May, and then further north toward the Great Lakes states during the summer.

One of the basic underlying reasons for this northward migration of the severe weather threat during the spring and summer is the fact that the jet stream follows a similar seasonal cycle.

The jet stream is a ribbon of high wind speeds located near the top of the troposphere, about 6 miles above the surface of the Earth. The jet stream position is anchored to the southern edge of the dome of cold air that is centered on the North Pole. During the depths of winter, that cold dome expands considerably, extending nearly to the Gulf of Mexico. As the winter ends and spring approaches, the hemisphere begins to warm up and the cold dome shrinks dramatically. Its southern edge moves to central Canada by early summer.

The jet stream is associated with vigorous vertical circulations — upward and downward motions. The upward vertical motions are instrumental in producing thunderstorms. Thus, when the jet stream migrates northward as the weather warms in spring and summer, so does the greatest concentration of severe weather outbreaks.

This very sort of situation characterized the severe outbreak last weekend in several southern states.

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, Severe Weather

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What can I do with home precipitation observations?

Rain gauge used for CoCoRHaS. (Photo credit: Steve Ackerman, Weather Guy)

Precipitation can widely vary over a region; consequently, local observations of precipitation are valuable weather observations. Contributing your personal observations via a community-based network of volunteers can help with weather and river forecasts.

One well-known observation network is the Community Collaborative Rain, Hail, and Snow Network, or CoCoRaHS (https://www.cocorahs.org). This is a group of volunteers working together to measure precipitation across the U.S., Canada, Puerto Rico, U.S. Virgin Islands, Guam, and the Bahamas. CoCoRaHS began on June 17, 1998 and now has over 26,000 observers. The variability of precipitation is such that observers are always needed. Joining is easy at https://www.cocorahs.org/application.aspx. There are many observers in Wisconsin and the surrounding region. Once you begin to participate, it is interesting to see the measurements of other observers in your area.

Participation measurement requires only a rain gauge and the CoCoRaHS web site has suggestions on ones to purchase as well as guidelines on where to set up your gauge.  Observers report the gauge’s measured daily 24-hour precipitation totals at 7 am. These are manual rain gauges, as CoCoRaHS does not use automated rain gauges due to the variation in the quality and reporting of the automated weather stations. These manual precipitation gauges are accepted by the National Weather Service (NWS) as comparable to their official instruments. 

The advantage of many automated rain gauges is that they can record the time and relative intensity of precipitation. Some CoCoRaHS observers record this in their notes but the official data are the 24-hour manual rain gauge reports.

Join CoCoRaHS and contribute observations that will be of interest to the community, hydrologists, NWS, the Wisconsin State Climatology Office, UW-Extension, and media outlets such as The Midwest Farm 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: Meteorology

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What are the impacts of the recent cuts to NOAA and NWS?

The National Oceanic and Atmospheric Administration (NOAA), which includes the National Weather Service (NWS), is a vital partner in the nation’s weather enterprise. They provide essential products and services that benefit the U.S. economy and the health and safety of us all. Citizens and businesses rely on accurate and timely forecasts.  Due to the inexplicable decisions of the current administration, many NOAA and NWS employees have been terminated without cause.

These recent terminations will have far-reaching consequences for public safety and the nation’s economic well-being. NOAA professionals on ‘probationary status’ are not just new employees; some have recently been awarded with career advancements resulting from their outstanding work that helps safeguard our communities by helping to keep all of us informed, prepared, and safe.

The weather enterprise of our nation includes a critical private-public partnership. The outcome of this partnership includes timely weather forecasts and information that protect people, improve livelihoods, save money, and add tremendous economic value across our nation. This successful partnership includes opportunities and responsibilities of the federal government, private business, and universities. NOAA provides crucial foundational support to this partnership, including collecting scientific observations, maintaining computer facilities, improving weather models, and providing public weather forecasts and warnings: products and services that are freely accessible to all. Universities train the future workforce while conducting research that advances our understanding of weather and climate processes. Private businesses use this foundation of data, science, and services to create value-added products for their clients.

This public-private partnership has been cultivated over many years and through persistent efforts of organizations such as the American Meteorological Society. The recent injudicious firings of NOAA and NWS employees will dismantle a critical public service and pull apart a crucially needed and successful public-private partnership.

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, Uncategorized

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Interview on WRN

“The Weather Guys” were recently interviewed by Bob Hague of the Wisconsin Radio Network. Check out the full interview on the WRN website:

https://www.wrn.com/uw-madisons-weather-guys-concerned-about-layoffs-at-noaa-and-nws

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: Uncategorized

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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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