How do satellites help forecast the weather?

Posted on April 14, 2025 by WeatherGuys Editor

Satellite data help forecast the weather in two ways: expert forecasters interpret the satellite images and numerical weather-prediction models assimilate the data they collect. Image analysis plays an important role in short-term forecasts, those that predict the weather 1 to 3 hours into the future, while numerical weather predictions are more useful in 12-hour to 3-day forecasts.

A 16-panel view of the continental US from the GOES-18 satellite. Each panel shows a different band on the ABI instrument. (Image credit: CIMSS)

While weather forecasters routinely analyze current satellite observations, most data never reach forecasters’ eyes. Most satellite observations are assimilated into numerical weather-prediction models. Today’s weather forecast models rely on satellite data to make accurate weather predictions. These satellite observations include the vertical distribution of temperature and humidity, cloud distributions, land and sea surface temperatures, location of volcanic ash, fires, and wind speeds and directions.

Professor Verne Suomi (1916-1995) is internationally regarded as the Father of Satellite Meteorology. In the early 1960’s he advocated for the benefits that would be gained by observing a single weather phenomenon from space at frequent time intervals. Suomi and Robert Parent, a professor in electrical engineering, started the Space Science and Engineering Center (SSEC), where they invented the Spin Scan Radiometer. Suomi and Parent saw their spin-scan camera launched in 1966 on NASA’s new geostationary Advanced Technology Satellite (ATS-1). These concepts enabled the tracking of movement and development of weather systems, and thus revolutionized satellite meteorology. For many years the spin-scan radiometer was the instrument on the NOAA’s geostationary weather satellites that generated the time sequences of cloud images seen on television weather shows and now the internet. The Space Science and Engineering Center at the University of Wisconsin-Madison, is regarded as the Birthplace of weather satellites.

We have much to learn about the vagaries of weather. One of the most important means of doing so involves the fleet of spacecraft dedicated to the task of watching our atmosphere. Future satellite systems will continue to improve our ability to monitor and forecast storms, and Wisconsin will continue to play a key role in the development of weather satellites and their use in weather forecasting.

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 vapor pressure?

Posted on April 7, 2025 by WeatherGuys Editor

Weather reports often include the dew point temperature and the relative humidity. These are just two of several ways to express the amount of water vapor in the atmosphere. Vapor pressure is another way. Each method has advantages and disadvantages.

Gas molecules exert a pressure when they collide with objects. The atmosphere is a mixture of gas molecules and each type of gas makes up a part of the total atmospheric pressure. The pressure the water molecules exert is another useful method of representing the amount of water vapor in the atmosphere. The pressure caused by these water vapor molecules is called the vapor pressure. Atmospheric vapor pressure is expressed in millibars (mb).

Observations of vapor pressure as a function of temperature on a ridge top at Black Rock Forest along the lower Hudson River in New York. The observations were made hourly from December 1994 through mid April 2001. Relative humidity is indicated by the color coding. Notice how the highest observed values of vapor pressure at each temperature form an arc that curves upward from left to right. This upper limit on vapor pressure at each temperature is the saturation vapor pressure. (Image credit: “Meteorology: Understanding the Atmosphere” by Steve Ackerman and John Knox)

Water vapor is at most only 4% of the total atmosphere. The average surface pressure as a result of all atmospheric gases is approximately 1,000 mb. Therefore, the vapor pressure attributable to water vapor alone is never more than about 4% of 1,000 mb, or 40 mb.

A variety of factors can change the vapor pressure. Increasing the air temperature will increase the vapor pressure. Changing the air temperature changes the average kinetic energy of the molecules and, therefore, the pressure exerted by the molecules. Increasing the number of water vapor molecules in a specific volume of air will also raise the vapor pressure. Therefore, when water evaporates into a volume of air, the vapor pressure increases.

The ratio of the actual vapor pressure exerted by molecules of water vapor versus the saturation vapor pressure at the same temperature indicates just how close the air is to saturation. This ratio is called the saturation ratio. Multiplying the saturation ratio by 100% yields the relative humidity.

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

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What are radiosondes?

Posted on March 31, 2025 by WeatherGuys Editor
A balloon launch from the Atmospheric, Oceanic, and Space Sciences rooftop during Grandparents University. (Photo credit: SSEC)

Radiosondes are instrument packages that measure the vertical profiles of air temperature, relative humidity, and pressure from the ground all the way up to about 19 miles. These radio-equipped meteorological instrument packages are carried aloft by a helium-filled “weather balloon.” Temperature and relative humidity are measured electronically and a small aneroid barometer measures pressure. The tracked position of a radiosonde (technically called a rawinsonde observation) is used to obtain wind speed and direction. At low air pressures in the stratosphere, the balloon expands so much that it explodes and the radiosonde drifts back to the ground underneath a small parachute.

Weather forecasting requires making observations and predicting changes by solving a complex set of equations that describe the physics and dynamics of our atmosphere. Accurate observations at a specific time over a large geographic region are critical to making accurate weather predictions.  Radiosondes provide needed information that forecasters and computer models use to determine current weather and make accurate weather forecasts. The National Weather Service normally, and historically, launches radiosondes twice daily (00 and 12 UTC, or 5 a.m. and 5 p.m. CST) in about 100 locations across the US territory. These measurements are combined with observations from across the globe made at the same time. These upper air measurements, or soundings, provide critical weather observations that cannot be gotten any other way. These soundings are particularly valuable as severe weather bears down on a location.

The large budget cuts recently imposed by the current administration have resulted in massive job cuts, as well as a reduction in weather balloon launches at several sites across the US. Knowing less about the current state of the atmosphere will impact forecast accuracy. Also, the suspended weather balloon launches impact the weather forecast downstream from where it is launched. The missing observations will impact the quality of the forecasts of major and impactful weather events.

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, Severe Weather

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

Posted on March 25, 2025 by WeatherGuys Editor

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?

Posted on March 17, 2025 by WeatherGuys Editor
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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