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

Comments Off on What is vapor pressure?

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

Comments Off on What are radiosondes?

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

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

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

Comments Off on What can I do with home precipitation observations?

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

Posted on March 10, 2025 by WeatherGuys Editor

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

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