Are day and night of equal hours on the equinox?

Posted on September 25, 2023 by WeatherGuys Editor
The two solstices happen in June (20 or 21) and December (21 or 22). These are the days when the Sun’s path in the sky is the farthest north or south from the Equator. A hemisphere’s winter solstice is the shortest day of the year and its summer solstice the year’s longest. In the Northern Hemisphere the June solstice marks the start of summer: this is when the North Pole is tilted closest to the Sun, and the Sun’s rays are directly overhead at the Tropic of Cancer. The December solstice marks the start of winter: at this point the South Pole is tilted closest to the Sun, and the Sun’s rays are directly overhead at the Tropic of Capricorn. (In the Southern Hemisphere the seasons are reversed.) (Image credit: Britannica)

This year, the autumnal equinox occurred on Saturday, Sept. 23, at 1:50 a.m. Central Time. During the equinox, the sun shines directly on the equator as its position moves from one hemisphere to the other. The word “equinox” is derived from the Latin word “aequus,” which means “equal,” and “nox,” which is the Latin word for “night.” During the 24 hours of the equinox, there are about 12 hours of day and 12 hours of night.

You may hear that daylight and nighttime are of equal length on the equinox. But during the equinox at our midlatitude location, there are approximately eight more minutes of daylight for two reasons: the sun’s shape and atmospheric refraction.

Sunrise time is most frequently defined as when the leading edge of the sun first touches the eastern horizon. Sunset occurs when the sun’s trailing edge touches the western horizon. This provides an extra 2½ to 3 minutes of daylight at our latitude.

In addition, our atmosphere acts like a lens, bending the light from the sun. When the sun is near the horizon, this refraction of the sunbeams has the effect of making the sun appear about a half a degree from its true position. When the sun appears to be on the horizon, it is actually just below the horizon geometrically. Atmospheric refraction advances the time of sunrise and delays the sunset, adding approximately six minutes of daylight. So, we have more daylight than night at the equinox.

The fall equinox marks the midpoint between the summer and winter solstices, and our daylight hours will continue to decrease as we approach the shortest day of the year, with about nine hours of daylight on Dec. 21. Our temperatures will also be getting colder.

Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at 11:45 a.m. 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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Do hurricanes affect Wisconsin?

Posted on September 18, 2023 by WeatherGuys Editor
Map showing the previous tracks of tropical systems that have traveled close to Wisconsin. Blue line segments indicate where the systems where tropical depressions and grey line segments indicate where they weakened to extratropical cyclones. (Image credit: NOAA/NWS Sullivan, WI)

A tropical cyclone is a rotating low-pressure system that has organized thunderstorms but no fronts. When a tropical cyclone’s maximum sustained winds reach 74 mph, it is called a hurricane. Hurricanes have never directly impacted the Upper Midwest region of the U.S.; however, the remnants of a hurricane or tropical storm have impacted the weather in the Midwest, including Wisconsin.

If a hurricane is particularly strong at landfall, it can move far enough northward to cause a significant rain event for areas in the Midwest. For the most part, such storms originally make landfall in Texas, Louisiana or Mississippi. These storms can be tracked by satellites or surface weather observations because they maintain an identifiable circulation pattern along their entire path.

A University of Iowa study analyzed discharge measurements of the U.S. Geological Survey (USGS) stream gauge stations from 1981 to 2011. Their study analyzed the stream gauge water discharges with the passage of the storms over the Midwest and eastern states and constructed maps of the relationship between inland flooding and tropical cyclones. The analysis demonstrated that these tropical storms can impact large areas of the United States, including Illinois, Wisconsin and Michigan. The natural hazards associated with these weather events are similar to flooding, which can impact 10 to 15 states hundreds of miles from the coast.

The amount of financial damage caused by such storms in the Midwest and the eastern United States will be a subject of future study.

Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at 11:45 a.m. the last Monday of each month. Send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.

Category: History, Meteorology, Severe Weather, Tropical

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Can we measure air pollution from satellites?

Posted on September 11, 2023 by WeatherGuys Editor
The TEMPO instrument is a UV-visible spectrometer, and will be the first ever space-based instrument to monitor air pollutants hourly across the North American continent during daytime. It will collect high-resolution measurements of ozone, nitrogen dioxide and other pollutants, data which will revolutionize air quality forecasts. (Image credit: NASA and Smithsonian Astrophysical Observatory)

The National Oceanic and Atmospheric Administration (NOAA) operates weather satellites with instruments capable of locating fires and determining fire characteristics such as size and intensity. These satellites also are critical to observing and monitoring smoke from those fires.

NASA recently launched a new satellite instrument to monitor air pollution. The Tropospheric Emissions: Monitoring of Pollution, or TEMPO, was launched into geostationary orbit on April 6, 2023. TEMPO measurements join an international satellite constellation of observations that will track pollution around the globe. The instrument measures sunlight reflected off Earth’s surface and solar energy scattered by clouds and the atmosphere. Gases in the atmosphere absorb sunlight at particular wavelengths, and the measured color spectra is used to determine the concentrations of several gases in the air, including nitrogen dioxide.

TEMPO is the first space-based instrument to monitor major air pollutants hourly and at high spatial resolution of 4 square miles. Observations of nitrogen dioxide concentration at this temporal and spatial detail will enable scientists to study rush hour pollution and the movement of pollution from forest fires. It can also help measure nitrogen dioxide released when the application of fertilizer on farms is followed by rainfall.

TEMPO data will help scientists evaluate the health impacts of pollutants and aid in the creation of air pollution maps at the neighborhood scale, improving understanding of disparities in air quality within a community. NASA will share the data with other agencies that monitor and forecast air quality, such as the Environmental Protection Agency (EPA) and the National Oceanic and Atmospheric Administration (NOAA).

Scientists at the University of Wisconsin’s Space Science and Engineering Center are members of NASA’s TEMPO science team and will monitor the presence of air pollutants over North America to help improve air quality forecasts.

Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at 11:45 a.m. 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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Are daily weather forecasts affected by climate change?

Posted on September 5, 2023 by WeatherGuys Editor

This is a very interesting question whose answer helps to further elucidate the difference between climate and weather.

As it turns out, predictions of the coming weather are nearly exclusively dependent on the observed conditions of the atmosphere in the day prior to the forecast period. These conditions are known formally as initial conditions. Those initial conditions are only partially sampled by the many observational platforms (routine surface observations, upper air observations and various satellite platforms, for instance) that have been devised over the years.

These initial conditions are fed into sophisticated computer programs known as numerical weather prediction models, and the ensuing calculations give meteorologists guidance in creating their forecasts.

Even though the climate is changing in the background, accurate measurements of these initial conditions is not substantially affected by that change, and so the forecast models are unaffected. Consequently, it is not likely that climate change has any discernible effect on the quality of weather forecasts.

The NWS routinely provides 7-day weather forecasts via weather.gov

The behavior of the climate system, on the other hand, is governed by a different set of conditions, known as boundary conditions. These include fundamental measures such as the length of the day, the amount of energy coming from the sun and the chemical composition of the atmosphere. Though these conditions do change, they only do so on very long time scales — scales not important to day-to-day weather forecasts.

Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at 11:45 a.m. the last Monday of each month. Send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.

Category: Climate, Meteorology, Uncategorized

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How is a heat index determined?

Posted on August 28, 2023 by WeatherGuys Editor

The heat index (HI) indicates how hot it feels.

This map shows air temperatures modeled at 6.5 feet (2 meters) above the ground the afternoon of August 23, 2023 generated by combining satellite observations with temperatures predicted by the GEOS model. The darkest reds indicate temperatures above 104°F (40°C). Heat indices were even HOTTER. (Image credit: NASA Earth Observatory)

The HI is calculated using an equation that is a function of air temperature and the relative humidity. The HI is sometimes referred to as the “feels-like” temperature.

When our bodies get hot, we cool down by sweating. The sweating does not directly cool our bodies; it is the evaporation of the sweat that cools us down. If the air has a high humidity, then the rate of evaporation is reduced. This hampers the body’s ability to maintain a nearly constant internal body temperature.

The HI helps the National Weather Service (NWS) to determine if they should issue a weather advisory. A weather advisory is issued to bring to the public’s attention a situation that may cause some inconvenience or difficulty for travelers or people who have to be outdoors. A heat advisory is one type of weather advisory.

The NWS will issue a heat advisory when the heat index is predicted to be 100 degrees. You are then advised to limit vigorous outdoor activity and drink plenty of fluids. The NWS will initiate alert procedures when the heat index is expected to exceed 105 to 110 degrees (depending on local climate) for at least two consecutive days.

NWS Heat Index Chart

The HI is calculated assuming you are standing in the shade and there are light winds. The NWS uses the Wet Bulb Globe Temperature (WBGT) to determine the heat stress in direct sunlight, which considers temperature, humidity, sun angle, cloud cover and wind speed in its calculation.

Exposed to direct sunlight, the WBGT can be up to 15 degrees hotter than the HI. When the WBGT is high, and you have to be outside, wear a hat and lightweight, light-fitting, light-colored clothes. Drink plenty of water and take breaks in shady areas.

Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at 11:45 a.m. the last Monday of each month. Send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.

Category: Meteorology, Severe Weather, Weather Dangers

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