What is the heat index?

NOAA’s Heat Index

The heat index indicates how hot it feels.

It is expressed as a function of air temperature and the relative humidity. The heat index temperature is for standing in the shade and light winds; when exposed to direct sunlight, the heat index value can be increased by up to 15 degrees.

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. This is why we are uncomfortable on hot, muggy days.

When the temperature is high but the relative humidity is low, the heat index can be less than the actual temperature. This is because cooling by evaporation of sweat is very efficient in these situations. However, high relative humidities prevent evaporation and make it seem hotter than it really is because our bodies cannot cool down. In these cases, the heat index is greater than the actual temperature.

The National Weather Service (NWS) initiates alert procedures when the heat index is expected to exceed 105 to 110 degrees, depending on local climate, for at least two consecutive days. 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. Never leave children or pets in a locked car.

A heat wave is a period of abnormally and uncomfortably hot and usually humid weather. The World Meteorological Organization defines a heat wave as when the daily maximum temperature for more than five consecutive days exceeds the average maximum temperature by 9 degrees.

Category: Meteorology, Seasons, Severe Weather

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What is mud rain?

Mud rain is rain that contains a noticeable concentration of particles of sand or dust.

Saharan dust collected in a rain gauge in the Caribbean.

The soil can be of local origin or it can originate from very distant regions. As rain falls through the dust layer, the raindrops collect the soil particles. When the rain droplets hit objects on the ground, the water evaporates and leaves behind dry mud spots.

Another consequence can be health hazards to those with respiratory issues because, as the fine particles of dust settle out, they can reduce the quality of the air we breathe.

At the end of June, a large dust cloud arrived in North America from the Sahara Desert. Micrometer-sized soil particles from the desert were lifted by the wind to high altitudes and transported over thousands of miles. The dust was carried by atmospheric circulation patterns across the Atlantic Ocean. Satellite observations tracked the movement of this dust, or aerosol, from Africa, across the Atlantic Ocean and into the Upper Midwest. 

 Aerosol Optical Depth (AOD) measured by satellites outlined dust flowing from Africa over the Atlantic Ocean to North America in June 2020. Credit: NASA

Desert soil is carried across the ocean every year, but this particular June dust plume spanned thousands of miles. Iron and other nutrients in dust can lead to phytoplankton blooms as the dust settles into nutrient-limited waters. As the dust covered multiple regions of the southern U.S., it turned daytime skies very hazy and sunsets red and orange.

Dust can be transported across the globe. Desert dust from the Sahara and Gobi deserts has been observed on the ice sheet of Greenland. Ice cores in Greenland provide a history of the dust deposition as they appear as layers in the ice. The mineralogy of the dust in the ice allows scientists to trace the desert of origin. The source also allows scientists to determine global wind patterns from thousands of years ago.

Steve Ackerman and Jonathan Martin, professors in the UW-Madison Department of Atmospheric and Oceanic Sciences (AOS), are guests on WHA radio (970 AM) at 11:45 a.m. the last Monday of each month.

Category: Meteorology, Phenomena

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What is the longest lightning bolt?

GOES East (GOES-16) data used to document the longest single lightning flash on record which occurred across southern Brazil in October 2018. (Credit: NOAA)

The World Meteorological Organization, or WMO, investigates and certifies meteorological records. On Friday, a WMO committee of experts confirmed a new world record for the longest single lightning flash. The single flash occurred on Oct. 31, 2018 and covered a horizontal distance of 440.6 miles, plus or minus 5 miles, across parts of southern Brazil.

This new record for the longest detected distance for a single lightning flash smashes the previous record of 199.5 miles, which occurred on June 20, 2007 across Oklahoma.

The WMO committee also certified a new record for the greatest duration for a single lightning flash: 16.73 seconds over northern Argentina on March 4, 2019. The new record for duration of a single lightning flash is more than double the previous record of 7.74 seconds on Aug. 30, 2012 over France.

These long flashes have come to be known as “megaflashes” and are lightning discharges that reach more than 50 miles in length. They originate in massive convective storm complexes known as mesoscale convective systems.

New satellite observations to study lightning were used to help validate the new record. It is these new satellite technologies that enable the observation and study of these long-lived lightning events, which previously could not be measured. The space-based instruments also provide near-global coverage of lightning flashes. These measurement capabilities are relatively new, and thus it is likely that these new records will fall as lightning detection technology continues to improve.

Lightning is very dangerous. While your chances of getting hit by lightning are only about 1 in a million in a given year, it is good to keep some safety tips in mind. The National Weather Service advocates the simple rule: “When thunder roars, go indoors!”

Category: Meteorology, Phenomena, Severe Weather

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Does the sunset last longer around the summer solstice?

This June 20 GOES East satellite image from illustrates why the North Pole receives 24 hours of daylight on the summer solstice and the South Pole receives zero. This year it also revealed dust in the Saharan Air Layer (SAL). (Credit: SSEC)

Yes. The summer solstice — in Latin, “sol,” or “sun,” and “stice,” or “come to a stop” — is the day of the year with the most daylight.

The first day of the astronomical Northern Hemisphere summer is the day of the year when the sun is farthest north. In 2020, this occurred at 4:44 p.m. Saturday.

As Earth orbits the sun, its axis of rotation is tilted at an angle of 23.5 degrees from its orbital plane. Because Earth’s axis of spin always points in the same direction — toward the North Star — the orientation of Earth’s axis to the sun is always changing as Earth orbits around the sun. As this orientation changes throughout the year, so does the distribution of sunlight on Earth’s surface at any given latitude.

We define sunset as the time the sun sinks below the horizon. The sun rises and sets farthest north at our summer solstice. The farther the sun sets from due west along the horizon, the shallower the angle of the setting sun. That translates to a longer duration for sunset at the solstice.

While the summer solstice is the day of the year with the most daylight, the earliest sunrise and latest sunset do not occur on the summer solstice. The earliest sunrise occurred about a week ago and the latest sunset occurs later in June.

The time when the sun reaches its highest point in the sky is called solar noon. Solar noon rarely occurs exactly at clock noon; it’s sometimes before and sometimes after. According to our clocks, solar noon comes later on our June solstice than it does one week before. Therefore, according to our clocks, the sunrise and sunset times also come later.

Category: Seasons

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Was that a tropical storm last week?

The path of Tropical Storm Cristobal as it was predicted to move through Wisconsin Tuesday and Wednesday, unleashing heavy rains and gusty winds. Photo credit: NOAA/National Hurricane Center

The western Great Lakes region was visited by a rare tropical storm Tuesday when Cristobal made its way into southern Wisconsin.

Rainfall from eastern Iowa to central Wisconsin was intense, with Madison, La Crosse, Wausau and Rhinelander all recording just under 2 inches of rainfall overnight Tuesday into Wednesday. Even more impressive rainfalls were recorded in eastern Iowa, as Dubuque got 2.16 inches, Cedar Rapids got 2.35 inches and Iowa City was hit with 3.47 inches.

Most impressively, the sea-level pressure (SLP) dropped to 989 millibars on Tuesday night — the lowest June SLP ever recorded in Madison. Green Bay’s SLP dropped to 986 — its record for June, breaking one set in 1917. As we have mentioned before in this column, differences in SLP are what drive the wind and so with this record low SLP, it is not surprising that wind gusts in Madison that night reached 30 mph at the airport and were likely higher in other local spots.

Dewpoint temperatures climbed into the low 70s overnight as well — a sure sign of tropical air. In fact, in the Janesville/Beloit area the overnight dewpoints were 75 degrees — more characteristic of Miami or New Orleans.

Though it is not the first time such a tropical system has affected the state, it is a rare event as various sources suggest no more than four other such events have occurred in recorded Wisconsin history.

Ed Hopkins, assistant state climatologist, informed us that the deadly Galveston Hurricane of September 1900 was one of the four. An unnamed hurricane from October 1949, Carla in 1961 and Gilbert in 1988 round out the rest of the storms that had impacted Wisconsin — though in each of these cases it was the southeastern corner of the state that was grazed.

Category: Meteorology, Seasons, Severe Weather

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