What is the heat index?

Posted on June 27, 2016 by WeatherGuys Editor
Carter Reitz, 9, of DeForest, cools off during opening day at Goodman Pool on June 10, the first 90-degree day of the season. The heat index -- which can be higher or lower than the actual temperature -- indicates how hot it feels. (Photo credit: Amber Arnold, State Journal)

Carter Reitz, 9, of DeForest, cools off during opening day at Goodman Pool on June 10, the first 90-degree day of the season. The heat index — which can be higher or lower than the actual temperature — indicates how hot it feels. (Photo credit: Amber Arnold, State Journal)

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; 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.

Summer heat waves are often associated with high heat indices and can be dangerous. For example, a heat wave in the Midwest in late July 1999 resulted in a heat index of 113 degrees in Chicago and resulted in more than 200 deaths in the Midwest. The heat wave that affected parts of the northeastern U.S., central Europe and Russia in the summer of 2010 contributed to the deaths of as many as 15,000 people in Moscow.

On July 31, 2015, the temperature in Bandar Mahshahr, Iran, was 115 degrees with a dew point temperature of 90, yielding a heat index of 165 degrees. The highest known heat index is 178 degrees, observed in Dhahran, Saudi Arabia, on July 8, 2003.

The National Weather Service 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.

Category: Meteorology, Seasons

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When was the year without a summer?

Posted on June 20, 2016 by WeatherGuys Editor
Mt. Etna, Europe's most active volcano, spews lave during an eruption on Nov. 16, 2013. The cold summer of 1816 is attributed to large volcanic eruptions. (Photo credit: Associated Press)

Mt. Etna, Europe’s most active volcano, spews lave during an eruption on Nov. 16, 2013. The cold summer of 1816 is attributed to large volcanic eruptions. (Photo credit: Associated Press)

The summer of 1816 is known as the year without a summer in eastern North America.

While extensive weather observations were not available, people’s journals documented the cold weather. Snow fell on June 5 in Vermont with temperatures in the low 30s, following a day with high temperatures in the upper 80s.

After the early June cold spell, farmers replanted as temperatures returned to normal. Another cold spell hit in July with freezing temperatures. Frost was also observed in August.

Weather in Europe and other regions of the globe was also abnormal as unusually cold and wet weather contributed to failed harvests.

The cold summer is attributed to large volcanic eruptions. Between 1812 and 1817 there were three major eruptions. The largest was Mt. Tambora, which erupted in Indonesia in April 1815.

The violent eruption pumped large amounts of debris and gases into the stratosphere. It took about a year for material from the eruption to spread globally. This cooled the planet because the tiny sulfuric acid drops reflected energy from the sun back out to space.

Not everything surrounding this event led to bad news. A group of friends vacationing in Switzerland found the cold summer unfavorable for outdoor activities, so they decided to have a ghost story contest. The winner was Mary Shelley and her story was published in 1818 — “Frankenstein.”

Category: Meteorology, Phenomena

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How do you measure how hot the summer is?

Posted on June 13, 2016 by WeatherGuys Editor
Jennifer Ronquillo, of Madison, plays with her daughters, Sienna, 3, and Neaveh, 1, at bottom, at Goodman Pool on opening day, Friday, Madison's first 90-degree day of the year.

Jennifer Ronquillo, of Madison, plays with her daughters, Sienna, 3, and Neaveh, 1, at bottom, at Goodman Pool on opening day, Friday, Madison’s first 90-degree day of the year.

After experiencing our first 90-degree day of the season on Friday, many people are wondering what we might expect this summer.

It turns out that the number or 90-degree days each summer is extremely variable here in Madison. From 1971 to 2015, the average number of days at or above 90 in Madison was 10.96. This average, however, struggles to convey a sense of the variability.

A better way to express that variability is by calculating the standard deviation, which, when added to or subtracted from the average, sets a range in which approximately two-thirds of the years will fall.

In this case the standard deviation is nine. Thus, we might expect that two-thirds of the years would range from having 20 to two days at or above 90. As it turns out, 33 of the last 45 summers have been in that range.

It  is interesting that six summers have had 20 or more hot days (1975, 1976, 1983, 1988, 1995 and 2012) – the record being held by 2012 with 39 days.

Over the last four and a half decades, there has been a trend toward fewer hot days each summer, with the averages being 15.8, 11.7, 8.2 and 7.3 days for the 1970s, ’80s, ’90s and ’00s, respectively. The half-completed 2010s, by virtue of the incredibly warm 2012, appear to be bucking this trend as, thus far, we have averaged 12 such days each summer in this decade, though only 2012 had as many as 12.

It remains to be seen what the rest of this summer and the decade will bring, but these data remind us how complicated the interplay between weather and climate can be since the global average temperature has been trending the other way in these same decades.

Category: Climate, Meteorology, Seasons

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What is the prediction for this year’s hurricane season?

Posted on June 6, 2016 by WeatherGuys Editor
This photo, taken from video provided by NASA, shows Hurricane Alex -- a rare January hurricane in the Atlantic -- seen from the International Space Station. The National Oceanic and Atmospheric Administration (NOAA) is predicting 10-16 named storms this hurricane season, which runs from June through November in the Atlantic Ocean basin. (Photo credit: NASA)

This photo, taken from video provided by NASA, shows Hurricane Alex — a rare January hurricane in the Atlantic — seen from the International Space Station. The National Oceanic and Atmospheric Administration (NOAA) is predicting 10-16 named storms this hurricane season, which runs from June through November in the Atlantic Ocean basin. (Photo credit: NASA)

Hurricane season in the Atlantic Ocean basin runs from June through November.

 

The National Oceanic and Atmospheric Administration (NOAA) is predicting 10 to 16 named storms this season.

To acquire a name, the storm must have wind speeds of 39 mph or higher. Of these, four to eight will turn into hurricanes; one to four of these storms are predicted to become major hurricanes with sustained winds of at least 111 miles per hour.

NOAA does not make a seasonal prediction on the path of hurricanes but will predict a hurricane’s path once the storm develops.

On average, the Atlantic Ocean sees 12 named storms, six hurricanes and three major hurricanes. So, if the forecast holds, we are in for an average hurricane season.

The last three years have seen weaker hurricane season, so this year may see high activity.

The forecast is based on current and expected conditions. First, an active season would be predicted if the sea surface temperatures are warmer than normal. Warmer temperatures increase evaporation off the sea surface and that provides the energy for the storm.

We are coming out of an El Niño year and there is a 70 percent chance entering a La Niña, a condition that favors more hurricane activity than average.

Perhaps surprisingly, the atmospheric pressure pattern over the Arctic also plays a role in the forecast: high pressure means a weaker jet stream, which favors hurricane development.

We already have had Hurricane Alex, which occurred in mid-January of this year. Hurricanes can exist outside of the defined hurricane season. Tropical Storm Bonnie was a weak but persistent tropical storm that formed on May 27.

Category: Severe Weather, Tropical

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What can be learned from the recurrence of extreme weather events?

Posted on June 2, 2016 by WeatherGuys Editor
Roberto Salas, left, and Lewis Sternhagen check a flooded car in Houston on May 26. This spring the Houston area has experienced five separate rain events that have been classified as "once in a hundred year" events, which have a 1 percent chance (or a 1-in-a -100 chance) of occurring in any given year. (Photo credit: Associated Press)

Roberto Salas, left, and Lewis Sternhagen check a flooded car in Houston on May 26. This spring the Houston area has experienced five separate rain events that have been classified as “once in a hundred year” events, which have a 1 percent chance (or a 1-in-a -100 chance) of occurring in any given year. (Photo credit: Associated Press)

In 2004, the Boston Red Sox won the World Series for the first time in 86 years and their loyal fans were ecstatic. In the very next year, the Chicago White Sox won their first World Series title in 88 years.

Never in the long history of baseball had two teams that had been denied championships for so long won those long-coveted titles in successive years. This circumstance was at least partially a function of the changes in baseball brought about by free agency — a fundamental change to the rules.

This spring the Houston area has experienced five separate rain events that have been classified as “once in a hundred year” events. A 100-year event has a 1 percent chance (or 1-in-a-100 chance) of occurring in any given year.

This designation is meant to suggest that the intensity of these events is not likely to return to Houston except every 100 years or so. Thus, to have experienced five such storms in a single season is extremely unlikely and points toward an alteration in the climate (a fundamental change in the “rules”) of the region.

In a similar way, readers might recall that here in Madison in March 2012 we recorded five days with temperatures at or above 80 degrees. In the prior 100 years, only five other March days had ever topped 80 degrees, so that was also exceptionally unusual and provided evidence also suggestive of a shift in the climate toward a warmer world.

As isolated incidents, these meteorological phenomena can be expected every so often even in an unchanging climate as there is a lot of internal variability in the atmosphere.

In the seemingly recurring sequence we have experienced in the past couple of decades, however, there is little doubt that such events are among the fingerprints of a changing climate — a reality that we simply must confront in a coordinated, scientifically informed manner. A great number of dedicated scientists at UW-Madison are actively seeking better understanding of the complicated weather/climate system.

Category: Climate, Meteorology, Severe Weather

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