What is a heat wave?

Posted on July 25, 2016 by WeatherGuys Editor

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

Heat waves are caused by very hot, stagnant air masses. Regions that suffer under intense hot spells are usually dominated by a surface high-pressure system with a mid-tropospheric ridge aloft. Dew points are also high, and to compound matters, wind speeds are often low.

Clear or partly cloudy skies allow intense solar energy to further heat the ground and the air mass. High humidity and stagnant air reduce the body’s ability to cool down through sweating. Lives are endangered when these conditions persist day and night for several days.

Each summer in the United States, about 175 to 200 deaths are attributed to heat waves. Most of these deaths occur in cities, particularly northern cities. Heat waves also have a strong economic impact.

A prolonged heat wave can cause the widespread use of air conditioning, leading to increased demands for power that stress gas and electric utilities. Transportation can be stymied when highway surfaces and railways buckle and warp in the heat. All types of outdoor work, such as landscaping and construction, experience reduced productivity. Agriculture is especially vulnerable as heat waves stunt crops and kill livestock.

The National Weather Service issues excessive heat warnings within 12 hours of the onset of extremely dangerous heat conditions. This warning is generally issued when the maximum heat index temperature is expected to be 105 degrees or higher for at least two days and nighttime air temperatures will not drop below 75.

These criteria for issuing a warning vary across the country, especially for northern regions that are not used to hot, humid conditions.

Category: Meteorology, Seasons, Weather Dangers

Comments Off on What is a heat wave?

What forces are in play when high winds knock down objects?

Posted on July 11, 2016 by WeatherGuys Editor

Wind is air moving from areas of high atmospheric pressure to low pressure. Violent destructive winds, as well as gentle summer breezes, result from a complex interplay of different forces.

One of these forces results from a pressure gradient, or how fast pressure changes over distance.

When pressure changes rapidly over a small distance, the pressure gradient force is large. Strong winds almost always result from large pressure gradients.

The greater the difference in pressure over a specific distance, the faster the air flows.

Strong winds can also flow out from thunderstorms, which was the case for the strong winds in southern Wisconsin last week that knocked down many tree limbs.

The wind is air in motion. The atmosphere is made up of gas molecules, mostly nitrogen and oxygen molecules.

These gas molecules are constantly in motion and exert a force when they strike an object, like our bodies or tree branches.

The force exerted by the molecules is a function of the speed, number and mass of the molecules.

Since wind is air in motion it has momentum. This momentum is transferred to the object the wind hits.

Thus, the force of the wind can push objects by moving them or knocking them over. Winds moving over and around objects can cause pressure changes around the object, which can also cause it to move.

The molecules that make up the atmosphere are always in motion and some are always striking your body.

Because these molecules are moving in all directions, this force is exerted in every direction and thus there is a balance of forces and the object doesn’t move.

Category: Meteorology, Severe Weather, Weather Dangers

Comments Off on What forces are in play when high winds knock down objects?

Why do thunderstorms scare dogs?

Posted on July 5, 2016 by WeatherGuys Editor
As an impending storm adds drama to the sky, Grant Bunchkowski of Oconomowoc and his dog, Champagne, take in the view near the city's Lac LaBelle in May 2013. Some dogs, like some humans, seem to have a great fear of thunderstorms. An article in "Psychology Today" suggests that some dogs may be seeking an electrical ground during a storm to avoid electrostatic shock. (Photo credit: John Hart, State Journal archives)

As an impending storm adds drama to the sky, Grant Bunchkowski of Oconomowoc and his dog, Champagne, take in the view near the city’s Lac LaBelle in May 2013. Some dogs, like some humans, seem to have a great fear of thunderstorms. An article in “Psychology Today” suggests that some dogs may be seeking an electrical ground during a storm to avoid electrostatic shock. (Photo credit: John Hart, State Journal archives)

Some dogs seem to have a great fear of thunderstorms, as do some humans.

Their fear can drive them through closed doors or windows, or send them cowering to an isolated spot in the house.

While it is unknown why some dogs become desperate to escape the storm, a few studies and personal experiences point to possible reasons for this fear.

Thunder, the loud noise that accompanies lightning, gives this nimbostratus cloud the name thunderstorm. Some dogs don’t like loud sounds, whether from a thunderclap or fireworks.

Static electricity may be another reason. An article in “Psychology Today” suggests that some dogs’ behavior during a storm means they are seeking an electrical ground to avoid electrostatic shock. Those dogs retreat to a particular location in the home during a storm, like the bathroom, bathtub or lean against a radiator.

Thunderstorms are associated with electric field changes. A bolt of lightning is an electrical discharge much like a shock you get when reaching to grab a metal door knob after walking on a rug, just much more energetic.

“St. Elmo’s fire” is evidence of this static electricity associated with thunderstorms. The masts of tall ships may acquire a blue or violet glow in the vicinity of thunderstorms. Pipes in homes and other metal objects have also displayed St. Elmo’s fire. The electric field around the mast, or pipes, causes ionization of the air molecules which generates a faint blue glow that is visible in low-light conditions.

A large dog with thick fur walking around on insulating foot pads is an ideal object for accumulating static charge, which will discharge.

The dog becomes conditioned to expect random shocks during the storm, probably not something anyone wants to endure.

Category: Meteorology, Phenomena, Weather Dangers

Comments Off on Why do thunderstorms scare dogs?

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

Comments Off on What is the heat index?

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

Comments Off on When was the year without a summer?