Mark Seeley’s Minnesota Weather Almanac, Second Edition, takes into account the state’s new thirty-year normals (1981-2010), presenting a breadth of scientific facts and fascinating stories.
In recognition of summer storm season, here are a few excerpts from the book explaining weather jargon that will have you talking like a weather expert.
The Heat Index, also known as the Comfort or Temperature-Humidity Index, evaluates the combined effects of temperature and humidity on the body’s ability to cool itself. According to the Heat Index, an air temperature of 85°F with a relative humidity of 60 percent feels the same as a temperature of 90°F with a humidity of 30 percent. For nighttime combined values of 75°F or above and daytime values of 105°F or more expected for 48 hours or longer, the National Weather Service usually issues an excessive heat advisory to warn about health risks, including fatigue, heat cramps, sunstroke, or heat exhaustion.
The dog days of summer are usually associated with the greatest heat of the year, characterized by thunderstorms and high dew points. The phrase’s origin is both ancient and astrological: the Greeks and Romans observed that one of the brightest stars, Sirius the Dog Star—located in the constellation Canis Major, Latin for “greater dog”—rose in conjunction with the sun during the six weeks of midsummer. The usual hot and sultry weather, which depleted people’s energy and wilted vegetation, was attributed to the evil effects of Sirius. In the United States, the dog days occur between mid-July and early September; in western Europe they run from July 3 to August 11.
Doppler radar is a type of weather surveillance that takes advantage of the Doppler effect. Based on the frequency change between outgoing and reflected radar signals, it determines the velocity of atmospheric targets moving directly toward or away from the unit. Doppler radar allows meteorologists to interpret wind speeds accompanying thunderstorms and to view rotating winds associated with funnel clouds.
The term heat lightning is derived from a mistaken belief that lightning is produced by an excessively heated atmosphere, based on observations of lightning under otherwise clear summer skies. What is viewed as heat lightning is actually a reflection of distant lightning flashes off the horizon. All lightning technically produces heat: a single stroke can warm the surrounding air to more than 50,000°F. The air’s rapid expansion causes sound waves, which are later heard as thunder. Sound travels approximately a mile every five seconds: to gauge the distance of the lightning flashes, count the number of seconds that pass between the flash and the resulting thunder, assuming about one-fifth mile for every second. Thus a 15-second interval between observed lightning and the sound of thunder indicates that the flash occurred about three miles away. Lightning strokes from more than ten miles away are rarely heard as thunder.
Four basic thunderstorm types
Thunderstorms occur in a variety of forms.
An isolated cumulonimbus or anvil-shaped cloud, known as a single-cell storm, is usually a convective cloud containing one updraft and one downdraft segment. Single-cell storms may produce some heavy rain, hail, or even a weak tornado, but they are usually short lived, lasting 30 or fewer minutes.
In a multicell cluster, a group of convective clouds moves together as a single unit, bringing multiple updraft and downdraft segments, highly variable rates of rainfall, and moderate hail. These systems may last for hours and can produce flash flooding or weak tornadoes.
A squall line is a row of convective clouds that share a common gust front along the leading edge, sometimes visible as a wall cloud. They can move at rapid speeds and produce heavy rainfall, moderate hail, and even tornadoes, occasionally leading to flash flooding.
A supercell, the most damaging type of thunderstorm, is a massive convective system of clouds that rotate as one unit, contain embedded strong updrafts and downdrafts, and produce large hail, frequent lightning, flooding, and moderate to severe tornadoes. Such storms may last for hours and travel across multiple states.