What is a derecho?

Posted on July 9, 2012 by Weather Guys Editor

A derecho (pronounced deh-RAY-cho, a Spanish word meaning “straight ahead”) is an hours-long windstorm associated with a line of severe thunderstorms. It is a result of straight-line winds, not the rotary winds of a tornado — hence its name. Derechos in the United States are most common in the late spring and summer (May through August).

The extreme winds of a derecho — up to 150 mph in the strongest storms — are often associated with a quasi-stationary front in mid-summer. If the atmosphere just north of the front is very unstable, the front may trigger rapidly developing thunderstorms. A line of thunderstorms that forms in the vicinity of the stationary front can, via its cold downdrafts, drag down high-speed air from above. This can cause the high winds of a derecho.

At the same time, the high winds push the line of thunderstorms outward, causing it to bend or “bow.” This results in a bow echo image on weather radar. Once they get going, derechos can cover lots of territory — up to 1,000 miles.

Derechos leave significant property damage in their wake, even flattening entire forests. In some cases, derechos wreak as much havoc as a hurricane or tornado. A June 29 derecho swept across the U.S. from west of Chicago to the East Coast, leaving as many as 5 million households without power. The storm traveled at speeds of over 60 mph, with wind gusts approaching 80 mph. At least 22 people were killed. About 40 percent of all thunderstorm-related injuries and deaths occur because of derechos.

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How much energy does it take to produce a torrential downpour?

Posted on July 2, 2012 by Weather Guys Editor

Recently, the Duluth-Superior metro area had devastating flooding a result of rainfall totals of 10 inches or more in some locations. You may have seen photographs of the damage wrought by the flood waters — washed-out roads, flooded homes, ruined crops, etc.

Even in the face of such dramatic damage it is easy to overlook the enormous amount of energy that is involved in simply processing the water involved in such enormous amounts of precipitation.

Water is the only chemical in our atmosphere that can readily exist in all three of its phases — solid, liquid and gas. When 1 gram of water transitions from the high energy vapor phase (in which water is just another invisible gas) to the more familiar liquid phase (rain), 600 calories of energy are released to the atmosphere.

If 10 inches of rain were ever to fall over Dane County (with an area of 1,221 square miles), that would amount to 125 billion kilograms of water which, of course, would have originated from 125 billion kilograms of water vapor.

Since each gram of that huge amount of water vapor would have released 600 calories of energy to the atmosphere when it was condensed, a total of 75 quadrillion calories would have been released to the atmosphere to produce a 10-inch rainfall over Dane County. That is enough energy to power the Madison metro area for just a shade under 30 years. Clearly, a lot is going on energetically behind the scenes.

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What is a flash flood?

Posted on June 25, 2012 by Weather Guys Editor

A flood occurs when water flows into a region faster than it can be absorbed into the soil, stored in a lake or reservoir or removed in runoff or a waterway into a drainage basin.

A flash flood is a sudden local flood characterized by a great volume of water and a short duration. It occurs within minutes or hours of heavy rainfall or because of a sudden release of water from the breakup of an ice dam or constructed dam.

Flash floods are dangerous because the waters in a flash flood often move at destructive speeds and they often occur with very little warning.

Rainfall intensity and duration are two key elements of a flash flood. Topography, soil conditions, and ground cover also play important roles. Steep terrain can cause rain water to flow toward and collect in low-lying areas, causing water levels to rise rapidly. If the soil is saturated with water, it cannot absorb more, so the excess water runs off the land quickly.

However, extremely dry soil conditions also can be favorable for flooding. Dry soil can often develop a hard crust over which water will initially flow as if the ground were concrete.

If a flash flood warning is issued, you should stay away from streambeds, drainage ditches and culverts. Move to high ground when threatened by flooding. Stay out of flooded areas. Never drive your car across a flooded road. Most flash flood-related deaths occur when people drive into floodwaters. Never underestimate the power of moving water.

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What is the summer solstice?

Posted on June 18, 2012 by Weather Guys Editor

The summer solstice (in Latin, sol, “sun,” and stice, “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 (on June 20 or 21). In 2012, this occurs on June 20 at 6:09 pm CDT.

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 this orientation changes throughout the year, so does the distribution of sunlight on Earth’s surface at any given latitude. This links the amount of solar energy reaching a location to the time of year and causes some months of the year to always be warmer than others — in other words, the seasons.

On the Northern Hemisphere’s summer solstice, the northern spin axis is tilted toward the sun, and latitudes north of the Arctic Circle (66.5 degrees N) have 24 hours of light. A common misconception is that Earth is closest to the sun in our summer. Actually, Earth is closest to the sun in December, which is winter in the Northern Hemisphere.

The summer solstice is often referred to as the first day of summer but there are other definitions of summer. For example, culturally we can consider summer to begin during the Memorial Day holiday weekend or after school gets out.

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What are pyrocumulus clouds?

Posted on June 11, 2012 by Weather Guys Editor

A pyrocumulus cloud forms from rising air that results from intense heating of the surface by phenomena such as wildfires or volcanic eruptions. The fires that generate these clouds can be man-made or natural. A big fire produces strong upward moving air currents that carry water vapor and ash upward. The water vapor can condense on the ash forming cloud drops. The vigorous upward motions produce these pyrocumulus clouds that look similar to thunderstorm clouds, which also form due to strong upward moving air.

A pyrocumulus is a fire cloud. In Latin, pyro means “fire” and cumulus means “pile up.” Cumulus is a type of cloud that is common in Wisconsin, particularly in summer. Cumulus clouds are those puffy-white clouds with tops that have a cauliflower appearance. Pyrocumulus clouds are grayish or brown in color because of the ashes and smoke of the fire. The tops of these clouds can reach as high as 30,000 feet. It is difficult to locate the bottom of a pyrocumulus cloud as it is often obscure by the ash generated by the fire or the volcanic eruption.

If lots of water vapor is available, the pyrocumulus can develop into a cumulonimbus, or thunderstorm. When a thundercloud forms, it is called pyrocumulonimbus. Like thunderstorms, pyrocumulonimbus can produce lightning because of the strong updrafts. Rain can also fall from these clouds, which could help extinguish the fire generating the cloud. Of course, the lightning might cause another fire.

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