Will this be the hottest summer on record?

Posted on July 30, 2012 by Weather Guys Editor

Our remarkably warm summer continues to take aim at some all-time record measures of heat here in Madison.

Just as we poked into the severe drought category on July 18, we were visited by fairly widespread heavy rains that night that delivered 1.43 inches of rain to Dane County Regional Airport. The morning of July 24 brought another 0.56 inches of rain and still more came that night into the morning of July 25. Thus, at least some relief of the drought has recently come our way.

Through the end of the day on July 24, Madison had had 28 days with high temperatures at or above 90 degrees. The all-time record number of such days is 40 recorded in 1955 (two were in June, 19 were in July, 15 were in August and four were in September that year).

In the last 41 years, only six summers (1975, 1976, 1983, 1988, 1995 and 2012) have had 20 or more such days. Of the five such summers before this one, the number of days at or above 90 degrees after July 31 was seven, 10, six, 16, and 10, respectively.

In other words, in such warm summers in the past, we have averaged 9.8 days at or above 90 in August and September. Given that we are very likely to have totaled 30 such days by the end of this July, if past trends in such years apply to 2012, it is quite likely that this summer will set the all-time record for most days at or above 90 degrees in Madison.

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Is Dane County wind affected by Lake Superior ice?

Posted on July 23, 2012 by Weather Guys Editor

A reader wondered whether his perception that it has become windier in southern Wisconsin over the past couple of decades has anything to do with the shorter Lake Superior ice season during the same time.

Though it has been demonstrated by recent research by our UW Atmospheric and Oceanic Science colleagues, Dr. Ankur Desai and Galen McKinley, that the shorter ice season on Lake Superior has led to warmer water in the summer and stronger winds locally near the lake — which, in turn change the currents in the lake itself — these effects are confined to the near vicinity of the lake.

Statistics on wind speed are not as common as those regarding temperature or precipitation and so longterm trends in surface windiness are not easily available. However, it is nearly certain that the local effects that control the changes near Lake Superior do not have a substantial effect on the winds over southern Wisconsin.

Autumn and winter winds in our part of the state are determined by our proximity to regions of low pressure that develop with regularity during the cold season and by turbulence resulting from daytime heating during the summer.

Of course, summertime winds are also greatly influenced by thunderstorm activity as we witnessed last Wednesday in the first significant, widespread rain in the southern part of the state since late May.

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How does our recent heat wave stack up against past events?

Posted on July 16, 2012 by Weather Guys Editor

With another wave of dangerous heat upon us, it is of interest to consider how the last heat wave rates alongside other memorable heat waves. First of all, each day from July 4-6, Madison’s high temperature was more than 100 degrees with the 104 on July 5 ranking as third highest of all time. Moreover, we set record high temperatures for five consecutive days from July 2-6.

The last time a three-day streak of temperatures over 100 occurred here was July 1936. During that heat wave, Madison set a record high temperature record of 107 (on July 13, 1936), while the state record of 114 was set that day at Wisconsin Dells. During that July 1936 heat wave, 14 states set their temperature records — incredible!

From June 27-July 9, we had 12 of 13 days with a high temperature of 90 or greater, with an embedded streak of 10 days in a row that ended July 6 (ranking it third all-time after a 15-day streak in July 1901 and a

13-day streak in July 1936).

Through July 9 we had 20 days with a high temperature at or above 90 this year. Through Sunday we had 23, with another one certain to come today. The record year for such days was 1988 when 90 was reached 35 times — well beyond the average of 13. By July 9, 1988, we had only 13 such days so we are well ahead of the pace for 90-degree days set in that record year.

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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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