What is a drought?

Posted on August 13, 2012 by Weather Guys Editor

A drought means different things to different people. Technically, a drought is a period of abnormally dry weather sufficiently long enough in a given area to cause a shortage of water, whether it is for crops, recreation, water supply utilities or other purposes. As you can imagine, a drought for someone who lives in a desert region would be very different than for a person living among Wisconsin’s many lakes.

There are several definitions of drought, but in general there are three types. A meteorological drought is any substantial and prolonged lack of rainfall over a period of time in a given region. A hydrological drought is a deficiency in surface or subsurface water supplies. An agriculture drought exists when there is not enough soil moisture to meet the needs of a crop at a particular time.

Because there are different types of droughts, there are different methods of measuring the severity of a drought. For example, a meteorological drought can be defined in terms of the percent deviation from the normal precipitation in the region, while a hydrological drought is defined in terms of stream flow, lake levels and groundwater levels.

An effective measure of the impact of drought on agriculture is the Palmer Drought Severity Index. This index, developed in the 1960s, is based on the supply and demand of a water balance equation. It takes into account the precipitation deficit of a location, the temperature of the region and the locally available water content of the soil.

Currently, the Palmer Drought Severity Index lists southern Wisconsin as having “severe drought” conditions, though conditions are improving with the recent rains.

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Are there different types of lightning?

Posted on August 6, 2012 by Weather Guys Editor

Lightning is a huge electrical discharge that results from the rising and sinking air motions that occur in thunderstorms. Lightning can be either cloud-to-cloud or cloud-to-ground and is accompanied by thunder. Lighting also has different appearances.

Staccato lightning is a cloud-to-ground lightning strike that is a short-duration stroke that often, but not always, appears as a single very bright flash and often has considerable branching. Forked lightning is a name, not in formal usage, for cloud-to-ground lightning that exhibits branching of its path. Ribbon lightning occurs in thunderstorms with high cross winds and multiple strokes. The wind will blow each successive stroke slightly to one side of the previous stroke, causing a ribbon appearance. Bead lightning is a type of cloud-to-ground lightning which appears to break up into a string of short, bright sections. It is relatively rare.

Heat lightning is a common name for a lightning flash that appears to produce no discernible thunder because it occurs too far away for the thunder to be heard. The sound waves dissipate before they reach the observer.

During the past two decades scientists have discovered and confirmed the existence of lightning that shoots upward into the upper atmosphere from thunderstorms. Red sprites and elves occur over cloud-to-ground lightning bolts and can extend to near the top of the atmosphere. They are too quick and weak to be seen by the naked eye. Blue jets, in contrast, are limited to the stratosphere and last long enough to be seen by pilots. Much is still not understood about these electrical phenomena.

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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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