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Category Archives: Meteorology
What is a derecho?
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). Continue reading
Is the weather nicer on Memorial Day or Labor Day?
Memorial Day traditionally marks the beginning of the summer, while Labor Day marks the end.
People are bound to have their own recollections and opinions about which weekend more reliably delivers agreeable weather here in southern Wisconsin, and those opinions can be colored by many non-meteorological influences. Continue reading
Did someone predict the recent Wisconsin tornado?
We cannot yet forecast tornado occurrence with any accuracy. One problem is the small size of a tornado, which is a narrow column of strong winds that rotate around a center of low pressure.
Over the last 60 years, forecasts of the development of large-scale low-pressure systems, which often organize the ingredients needed to form a tornado, have steadily improved. Because of these advances, meteorologists are better able to predict those conditions a few days in advance, enabling forecasters to identify counties where there is a threat of severe weather sometimes as many as three days in advance. Two days in advance of the recent EF-2 tornado that hit southeastern Polk County, the National Weather Service’s Storm Prediction Center’s convective outlook issued a slight-risk for the area. Continue reading
Category: Meteorology, Severe Weather
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Is the California drought over?
Drought in the United States has been assessed by a number of different measures over the last several decades. One of the older such measures is the Palmer Drought Index introduced in 1965. The Palmer Index is based on a … Continue reading
Why is the grass wet in the morning, when it didn’t rain?
Recent spring mornings have produced wet lawns. To explain this, we start with the fact that air contains water in the gas phase, called water vapor. Dew is made of liquid water that has condensed from some of the water vapor in the air.
Dew occurs when objects cool. A common example of dew formation is when drops of liquid form on the glass of an ice-cold drink. Dew forms when the object, such as the glass, cools down to the dew point temperature. Continue reading
Category: Meteorology, Phenomena
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