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Why does the United States have a National Weather Service?
While successfully prosecuting the Civil War against the Confederacy, Gen. Ulysses S. Grant* had learned that weather information – even if NOT in the form of a forecast – was extremely valuable for operations. Coincidentally, in the years following the … Continue reading
Category: History, Meteorology
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What is this upside-down icicle?
The accompanying photo was taken by Daniel Dettmers in the morning of February 4 on frozen Lake Kegonsa. The high on the previous day was 37°F. This caused puddles of water to sit on the ice of Madison’s regional lakes during the day. Tuesday morning’s low temperature was below 20°F with calm winds. These are just the right conditions to form what are called ‘ice spikes,’ as shown in the photograph.
When water freezes, it expands and becomes less dense. Ice floats on water. But if the lake ice is thick, when puddles form on a warm day, they sit on the ice surface. With the cold nighttime temperatures, the surface of the puddle freezes, trapping liquid water below. As the puddle freezes, it can leave a small hole in the surface of the ice. Continue reading
Is it getting windier in Wisconsin?
Wind speed and direction are variables that change over space and time, and conditions can change considerably from month to month, as well as from year to year. Scientifically assessing any long-term changes in weather elements requires a long-term data set of accurate measurements. Temperature data goes back hundreds of years, and even thousands of years, using ancillary data such as from tree rings.
Wind is a more difficult parameter to study and analyze. The observations of wind speed and direction need to be made at the same height above the surface. The type of surface also impacts the measurement, as the wind sensor should not be too close to trees or buildings. Finally, large annual fluctuations make long-term trends difficult to detect. Continue reading
Is there such a thing as “thundersnow”?
A reader recently asked us if thunder ever occurs with snowfall. It turns out that such “thundersnow” does, in fact, occur occasionally in very intense winter storms. Clouds and precipitation develop when the air is forced to rise to higher heights where the pressure is always lower. The rising air expands into its lower pressure environment and the expansion results in a cooling of the air. This cooling raises the relative humidity of the air and sometimes brings it to saturation, at which point invisible water vapor condenses into liquid water or goes straight to the solid ice phase. During winter, the dynamical forces that create ascending air are very strong and well organized on large scales. However, the stability of the stratification is stronger, partly because the air is generally much drier, which discourages thunderstorm development. During summertime the large-scale is less organized but there is more abundant water vapor, weaker stratification and stronger individual updrafts of air that form intense thunderstorms. Thundersnow is not very common because it requires moist, poorly stratified air (more characteristic of the warm season) and strong large-scale dynamics (more characteristic of the cold season) to occur simultaneously. Continue reading
What causes the Santa Ana winds?
Santa Ana winds are dry, warm, and gusty winds that blow from the interior of southern California toward the coast and offshore. They are a type of downslope wind, which is a wind directed down a slope produced by processes larger in scale than the slope.
Santa Ana winds can occur when the pressure gradient caused by a high-pressure region over the Rockies, in combination with friction, forces air from the mountainous West down the San Gabriel Mountains in southern California.
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