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Category Archives: Severe Weather
What are radiosondes?
Radiosondes are instrument packages that measure the vertical profiles of air temperature, relative humidity, and pressure from the ground all the way up to about 19 miles. These radio-equipped meteorological instrument packages are carried aloft by a helium-filled “weather balloon.” … Continue reading
Why does the severe weather threat increase as spring and summer approach?
As the threat of winter snows recedes across the country, it is replaced by the threat of severe weather (i.e. thunderstorms with hail, damaging winds and tornadoes).
The severe weather season, though broadly spanning March through August across the United States, is actually quite regional. It begins in March in the southern states, moves to the southern Plains during April and May, and then further north toward the Great Lakes states during the summer. Continue reading
Category: Meteorology, Seasons, Severe Weather
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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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What is a meteorological bomb?
Storm systems that form in middle or high latitudes, in regions of large temperature contrast are called extratropical cyclones. This contrasts with tropical cyclones, such as hurricanes, which form in regions or relatively uniform temperatures.
Extratropical cyclones are surface low-pressure systems where the air at the surface flows counterclockwise in the northern hemisphere and clockwise in the southern hemisphere. The winds around these centers of low-pressure spiral inward at the surface, forcing rising motions. The rising air can result in clouds and precipitation. The process of extratropical cyclone development and intensification is referred to as cyclogenesis. In North America there are favorable regions for cyclogenesis, including the eastern slopes of mountain ranges, the Atlantic Ocean off the Carolina Coast, and the Gulf of Mexico. The jet stream also influences cyclogenesis. Continue reading
