Category Archives: Severe Weather

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 →

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 →