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Monthly Archives: January 2025
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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Beyond the headlines, what else has been happening in the weather lately?
There has certainly been a lot of interesting and, in many cases, devastating weather around the country in the past couple of weeks. The heavy snow in parts of the country that don’t often see it along with the California wildfires have caught the attention of lots of us in the first days of the new year.
But in the background is a rather remarkable one-week stretch that occurred in the last week of December. Continue reading
Why are clouds relatively flat on the bottom?
Most clouds, especially those with flat bottoms, form in rising air. The air can be forced to rise due to convection, frontal lifting, or when air near the surface flows together from different directions. As a volume, or parcel, of air rises, it expands and cools. In addition, the relative humidity of the rising air increases. As the parcel approaches the point of saturation, water vapor condenses to form tiny water droplets or ice particles, creating a cloud. Saturation occurs at a distinct altitude, which varies depending on the temperature and humidity structure of the atmosphere. Below this condensation level clouds do not form.
Often low clouds, like stratus and cumulus, appear to have flat bases. These clouds form as air near the ground is rising. As the air rises, it expands as pressure decreases with altitude. This expansion results in a cooling, which causes the relative humidity in the rising parcel to increase. The temperature of the rising air approaches the dew point temperature. When it reaches the height where those two temperatures are equal, the relative humidity is 100% and a cloud forms. Meteorologists call this altitude the lifting condensation level. Continue reading
