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What causes tornados and do they have a lifecycle?
A tornado is a powerful column of winds that rotate around a center of low pressure. The winds inside a tornado spiral inward and upward, often exceeding speeds of 300 mph. We don’t know if a particular storm will produce a tornado but we do know the necessary conditions needed for tornado formation.
The required conditions for a thunderstorm to produce a tornado are warm humid air near the surface with cold dry air above. These conditions make the atmosphere very unstable, in the sense that once air near the ground is forced upward, it moves upward quickly and forms a storm. Severe thunderstorm conditions also include a layer of hot dry air between the warm humid air near the ground and the cool dry air aloft. This hot layer acts as a lid that allows the sun to further heat the warm humid air, making the atmosphere even more unstable. Continue reading
Category: Meteorology, Phenomena, Severe Weather
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What is vapor pressure?
Weather reports often include the dew point temperature and the relative humidity. These are just two of several ways to express the amount of water vapor in the atmosphere. Vapor pressure is another way. Each method has advantages and disadvantages.
Gas molecules exert a pressure when they collide with objects. The atmosphere is a mixture of gas molecules and each type of gas makes up a part of the total atmospheric pressure. The pressure the water molecules exert is another useful method of representing the amount of water vapor in the atmosphere. The pressure caused by these water vapor molecules is called the vapor pressure. Atmospheric vapor pressure is expressed in millibars (mb). Continue reading
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 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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