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Answer: There can be many influences on raindrop size, but a principal factor involved is the intensity of upward motion within the rain cloud. Smooth, stratiform clouds with a broad, gentle rising motion tend to produce small drops, while convective showers and thunderstorms have strong updrafts that can offset greater terminal velocities, allowing droplets to grow larger before falling.
Apr. 5, 2009 | Tags: clouds, thunderstorms

Answer: Most strong or long-lived tornadoes are produced by supercell thunderstorms, and in the vast majority of cases the orientation of wind directions and speeds through the lower and middle atmosphere in the northern hemisphere lead to counterclockwise-rotating storms and tornadoes. A rare exception is documented at tornado.sfsu.edu/geosciences/StormChasing/cases/Sunnyvale/Sunnyvale.html.
Apr. 4, 2009 | Tags: thunderstorms, tornadoes, winds

Answer: Many thunderstorms get their energy and the explosive upward motions that help to separate electrical charges by way of instability associated with strong decreases of temperature with increasing height. This is more likely during the afternoon or evening when the earth's surface has been heated by the sun for much of the day, but morning storms can and do occur due to other processes.
Apr. 4, 2009 | Tags: thunderstorms

Answer: Small, weak tornadoes of the "gustnado" or "landspout" variety can rotate clockwise (in the northern hemisphere) if they are created when a pre-existing clockwise swirl is acted on, stretched vertically and intensified by upward motions associated with a storm cell. Such swirls can be created topographically, along frontal boundaries, or along the periphery of a thunderstorm outflow.
Apr. 4, 2009 | Tags: thunderstorms, tornadoes

Answer: In mid-latitude locations like North Carolina, March is a transition period between winter and spring, with both a lingering potential for wintry weather with strong cold outbreaks, and early intrusions of warm, unstable air that can interact with frontal systems and the jet stream to produce severe thunderstorms. At RDU, we have had temperatures in March ranging from 11 to 92!
Mar. 16, 2009 | Tags: severe weather, thunderstorms

Answer: You're referring to the scale of radar reflectivity that depend on how much radio energy is being scattered back toward the radar. Generally, this is greatest for higher rain rates and larger rain drops. However, rain alone almost never reaches into the values that are color coded black (sometimes purple or gray). We do see occasional thunderstorms where these colors indicate hail.
Feb. 16, 2009 | Tags: hail, thunderstorms

Answer: Many tornadoes follow fairly linear paths that correspond to the direction of movement of their parent thunderstorm cells. However, there are cases in which the storm cells encounter frontal boundaries with large-scale wind shifts, or smaller outflows and circulations associated with nearby storms cells. When this happens, the tornado can curve or, at times, follow a more complex path.
Dec. 20, 2008 | Tags: thunderstorms, tornadoes, winds

Answer: Most severe weather events (large hail, severe thunderstorm winds, and tornadoes) do involve unstable air, but there are other ingredients required. Those that are usually in play include sufficient moisture at appropriate altitudes, favorable wind profiles, and a forcing mechanism such as a front, low pressure area or upper level disturbance to initiate upward motion.
Dec. 12, 2008 | Tags: hail, severe weather, thunderstorms, tornadoes, winds

Answer: It doesn't work out that way every time, so the rule of thumb isn't ironclad by any means. However, weather patterns that sweep enough warm, unstable air northward to produce winter thunderstorms also tend to transport cold airmasses well to the south. If an appropriate disturbance moves through the region while the deep cold air is still in place, snow can result.
Dec. 12, 2008 | Tags: past weather, snow, thunderstorms, winter weather

Answer: Tornadoes can occur any time of year, but the atmosphere tends to be more unstable and supportive of intense thunderstorms during the warmer months. However, fronts and jet stream dynamics also play a role, and these are often lacking in summer over central North Carolina. Here, about 43 percent of our tornadoes form in the spring, 26 percent during summer and 23 percent in the fall.
Nov. 10, 2008 | Tags: past weather, thunderstorms, tornadoes