Severe Weather Notes
-For a thunderstorm to form, three conditions must exist.
1. There must be an abundant source of moisture in the lower levels of the atmosphere. 2. Some mechanism must lift the air so that the moisture can condense and release latent heat. 3. The portion of the atmosphere through which the cloud grows must be unstable. -The air in a thunderstorm will keep rising until: 1. It meets a layer of stable air that it cannot overcome 2. The rate of condensation, which diminishes with height, is insufficient to generate enough latent heat to keep the cloud warmer than the surrounding air -Typical thunderstorms last only about 30 minutes and individual storms are only about 24 km in diameter. •Thunderstorms are often classified according to the mechanism that caused the air to rise. •An air-mass thunderstorm is a thunderstorm that results from the air rising because of unequal heating of Earth’s surface within one air mass. -Mountain thunderstorms occur when an air mass rises as a result of orographic lifting, which involves air moving up the side of a mountain. -Seabreeze thunderstorms are local air-mass thunderstorms caused, in part, by extreme temperature differences between the air over land and the air over water. -Frontal thunderstorms are thunderstorms that are produced by advancing cold fronts and, more rarely, warm fronts. -Cold-front thunderstorms get their initial lift from the push of the cold air which can produce a line of thunderstorms along the leading edge of the cold front. -Because they are not dependent on daytime heating for their initial lift, cold-front thunderstorms can persist long into the night What are the stages of development of a thunderstorm? Cumulus Stage –In the cumulus stage, air starts to rise nearly vertically upward. –Transported moisture condenses into a visible cloud and releases latent heat. –As the cloud droplets coalesce, they form larger droplets, which eventually fall to Earth as precipitation. Mature Stage –As precipitation falls, it cools the air around it which becomes more dense than the surrounding air, so it sinks creating downdrafts. –The updrafts and downdrafts form a convection cell. –In the mature stage, nearly equal amounts of updrafts and downdrafts exist side by side in the cumulonimbus cloud. Dissipation Stage –The supply of warm, moist air runs out because the cool downdrafts cool the area from which the storm draws energy. –Without the warm air, the updrafts cease and precipitation can no longer form. –The dissipation stage is characterized primarily by lingering downdrafts. –A storm surge occurs when hurricane-force winds drive a mound of ocean water, sometimes as high as 6 m above normal sea level, toward coastal areas where it washes over the land.
–In the northern hemisphere, a storm surge occurs primarily on the right side of a storm relative to its eye, where the strongest onshore winds occur. –Floods are an additional hurricane hazard, particularly if the storm moves over mountainous areas, where orographic lifting enhances the upward motion of air. –The National Hurricane Center, which is responsible for tracking and forecasting the intensity and motion of tropical cyclones in the western hemisphere, issues a hurricane warning at least 24 hours before a hurricane strikes. –The center also issues regular advisories that indicate a storm’s position, strength, and movement. |
What are the ingredients to making a tornado?
supercells •are self-sustaining, extremely powerful severe thunderstorms, which are characterized by intense, rotating updrafts. •Only about ten percent of the roughly 100 000 thunderstorms that occur each year in the United States are considered to be severe; even fewer become supercells. •Downbursts are violent downdrafts that are concentrated in a local area and can contain wind speeds of more than 160 km/h. •Hail is precipitation in the form of balls or lumps of ice that can do tremendous damage. •Hail forms because of two characteristics common to thunderstorms. •A tornado is a violent, whirling column of air in contact with the ground. •Before a tornado reaches the ground, it is called a funnel cloud. •Tornadoes are often associated with supercells. •The air in a tornado is made visible by dust and debris drawn into the swirling column, or by the condensation of water vapor into a visible cloud. •A tornado forms when wind speed and direction change suddenly with height, a phenomenon known as wind shear. Tornado Classification –The Fujita tornado intensity scale classifies tornadoes according to their path of destruction, wind speed, and duration. –The scale ranges from F0, which is characterized by winds of up to 118 km/h, to the violent F5, which can pack winds of more than 500 km/h. –Most tornadoes do not exceed the F1 category. –Only about one percent ever reach the violent categories of F4 and F5. Tornado Distribution –While tornadoes can occur at any time or place, some places are more conducive to their formation. –Most tornadoes form in the spring during the late afternoon and evening, when the temperature contrasts between polar air and tropical air are the greatest. –Tornadoes occur most frequently in a region called “Tornado Alley,” which extends from northern Texas through Oklahoma, Kansas, and Missouri. •Tropical cyclones are large, rotating, low-pressure storms that form over water during summer and fall in the tropics. What does it take to make a hurricane? -Tropical cyclones thrive on the tremendous amount of energy in warm, tropical oceans -This latent heat from water that has evaporated from the ocean is released when the air begins to rise and water vapor condenses. -Rising air creates an area of low pressure at the ocean surface. -The cyclonic rotation of a tropical cyclone begins as warm air moves toward the low-pressure center to replace the air that has risen. –Tropical cyclones require two basic conditions to form: •An abundant supply of very warm ocean water •Some sort of disturbance to lift warm air and keep it rising Stages of a hurricane
–Tropical cyclones usually begin as disturbances that originate either from the ITCZ or as weak, low-pressure systems called tropical waves. –Only a small percentage these ever develop into hurricanes because conditions throughout the atmosphere must allow rising air to be dispersed into the upper atmosphere. –When a disturbance over a tropical ocean acquires a cyclonic circulation around a center of low pressure, it is known as a tropical depression. –When wind speeds around the low-pressure center of a tropical depression exceed 65 km/h, the system is called a tropical storm. If air pressure continues to fall and winds around the center reach at least 120 km/h, the storm is officially classified as a hurricane •The Saffir-Simpson hurricane scale classifies hurricanes according to wind speed, air pressure in the center, and potential for property damage. •The Saffir-Simpson hurricane scale ranges from Category 1 hurricanes to Category 5 storms, which can have winds in excess of 155 mph. •Most of the deadliest hurricanes that strike the United States were classified as major hurricanes. Running our of energy –A hurricane will last until it can no longer produce enough energy to sustain itself. This usually happens when: •The storm moves over land and no longer has access to the warm ocean surface from which it draws its energy. •The storm moves over colder water. |
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