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13-1 Thunderstorms Learning Target I can explain how local conditions impact the formation of thunderstorms. Average annual occurrences of thunderstorms in the United States • What region of the U.S. shows the lowest average of thunderstorms per year? • What regions show the second lowest average per year? • What region shows the highest average per year? • Hypothesize why these regions rank in this order? How do Thunderstorms Form? Thunderstorms requires 3 ingredients to form: –Source of moisture –A lifting mechanism –Unstable atmosphere Sources of Moisture • Typical sources of moisture are large bodies of water such as the __________ and Pacific Gulf of Mexico _________oceans as well as the ____________ Atlantic Water temperature affects how much moisture is in the atmosphere • Warm water puts more moisture into the atmosphere than with cold water b/c…? • Warm water’s higher thermal energy makes evaporation from liquid water to vapor easier • Warm ocean currents occur along east coasts of continents (including the Gulf). • Cool ocean currents occur along west coasts. Water Sources for thunderstorms and their relative temperatures Cool ocean currents in Pacific Warm ocean currents from Gulf and Atlantic • What factors contribute to the southeastern U.S. having a higher number of thunderstorms? • Increased moisture in atmosphere from the warmer waters of the gulf and Atlantic moisture sources. How do Thunderstorms Form? Remember…Thunderstorms requires 3 ingredients to form: – Source of moisture – Lifting mechanisms – Unstable atmosphere Lifting Mechanisms • Basic premise – moist air must rise and condense for thunderstorms to develop. • Mechanisms that cause air to rise? • Uneven heating of Earth’s surface creates warmer air that is less dense than surrounding air, and thus it rises, cools, and may condense • Orographic lifting – air forced up over landforms • Fronts – particularly cold fronts for thunderstorms How do Thunderstorms Form? Remember…Thunderstorms requires 3 ingredients to form: – Source of moisture – Lifting mechanisms – Unstable atmosphere Unstable Atmosphere Simply put, “unstable” air is air that is moving vertically. • An unstable air mass is characterized by warm moist air near the surface and cold dry air aloft. • The warm moist air is less dense and rises. As it rises it cools and some of the water vapor condenses, forming a cumulus cloud. • Condensation releases latent heat and allows continued lifting which can result in tall cumulonimbus cloud formation. 13-1 Thunderstorms Learning Targets •I can identify the processes that form thunderstorms •I can describe the life cycle of a thunderstorm - (a.k.a. three stages) Life Cycle of a Thunderstorm • Three stages: – Cumulus Stage – Mature Stage – Dissipation Stage Cumulus Stage • Characterized by updrafts • Water vapor is transported to upper, cooler regions. • The water vapor condenses forming a cumulus cloud. • Condensation releases latent heat. • Latent heat fosters continued lifting, more condensation, more latent heat, and so on. Cumulus Stage Air within the cloud is dominated by updrafts Mature Stage • Strong updrafts and downdrafts side by side in the cumulonimbus • Precipitation cools the air as it falls. • Newly cooled air sinks with precipitation creating downdrafts. • The convection of updrafts and downdrafts create strong surface winds. • The friction between the up and downdrafts create separate areas of charge (lightning potential) Mature Stage • Strong updrafts and downdrafts coexist. • Most dangerous stage- wind-lightening Dissipation Stage • Cool downdrafts spread in all directions when they reach Earth’s surface. • This cools the areas the storm draws its thermal energy from. • The updrafts cease and clouds can no longer form. Dissipation Stage Cooling rain and downdrafts cools the surface area that was providing the thermal energy for updrafts. This stage ends when the last precipitation reaches Earth.