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Earth / Environmental Science Ch. 20 – WEATHER PATTERNS & SEVERE STORMS AIR MASSES • • • In middle latitudes, summer heat waves and winter cold spells are familiar experiences Summer waves - high temperature and high humidity, (few days it has stormy weather & relatively cool water) Winter cold spell (or waves) - characterized by period of frigid temperature under clear skies, usually followed by cloudy, snowy and relatively warm weather. Air Masses and Weather • • Immense body of air that is characterized by similar temperature and moisture at a given latitude It can be 1600km (or more) across and several kilometers thick Movement of Air Mass • • Through out air mass is going southward, it becomes warmer. (See Figure 2, p.559) As it moves the characteristic of an air mass change and so does the weather in the area which the air mass moves. Classifying Air Masses • • • Air masses that influence the weather in North America are mP, cP, mT, cT (see Figure 3, p.560) Polar (P) air mass forms at high latitude (toward Earth’s poles) and its character is cold Tropical (T) air mass forms at low latitude and its character is warm • • • Continental (c) air mass forms at land and its character is dry Maritime (m) air mass forms at water and its character is humid (P & T indicates its temperature; c & m indicates the surface where it was formed) Weather in North America • • • Influenced by cP & mT air masses cP is from North (e.g., Canada, Alaska, Arctic area). mT is from warm water (e.g., Gulf of Mexico, Caribbean Sea, and the adjacent Atlantic Ocean) Continental Polar Air Mass (cP) • • • Uniformly cold and dry winter and cool and dry in summer It brings clear skies and cold temperatures (associated with cold waves) The difference in temperature between Great Lakes and adjacent land area (especially when cP air mass pushes southward across the lake) produces heavy snow Maritime Tropical Air Mass (mT) • • • Warm and loaded with moisture; usually stable. mT air is source of much of the precipitation received in eastern 2/3 of U.S. In summer, it invades the central and eastern U.S. and brings high temperature and oppressive humidity Maritime Polar Air Mass (mP) • • • • When cP (cold, dry, stable) is crossing North Pacific, it changes and becomes mP (mild, humid, unstable) (see Figure 7, p.562) It is often accompanied by low clouds, showers When it advances inland against the western mountains, uplift of the air produces heavy precipitation on windward slope of mountain In winter, at northeastern U.S., nor’easter (northeaster) is formed Continental Tropical Air mass (cT) • • • It least influences the weather of North America Only occasionally do cT air masses affect the weather outside their source region It causes extremely hot, drought-like conditions in Great Plain & it results in mild weather in Great Lake region (Indian summer) FRONTS Formation of Fronts • • • It forms between two contrasting air masses. When two air masses meet, they form a front (which is a boundary that separates two air masses) When air masses on both sides are moving and going same, it acts like a barrier However, in most cases the distribution of pressure across front causes one air mass to move faster than the other. Type of Fronts - Warm Fronts • • • • It forms when warm air moves into an area formerly covered by cooler air Its movement is slow so, its slope is very gradual (see Figure 10, p.565) and it cools to produce clouds and frequently precipitation (at large area for extended period) Clouds change from Nimbostratus (Ns) Altostratus (As) Cirrostratus (Cs) Cirrus (Ci) A gradual increase in temperature occurs with the passage of a warm front. Type of Fronts - Cold Fronts • It is formed when cold, dense air moves into a region occupied by warmer air. (See Figure 11 p.566) • When the cold front moves, it becomes more rapidly & steeper than warm front Cold front can lead to heavy downpours, gusty winds and towering clouds often can be seen in distance After cold front passes, weather clears soon and temperatures drop and wind shifts • • Type of Fronts - Stationary Fronts • The flow of air on either side of front is neither toward the cold • air mass nor toward the warm air mass In such cases, the surface position of the front doesn’t move, and stationary front forms. Type of Fronts - Occluded Fronts • • • When an active cold front overtakes a warm front, an occluded front forms (see Figure 12, p.567) It develops as the advancing cold air wedges the warm front upward. Most precipitation is associated with the warm air being forced upward Middle Latitude Cyclones • • • • • Middle-latitude cyclones are large centers of low pressure that generally travel from west to east and cause stormy weather Most of it has a cold front, extending from the central area Its airflow is in a counterclockwise direction and in toward the center of the low pressure See the Figure 14A, p.569 (First, front is formed; second, over time it takes wave shape; third, air flow and pressure change resulting in a counterclockwise flow of air And cold front closes in on the warm front and cold front lift produces an occluded front and finally it weakens). The Role of Airflow Aloft • • • These rotating surface wind systems are actually generated by upper-level flow More often than not, air high up in the atmosphere fuels a middle-latitude cyclone The surface air that feeds cyclone generally originates as air flowing out of an anticyclone (see Figure 15, p.570) SEVERE STORMS Thunderstorms • • A thunderstorm is a storm that generates lighting and thunder. It frequently produces gusty winds, heavy rain, and hail Produced by a single cumulonimbus cloud Occurrence of Thunderstorms • • • • 2000 thunderstorms in progress on Earth at any one time. More that 16 million occur annually around the world At tropics (where it is warm, and there is plenty of moisture instability) about 45,000 thunderstorms occur. United States - 100,000 thunderstorms each year (frequently Florida, Gulf Coast) Development of Thunderstorms • • • Thunderstorms form when warm, humid air rises in unstable environment. Thunderstorms develop in this order: o First, cumulus stage (which involves strong updraft (or upward movements of air) and supply warm and moist air). o Second, mature stage (which point in the development of thunderstorm) at this point, thunderstorm is most active (Gusty wind, lightning, heavy precipitation, hail are produced). o Third, dissipating stage - at this time downdraft (or downward movement of air) throughout the cloud. During this final stage, the air which was cooling by precipitation and high above colder air makes storm die down (see Figure 17 A,B and C) Tornadoes • These are violent wind storms that take the form of a rotating column of air called a vortex. • The vortex extends downward from a cumulonimbus cloud. Occurrence and Development of Tornadoes • • • • In U.S. in April through June, the frequency of storms is greatest December and January is less frequent. Most tornadoes form in association with thunderstorms Important process in the formation of many tornadoes is the development of a mesocyclone - which is a vertical cylinder of rotating air that develops in the updraft of a thunderstorm (it causes wind lower in the atmosphere to roll). (see Figure 18 p573) Tornado intensity • • • • Pressures outside of the storm are higher than inside storm (10% more) This difference causes air to rush into tornado from all directions As the air streams inward, it spirals upward around the core. Eventually, the air merges with the airflow of the cumulonimbus cloud that formed the storm. Because of the tremendous amount of pressure change associated with a strong tornado, maximum wind can sometimes approach 480 kilometers per hour. Fujita scale – used to estimate tornado intensity (See Table 1 p.574) Hurricanes • Whirling tropical cyclones that produce winds of at least 119km/h. Occurrence of Hurricanes • • Most hurricanes form 5-20 degrees North & South latitude. (especially North Pacific is greatest) Coastal region of southern and eastern U.S. experience fewer than five hurricanes per year. Development of Hurricanes • • • • Hurricanes develop most often in the late summer when water temperatures are warm enough to provide the necessary heat and moisture to the air It begins as a tropical disturbance (depression) An inward rush of warm, moist surface air moves toward the core of storm The air then turns upward and rises in the eye wall (which is a doughnut-shaped wall that surrounds the core) The eye of the hurricane (which is the center point) is where precipitation ceases and winds subside. This is the warmest part of the storm. Hurricane Intensity • • • Using Saffir-Simpson Hurricane Scale (See Table 2 p.577) When hurricane eye moves onto land, storm surge (which is 65~80km wide dome of water) is created. When a hurricane moves over the cool ocean, it is weakened because the cool ocean cannot supply adequate heat and moisture.