Ch 20: Weather Patterns and Severe Storms Overview TAKE NOTES ON  bullet points!!

Air Masses and Weather Air Mass – immense body of air that is characterized by similar temperatures and amounts of moisture at any given altitude Because of its size, it may take several days for an air mass to move over an area (giving that area fairly constant weather) When an air mass moves out of the region over which it formed, it carries its temperature and moisture conditions with it As it moves, the characteristics of an air mass change, and so does the weather in the area over which the air mass moves

Influence of a Canadian Air Mass

Classifying Air Masses The area over which an air mass gets its characteristic properties of temperature and moisture is called its source region Air masses are named according to their source region Polar (P) air masses (cold air) form at high latitudes, while tropical (T) air masses (warm air) form at low latitudes In addition to their overall temperature, air masses are classified according to the surface over which they form Continental (c) form over land (dry air) and maritime (m) form over water (humid air) The four basic types of air masses in North America: Continental Polar (cP) – cold and dry Continental Tropical (cT) – warm and dry Maritime Polar (mP) – cold and moist Maritime Tropical (mT) – warm and moist

Air Mass Source Regions

Weather in North America Much of the weather in North America, especially weather east of the Rocky Mountains, is influenced by continental polar (cP) and maritime tropical (mT) air masses Continental polar air masses are uniformly cold and dry in winter and cool and dry in summer; lake-effect snow is caused when one of these air masses passes over the moisture of the lake and drops the precipitation down on the leeward side of the lake Maritime tropical air masses are warm, loaded with moisture, and usually unstable; they are the source of much of the precipitation in the eastern two thirds of the U.S. During the winter, maritime polar (mP) masses that affect weather in North America come from the North Pacific Continental tropical air masses have the least influence on the weather of North America Only occasionally do continental tropical (cT) air masses affect the weather outside their source region

Continental Polar and Maritime Polar Air Masses

Formation of Fronts When two air masses meet, they form a front, which is a boundary that separates two air masses Fronts can form between any two contrasting air masses In contrast to the vast sizes of air masses, fronts are narrow (~15-200 km wide) In the ideal case, the front would move in the same direction with warmer air overlying cooler air However, the distribution of pressure across a front causes one air mass to move faster than the other, causing it to advance into the slow air mass

Warm Fronts A warm front forms when warm air moves into an area formerly covered by cooler air The surface position of a warm front is shown by a red line with semi-circles that point toward the cooler air The first sign of the approaching warm front is the appearance of cirrus clouds Because of their slow rate of movement and very low slope, warm fronts usually produce light-to-moderate precipitation over a large area for an extended period A gradual increase in temperature occurs with the passage of a warm front, as does a wind shift from the east to the southeast

Warm Front

Cold Fronts A cold front forms when cold, dense air moves into a region occupied by warmer air On a weather map, the surface position of a cold front is shown by a blue line edged with blue triangles that point toward the warmer air mass As a cold front moves in, it becomes steeper and advance more rapidly than warm fronts The forceful lifting of air along a cold front can lead to heavy downpours and gusty winds As a cold front approaches, towering clouds can often be seen in the distance The weather behind a cold front is dominated by a cold air mass, so weather clears shortly after the front has passed

Cold Front

Stationary Fronts Occasionally, the flow of air on either side of a front is neither toward the cold air mass nor toward the warm air mass, but almost parallel to the line of the front The surface position of the front does not move, and a stationary front forms On a weather map, stationary fronts are shown by blue triangles on one side of the front and red semicircles on the other Sometimes, gentle to moderate precipitation occurs along a stationary front

Occluded Fronts When an active cold front overtakes a warm front, an occluded front forms The weather associated with an occluded front is complex: most precipitation is associated with the warm air’s being forced upward Remember, fronts, like all aspects of nature, do not always behave as we expect them to

Formation of an Occluded Front

Middle-Latitude Cyclones The main weather producers in the U.S. are middle-latitude cyclones, denoted by the letter L Middle-Latitude Cyclones – large centers of low pressure that generally travel from west to east and cause stormy weather The air in these weather systems moves in a counterclockwise direction and in toward the center of the low Most middle-latitude cyclones have a cold front, and frequently a warm front, extending from the central area Forceful lifting causes the formation of clouds that drop abundant precipitation

Middle-Latitude Cyclone Model

Cloud Patterns Associated With Middle-Latitude Cyclones

The Role of Airflow Aloft Airflow aloft plays an important role in maintaining cyclonic and anticyclonic circulation The rotating wind systems are actually generated by upper-level flow Surface convergence must be offset by outflow somewhere higher in the atmosphere As long as the spreading out of air high up is equal to or greater than the surface inflow, the low-pressure system can be sustained More often than not, air high up in the atmosphere fuels a middle-latitude cyclone In an anticyclone, air spreading out at the surface is balanced by air coming together from high up

Role of Airflow Aloft in Cyclonic Activity

Thunderstorms Thunderstorm – a storm that generates lightning and thunder; frequently produce gusty winds, heavy rain, and hail Thunderstorms may be produced by a single cumulonimbus cloud, or it may be associated with clusters of cumulonimbus clouds along a cold front At any given time, there are an estimated 2000 thunderstorms in progress on Earth (~45,000 a day and 16 million a year); the U.S. experiences ~100,000 a year Thunderstorms form when warm, humid air rises in an unstable environment The life span of a single cumulonimbus cell within a thunderstorm is only about an hour or two, but as the storm moves, it is constantly getting fresh supplies of warm, humid air

U.S. Distribution of Thunderstorms

Stages in the Development of a Thunderstorm

Tornadoes Tornadoes – violent windstorms that take the form of a rotating column of air called a vortex; the vortex extends downward from a cumulonimbus cloud In the U.S., ~770 tornadoes are reported each year; greatest occurrence between April and June Most tornadoes form in association with severe thunderstorms An important process in the formation of many tornadoes is the development of a mesocyclone (see diagram) Pressures within some tornadoes can be as much as 10% lower than the surrounding area, causing air near the ground to be “sucked” into the vortex One scale used to measure tornado intensity is the Fujita tornado intensity scale

Tornado Suction Vortices

Formation of a Mesocyclone

U.S. Tornado Incidences

Hurricanes (2) Hurricanes – whirling tropical cyclones that produce winds of at least 119 kilometers per hour (also known as typhoons, cyclones, and tropical cyclones) At sea, they can generate 15-meter high waves capable of destruction hundreds of kilometers away A hurricane is a heat engine that is fueled by the energy given off when huge quantities of water vapor condense Hurricanes develop most often in the late summer when water temperatures are warm enough to provide the necessary heat and moisture to the air

Eye Wall – doughnut-shaped wall that surrounds the center of the storm, where the greatest wind speeds and heaviest rainfall occur Eye – center of the storm where precipitation ceases and winds subside The intensity of a hurricane is described using the Saffir-Simpson scale Storm Surge – a dome of water about 65 to 80 km wide that sweeps across the coast where a hurricane’s eye moves onto land A hurricane weakens when it moves over cooler ocean water and land

Hurricane Cross-Section

Hurricane Katrina

Assignment Read Chapter 20 Do Ch. 20 Assessment #1-37 (pg. 583-584) #1-4 (pg. 585) Study for Chapter 20 Quiz!