GEOG 1114 1st Edition Lecture 15 Outline of Last Lecture Atmospheric Disturbances Air masses and Fronts definitions characteristics Types of Air Masses Types of Fronts Outline of Current Lecture Atmospheric Disturbances continued Cyclones Anticyclones Low Latitudes Minor Atmospheric Disturbances Current Lecture Cyclones Characteristics Development Consist of convergent surface air that ascends and diverges in the upper atmosphere Large migrating or traveling low pressure systems up to 1000 miles Called depressions in Europe Develop in mid latitude zones as a result of converging counterclockwise air flow in N H that attracts cool air from the north and warm air from the south Carried by the westerlies Dominate day to day weather and bring precipitation These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute When a front passes over temperature decreases sharply winds sift pressure falls clear skies are replace by cloudiness and precipitation Life Cycle of a Mid Latitude Cyclone Front develops Waves appear Cyclonic circulation is well developed Occlusion begins Occluded front fully developed Cyclone dissipates Occurrence and Distribution of Mid Latitude Cyclones At any given time there can be from 6 to 15 mid latitude cyclones in the Northern Hemisphere Same for Southern Occur in westerlies move generally from west to east More numerous and faster moving in winter than in summer Anticyclones Consists of subsiding air from aloft and diverging at surface Descending air is warmed adiabatically thus reducing the moisture content and produce clear skies and fair weather Develop over arctic regions during the winter They bring cold dry fair weather when they descend In summer high pressure systems are associated with subtropical highs that form in the Caribbean These descend into the eastern third of the US bringing moist maritime tropical air into the US In Low Latitudes Weather in the Low Latitudes is characterized by Monotony Any changes in weather in the continental tropics results from the daily cycle of heating and cooling of moist air Many regions have strong seasonal contrasts and exhibit weather phenomena found nowhere else in the world Tropical Hurricanes Intensive low pressure centers develop in late summer Characterized by steep pressure gradients high wind speeds and circulation If wind speeds 75 mph these cyclonic disturbances are classified as tropical hurricanes Develop when oceans reach their maximum temperature In NA and Caribbean Hurricane In W pacific Typhoons In Indian Ocean Australia Cyclones Have non stormy eye Start as tropical disturbance form in 0 to 3 latitude There are as many as 100 year but only a few have the energy wind speeds and circulation to develop into hurricanes Minor Atmospheric Disturbances Thunderstorms Violent convective storms accompanied by thunder and lightning Associated with vertical air motion high humidity and instability which are conditions that produce towering cumulonimbus clouds and pptn in response to frontal uplift Sometimes produced by individual clouds formed by convective heating and uplift summer Often produced with orographic and convergent uplift as well and can be associated and can accompany hurricanes Tornadoes an intense vortex as deep low pressure cell surrounded by a violently whirling cylinder of wind Small and localized but can be very destructive Associated with severe thunderstorms activity Tornadoes break out most commonly along thunderstorm or squall lines ahead of the cold fronts where temperature contrasts are large and instability great Vortex develops in the cumulonimbus clouds associated with these fronts in association with a vertical wind shear a significant change in wind direction from the bottom to tope updraft characterized by circulation These storms have the most extreme pressure gradients known as much as 100 milibar difference from the center of the tornado to the air just outside the funnel This pressure gradient produces rotating winds of extraordinary speeds called a mesocyclone The tornado extends down from the wall cloud and as it approaches the surface it is retarded by friction and may take a course of its own They occur at random and become visible when water vapor condenses producing the familiar funnel could Dust and debris swept up form the ground create a much darker and more ominous looking funnel Typically advance at a speed of 20 30 mph Its track can be about 3 6 miles wide but a few are much longer They often appear in late afternoon but many occur during night 90 of all tornadoes occur in US most of which occur in Oklahoma