Final Exam Highlighted words represent italicized or bolded key words from the textbook These are significant Chapter 10 Thunderstorms Thunderstorms o Convective storms that form with rising air o Birth when warm moist air rises in a conditionally unstable environment o Thing that may cause air to rise Unequal heating at surface Effect of terrain lifting air along shallow boundaries of converging surface winds Diverging upper level winds coupled with converging surface winds and rising air Warm air rising along a frontal zone o Severe Thunderstorms are defined as having hail with a diameter of inch and or surface gusts over 50 knots o Scattered Thunderstorms form on warm humid days and are often referred to as ordinary cell thunderstorms because they form in humid air masses away from significant weather fronts o Ordinary Cell air mass Thunderstorms or simply ordinary cell thunderstorms tend to form in a region where there is limited wind shear where wind speed and wind direction do not abruptly change with increasing height above the surface o Cycle of development Ordinary Cell Thunderstorms Cumulus Stage growth stage as a parcel of warm air rises it cools and condenses into a single cumulus cloud or cluster As the cluster builds condensation gives off latent heat making the cloud less dense than the air surrounding it As the rising air can no longer stay suspended and falls the drier air is pulled into the cloud a process called entrainment This causes some raindrops to evaporate chilling the air The chilled air then begins to descend as a downdraft The appearance of downdrafts marks the beginning of the mature stage During the mature stage the thunderstorm is the most intense When the top of the storm begins to reach the stable part of the atmosphere it takes on the anvil shape spreading ice crystals horizontally When the downdraft reaches the surface a boundary between warm and cold air is created called a gust front Dissipating stage occurs 15 30 minutes after the mature stage when the updrafts weaken as the gust front moves away from the storm The downdrafts inside cut off the humid updrafts the storms fuel supply Multi cell Thunderstorms o Gust Front o Microbursts Multi cell thunderstorm is defined as one that contains a number of cells each in a different stage of development Tend to form in a region of moderate to strong vertical wind shear Rising air may intrude well into the stable atmosphere and create an overshooting top Below the downdrafts the dense air may cause surface pressure to rise causing a mesohigh The gust front is created as the leading edge of the falling down draftspush out in all directions High winds behind a strong gust front are called straight line winds to distinguish them from the rotating winds of a tornado As warm moist air rises along the forward edge of the gust front a shelf cloud may form prevalent when the atmosphere is very stable near the base of the thunderstorm Occasionally elongated clouds form just behind the gust front and appear to spin slowly called roll clouds When the atmosphere is conditionally unstable the leading edge of the gust front may force the warm air upward producing a complex of multi cell storms each with new gust fronts These gust fronts may then merge into a huge gust front called an outflow bound A microburst is a downburst with wind extending only 4 kilometers or less Beneath an intense thunderstorm the downdraft may become localized so that it hits the ground and spreads horizontally in a radial burst of wind these downdrafts are called downbursts Wind shear usually associated with microbursts is rapid changes in wind speed or wind direction Most downdrafts are cool but blasts of hot air from a dissipating thunderstorm can raise the surface temperature called heat bursts Multi cell thunderstorms may form as a line of thunderstorms called a squall line They can form along cold fronts and extend for hundreds of miles Or form in warm air 100 300 kilometers ahead of a cold front these prefrontal squall line thunderstorms represent the most severe type of squall line The surging nature of the main cold front causes the air to develop into gravity waves Downdrafts from the squall lines may concentrate into a rather narrow band of fast flowing air called a rear flank inflow jet If the rear flank inflow jets reach the surface they can rush outwards producing damaging straight line winds that may exceed 90 knots A bow echo may form as strong winds push the squall lines outwards so it appears as a bow on the radar When the damage associated with the straight line winds extends for a considerable distance along the squall line s path the windstorm is called a derecho o Squall Line Thunderstorms o Mesocale Convective Complexes MCC Mesoscale Convective Complexes occur when multi cell thunderstorms grow in size and organize into large circular convective systems Tend to form during the summer where the upper level winds are weak Super cell Thunderstorms rotating updraft o Super cell thunderstorms are large long lasting thunderstorms with a single violently o Forms in regions with strong vertical wind shear in such a way that the outflow of cold air from the downdraft never undercuts the updraft o Wind shear can create horizontal spin which can cause rotation therefore leading to the formation of tornadoes called the mesocyclone rain free base descend o The rotating air column on the south side of the storm usually 5 10 kilometers across is o The rotating updraft is so strong precipitation cannot fall through it and it creates a o If low level humid air is drawn into the updraft a rotating cloud called a wall cloud may o Above warm surface air a wedge or tongue of warm moist air is streaming northward It is in this region we find a relatively narrow band of strong winds called the low level jet o Super cells are likely to form where The position of cold air above warm air creates a conditionally unstable atmosphere Strong vertical wind shear induces rotation Thunderstorms and the Dryline o Thunderstorms may form along or just east of a boundary called a dryline The dryline is a narrow zone with a sharp horizontal change in moisture o In the United States drylines are most frequently absorbed in the western half of Texas Oklahoma and Kansas Thunderstorms and Flooding In this region drylines occur most frequently during spring and early summer o Intense thunderstorms can be associated with flash floods Flash floods are floods
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