+ What’s in a Name? Cyclone refers to the circulation around a low-pressure center. Hurricanes Midlatitude cyclones Tornadoes
+ Thunderstorms Thunderstorms generate lightning, thunder, winds, and hail. There are two types of thunderstorms. Air-mass thunderstorms are associated with warm, humid air that rises in unstable environments; cumulonimbus clouds, and mT (maritime tropical) air masses. Severe thunderstorms may produce high winds, hail, flash floods, and tornadoes. They are the result of uneven heating, frontal lifting, and diverging winds.
+ Air-Mass Thunderstorms Air-mass thunderstorms—mT air masses
+ Air-Mass Thunderstorms Cumulus stage: Warm, humid air rises creating clouds that won’t evaporate. A cumulonimbus tower develops as water vapor moves from the surface to greater heights. Clouds pass freezing zone and Bergeron process starts. Rain accumulation is too great for updraft so it falls creating a downdraft via entrainment (influx of cool dry air. Remember cold air gets heavy and sinks).
+ Air-Mass Thunderstorms Mature stage: Thisis the most intense phase. This results in heavy rain and possibly small hail. Cool downdrafts exist next to updrafts. Dissipating stage: Thisstage is dominated by downdrafts and entrainment causing evaporation.
+ Air-Mass Thunderstorms Occurrence: Mountainous regions, such as the Rockies and the Appalachians, experience a greater number of air-mass thunderstorms.
+ Severe Thunderstorms Severe thunderstorms: Heavy downpours Flash flooding Straight line wind gusts Hail, lightning Wind shear- change in wind speed or direction at different heights Can overshoot (enter stratosphere) Downdraft preceding (gust front)
+ Supercell Thunderstorms Supercells These storms can produce extremely dangerous weather. They consist of a single, powerful cell that can extend to heights of 20 km or more. The clouds can measure 20–50 km in diameter. Mesocyclone: Verticalwinds may cause the updraft to rotate, which forms a column of cyclonically rotating air. Tornadoes often form.
+ Supercell Thunderstorms Squall lines: Squall lines are narrow bands of thunderstorms. cT (continental tropical) air is pulled into the warm sector of a midlatitude cyclone. Mammatus skies sometimes precede squall lines. (dark cloud rolls that have downward pouches) These can also form along a dryline, where there is an abrupt change in moisture.
+ Lightning and Thunder Lightning strokes: The flash (total discharge) lasts a few tenths of a second. It is what we see and it contains multiple strokes. The leader is the ionized air, which forms a conductive path. A step leader extends earthward in a short, nearly invisible burst. The return stroke extends upward from ground to cloud.
+ Lightning and Thunder Thunder: The air is heated quickly to as much as 33,000°C. It expands explosively, which produces sound waves that travels at 330 mps. If lightning is more than 20 km away, thunder is not heard.
+ Lightning and Thunder What causes lightning?
+ Tornadoes Tornadoes (twisters, cyclones): These are violent windstorms with a rapidly rotating column of air, or vortex. Pressures within tornadoes can be as much as 10% lower than immediately outside the storm. It may consist of single or multiple vortices.
+ The Development and Occurrence of Tornadoes Mesocyclones form Winds are stronger aloft producing rolling motion about a horizontal axis Stronger thunderstorm updrafts tilt the horizontal rotating air to a nearly vertical alignment
+ The Development and Occurrence of Tornadoes The mesocyclone, vertical cylinder of air, is established. It stretches vertically and narrows horizontally causing wind speeds to accelerate inward creating a vortex. FunnelCloud- vortex emerges from cloud Tornado- vortex touches the ground
+ The Development and Occurrence of Tornadoes Profile of a tornado: Average diameter 150–600m Travels ~45 kph Path about 26 km long Most travel to the NE Exist between <3 min to >3 hours Wind speeds between <150 kph to >500 kph Between 90- 300mph
+ Profile of a Hurricane Hurricanes: Hurricanes are intense centers of low pressure. They form over tropical/subtropical oceans. These storms have intense convective activity and rotary circulation and can have wind speeds in excess of 74mph. They are usually 100–1500 km in diameter, form between 5° and 20° latitude and have a steep pressure gradient. Hurricanes are called typhoons in the NW Pacific and cyclones in the SW Pacific and Indian Oceans.
+ Profile of a HurricaneEye wall: doughnut shaped wall of intense convective activity surrounding the eye.Eye: Center of hurricane where wind and rain cease
+ Hurricane Formation and Decay Hurricanes: Hurricanes are fueled by the latent heat of condensed water vapor. Most are formed from late summer to early fall. Sea-surface temperatures of 27°C or higher provide the necessary heat and moisture.
+ Hurricane Formation and Decay Hurricane formation: Tropical disturbances, such as disorganized cloud arrays and thunderstorms, occasionally grow larger and develop strong cyclonic rotation. Easterly waves gradually move from east to west.
+ Hurricane Formation and Decay Hurricane formation: As areas within tropical disturbances get warmer, several factors help form hurricanes. Surface pressure drops creating a region of weak low pressure and cyclonic circulation. Higher pressure develops at the top of the storm causing an outward flow of air from the top. If wind speeds do not reach 63 kph, it is called a tropical depression. When winds exceed 61 kph, it becomes a tropical storm and is given a name.
+ Hurricane Formation and Decay Hurricane decay: Decay occurs when latent heat is cut off. A hurricane moves over cooler water or land. When large scale flow aloft is unfavorable, they diminish in intensity.