Weather and Climate Lecture 13: Severe Weather and Hurricanes
Thunderstorms • A thunderstorm is simply a storm containing lightning and thunder. • Thunderstorms occur all over the world
Formation - Ingredients • Moisture (water vapor) in the lowest levels of the atmosphere • Cold, dry air aloft (2-3 miles above the surface) • Lifting mechanism
Air Mass (Single-Cell) Thunderstorms • Formed by uneven heating of the earth’s surface • Brief, but well-defined lifespan with three stages • Cumulus Stage • Mature Stage • Dissipating Stage
Mature Thunderstorms • The updraft can become so strong that it penetrates into a region of stable air, resulting in a overshooting top.
Lifespan of a Thunderstorm • A single cell, consisting of an updraft and downdraft, lasts 20 minutes. • But we have all observed thunderstorms that appear to last longer than 20 minutes…. WHY?
Multicell Storms • Sometimes when the cold downdraft reaches the surface, it may force the warm, moist surface air upward • This rising air can condense and build a new thunderstorm Multicell Cluster Multicell Line (Squall Line)
Straight-Line Winds • Associated with a cluster of severe thunderstorms • May exceed 90 knots • If wind damage extends 250 miles along storm path, it is called a derecho.
Tree Damage What does it look like from a downburst vs. a derecho?
Severe Thunderstorm • Hail ¾” or greater • Winds in excess of 50 knots (57.5 mph) • Tornado
Capping Inversion • An important component of creating a severe thunderstorm is the strength of the boundary layer cap. • Too strong nothing develops • Too weak numerous cumulus clouds • Just righthelps to funnel the air into a strong updraft
Tornado Wind Speed As the tornado moves along a path, the circular tornado winds blowing opposite the path of movement will have less speed. For example, if the storm rotational speed is 100 knots, and its path is 50 knots, it will have a maximum wind of 150 knots on its forward rotation side. Figure 15.31
Suction Vortices • Many violent tornadoes contain small whirls inside them.
Tornadoes vs. Mesocyclone • Development of mesocyclone • Stretching – gives rotating air a faster spin
Supercell View from East
Tornadoes • Vertical Wind Shear (pencil example) • Horizontal roll meets updraft and is tilted vertically
Rotating Clouds as Tornado Signal The first sign that a supercell may form a tornado is the sight of rotating clouds at the base of the storm, which may lower and form a wall cloud, shown in this picture. Figure 15.41