1. Cyclones and Anticyclones September 19, 2007

2. Name that cloud

3. Atmospheric Pressure Systems High-pressure system -also called anticyclone -circulating body of air -descending air -clockwise circulation in northern hemisphere

5. Low-pressure system -also called cyclone -circulating body of air -rising air -counterclockwise circ. in northern hemisphere

7. Semi-Permanent Pressure Cells

8. Thunderstorms A typical thunderstorm lasts a few hours

9. Severe Thunderstorms Can be single-cell or multi-cell storms Anticyclonic and cyclonic circulation within the cell - supercell Updrafts into the storm can reach 170mi/hr As supercells move, the anticyclonic an cyclonic circulation can separate creating 2 supercells This is how squall lines and mesoscale convective complexes (MCCs) are created

10. Supercell Thunderstoms Can last 12 hours Squall lines and MCCs can last longer Most common during afternoon and early evening The sun heats the ground and that heat rises to the lower atmosphere This intensifies the contrast between air temperatures on either side of the coming cold front

11. Super- cell

12. Supercell

14. Hail In a supercell, powerful updrafts carry water droplets to great altitude The cold high-altitude air freezes the water The frozen water then serves as a condensation nuclei for other water droplets forming hail Hail is cycled through the cloud growing larger It falls to the ground when it is heavier than the force of the updraft

15. Hail Can be pea to baseball sized Occur most often between April and September Peak between June and July Can occur in late fall and early spring, but usually very small

16. Thunderstorm Wind Straight-line (nontornadic) winds called derechos – very powerful and damaging Gust front – rush of cool air derechos moving ahead of the storm – can increase by 60mi/hr in seconds As rain begins, wind speed decreases Microbursts also common

17. Lightning Most dangerous aspect of a thunderstorm for human life More dangerous than tornadoes The Troposphere and Ionosphere are excellent conductors of electric current in the atmosphere Between them is the Stratosphere, which is a terrible conductor of electric current

18. Lightning When a poor conductor is between two good conductors, a capacitor is created A capacitor has the ability to store electrostatic charge The earth-ionosophere capacitor is 8000 mi in diameter and surrounds the Earth Vertical air currents and areas of high water vapor content produce channels of high conductivity

19. Lightning Cumulonimbus clouds that reach the Tropopause present a path for electrical discharge Discharge can occur slowly or very quickly, as lightning strokes Our atmosphere is constantly dissipating 450 megawatts of power