Cyclones and Anticyclones September 19 2007 Name that

Cyclones and Anticyclones September 19, 2007

Name that cloud

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

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

Semi-Permanent Pressure Cells

Thunderstorms • A typical thunderstorm lasts a few hours

Severe Thunderstorms • Can be single-cell or multi-cell storms • Anticyclonic and cyclonic circulation within the cell - supercell • Updrafts into the storm can reach 170 mi/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

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

Supercell

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

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

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 60 mi/hr in seconds • As rain begins, wind speed decreases • Microbursts also common

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

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

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