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 innorthern 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 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

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

Super-cell

Supercell

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 60mi/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