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