• Last warning: Exam 3 Monday, April 9: end of volcanoes, tsunami, and weather through tornadoes; review tomorrow, 5.15 PM, UPSTAIRS in Rm 223.

• Exam 4 is on the website (as always, only partially applicable)

ANNOUCEMENTS

homework

•If you have not yet done the homework, you do have until 5 PM; be sure not to have any spaces in the URL (if you copy and paste, you might see %20recent -- the %20 has to go for the website to load). So: http://earthquake.usgs.gov/eqcenter/recenteqsww/Quakes/us2007aqbk.php#summary

A couple of clicker review Qs

A cold front means that

1. A cold air mass is moving into a warm air mass

2. A warm air mass is moving into a cold air mass

3. Two air masses are next to each other, but neither is really moving

Which hemisphere…

1. Northern2. Southern

severe weather (the good stuff)

• defined as– thunderstorms– tornadoes– hurricanes– blizzards – heat waves– dust storms

thunderstorms

• we have an average of 40 of them per year; the high is central/southern Florida (>100/year)

• three critical conditions– water (vapor) in the lower atmosphere– change in temperature with altitude so that air

cools rapidly– air moving rapidly upward to take warm, moist air

up into the upper troposphere

1) moist air

need moisture in upper atmosphere, so need it to come from lower atmosphere

2) temperature change

if warm air cools slowly (or if there isn’t enough moisture) the forming cloud will evaporate as fast as it forms

3) updraft

taking the moisture upward has to be a continuing process

so what actually happens?

• rising moisture in warm air condenses

• growing cumulus cloud becomes cumulonimbus

• stage lasts about 10 minutes

• some lightning, little/no rain, but precipitation forming

Stage 1: developing stage (cumulus stage)

so what actually happens?

• mass of frozen precipitation becomes heavier than the cloud

• rising and falling masses of air

Stage 2: mature stage

• stage lasts about 20 minutes (can be much longer)

• black or dark green cloud• thunder/lightning/rain/

tornadoes

approximate altitude ofthe tropopause

so what actually happens?

• downdraft > updraft• rain decreases

Stage 3: dissipating stage

• lightning still a hazard• cloud dissipates

Hail….

• round or irregular pieces of ice• concentric rings

(This is in a person’s hand…)

Hailstone size Size (“) Updraft wind speed

pea 1/4 24 mph

penny 3/4 40 mph

quarter 1 49 mph

walnut 1 1/2 60 mph

golf ball 1 3/4 64 mph

tennis ball 2 1/2 77 mph

baseball 2 3/4 81 mph

tea cup 3 84 mph

grapefruit 4 98 mph

http://www.lightningsafety.noaa.gov/science.htm

If you would like to read more about lightning, go here:

Tornadoes…

• We hold the record here in the US– generally ~1200/yr– avg. 70 fatalities

• How do they form?– the classic answer: "warm moist Gulf [of Mexico] air meets cold

Canadian air and dry air from the Rockies" -