Middle School SCIENCE -- Mrs. White

-      Weather influenced by air 1000’s of miles away

-      ∆’s in weather caused by movement & interactions of air masses

-      Air mass – lg. body of air w/ similar temp., humidity (moisture content), & air pressure

· Classified by: temperature & humidity 

§ Characteristics depend on region where air mass is formed – source area :     (See Fig.1, p.46)

§ Represented on map by 2-letter symbol:

s 1^st letter = moisture content (humidity):

~    Maritime (m) ­­– forms over ocean: humid

~    Continental (c) – forms over land: dry

s 2nd letter = overall temp. of air mass:

~    Polar (P) – forms in polar regions: cold (H air pres.)

~    Tropical (T) ­­– forms in tropics: warm (L air pres.)

· Cold air masses: 2 types influence U.S. weather:

§ continental polar (cP) forms over Canada – cool, dry air

§ maritime polar (mP) forms over: North Pacific Ocean & North Atlantic Ocean – cool, humid air

· Warm air masses: 2 types influence U.S. weather:

§ maritime tropical (mT) forms over warm Pacific Ocean, Gulf of Mexico, & Atlantic Ocean – warm, humid air

-      Air masses are moved by global winds & jet streams (prevailing westerlies)

-      Front – boundary, or area where 2 air masses meet, but do not mix

·     Air masses have diff. densities & " do not usually mix:

§ Warm (less dense) air rises over cold air mass (denser)

§ Storms & changeable weather develop along fronts

·     Fronts found w/ weather in middle latitudes

·     4 types:

1)   Cold front

§ Forms where cold (dense) air moves under warm (less dense) air

§ Warm air is pushed up

§ Moves quickly

§ Brings thunderstorms cold, heavy rains, snow

§ Cooler weather follows

2) Warm front

§ Forms where warm (less dense) air moves over cold (dense) air

§ Warm air gradually replaces cold air

§ Moves gradually

§ Brings drizzly rain

§ Clear & warm weather follows

3) Occluded front

§ Forms when warm air moves is caught bet. 2 colder air masses

§ Coldest masses moves under warm, pushing it up, meeting other cold air masses

§ Cold air masses may mix; warm air is cut off

§ Brings lg. amt. of rain or snow

4) Stationary front

§ Forms when cold air mass meets warm air mass

§ Warm air mass is not pushed up

§ Air masses remain separated & do NOT mix

§ Brings days of cloudy, wet weather

-     Differences in air pres. affect weather:

·     Cyclone – area of low pres. where air masses come tog. (converges)

§     Winds spiral toward center, counterclockwise

§     Air rises & forms clouds & rain as it cools

·     Anticyclone – area of high pres. where air moves apart (diverges)

§     Winds spiral outward, clockwise toward low pres. area

§     As air sinks, it gets warmer ® dry, clear weather

-  Clouds

·        Collection of millions of tiny H₂O droplets (liquid) or ice crystals (solid)

·        Form as warm air rises & cools ® saturated air ® liquid/solid

·        Each type of cloud is associated w/ a diff. type of weather

·        Classified by form:

§        Cumulus – puffy, white clouds w/ flat bottoms

s        Forms when warm air rises

s        Indicates fair weather

s        Lg. clouds, often darker ® thunderstorms:

s        Cumulonimbus – type of cloud that causes thunderstorms  (cumulus  = heap or mass; nimbus = rain)

§        Stratus – clouds that form in layers

s        Cover lg. areas; may block sun

s        Indicates fair weather

s        Nimbostratus – dark stratus clouds that produce continuous rain (nimbus = rain)

s        Fog –  stratus cloud that forms near the ground   

§        Cirrus – thin, feathery clouds found at high altitudes

s        Form when wind is strong

s        Made up of ice crystals

·        Can be further classified by altitude (how high)

s        Cirro - form at high altitudes

s        Alto – form at middle altitudes

s        No special prefix for low forming clouds

-  Precipitation – H₂O in liquid or solid form

·        Rain

§        Begins as a droplet (needs a surface, such as dust to form)

§        Falls when droplet becomes big enough & heavy enough to fall

§        Rain ³ 0.5mm; drizzle & mist < 0.5mm

§        Drizzle/mist fall from stratus clouds

·        Sleet – rain that falls thru layer of freezing air Þ ice balls

·        Freezing rain – rain that falls thru cold air but does not freeze until it touches a cold surface

·        Snow - H₂O vapor that converts directly into ice crystal

·        Hail – forms only in cumulonimbus cloud

§        Begins as small ice pellet

§        Layered: coated w/ H₂O as falls; carried up in updraft where it re-freezes; process repeats until too heavy