Chapter 3 Notes
Weather
3.1 What is the weather
Weather – present state of the atmosphere
- Includes
o Air pressure (mB)
o Wind
o Temp
o Humidity
- water cycle is the basis of weather
- sun
o causes heat
o evaporates water
o heats air and creates wind
- 3 components
o Air
o Sun
o Water
Humidity – the amount of water vapor in the air
- fluctuates due to temp
- increase temp, increase water vapor it can hold
- decrease temp, decrease water vapor it can hold
o relative humidity
 amount of water vapor in the air compared to what that temp
can hold
 12 C – 50%
 @ 25 C – 25%
 @ 50 C – 10 %
o if a higher temp has a high % of humidity:
 temp drops – air cannot hold all that water vapor
 turns to precipitation
o saturated air – 100% relative humidity
o dew point – the temp at which air is saturated & condensation
forms
 ex. Dew on grass
Cloud formation
- warm air rises then cools
- condenses in troposphere
- collects dust, salt & smoke particles in the atmosphere
- three types
o stratus
 smooth, even sheets
-
-
 air cools below its dew point temps
 low altitudes
 fair weather or precipitation
 steady drizzle
 fog – type of stratus cloud
o cumulus
 puffy & white
 air currents rise
 can get high in atmosphere
 fair & stormy weather
o cirrus
 fibrous & curly
 high, white, feathery clouds containing ice crystals
 fair weather
 indicate storms to come
cirro – high
alto – middle
strato – low
nimbus – dark clouds associated with precipitation
o no sunlight passes throu
o a towering rain cloud – cumulonimbus
 cumulo – big & fluffy
 nimbus – dark, rainy
o low dark, gray with long steady rain – nimbostratus
 nimbo – dark, rainy
 stratus – low attitude, blanket-like
precipitation – 0.2 mm in size at least
o air temps create the type
air temps:
o rain – above freezing
o snow – below freezing
o sleet – snow melts, refreezes
o hail – lump of ice, water freezes in layers, tossed around by rising
convection currents
13.2 Weather Patterns
Air mass – large body of air that has the same properties as Earth’s surface where
it develops
- over land is dry compared to over water
Pressure systems
- 2 types:
o High pressure
 Lots of molecules sinking
 Makes it difficult for clouds to form
 Sign of fair weather
o Low pressure
 Lots of room for molecules to rise and collect
 Cloudy weather
- Amount of air molecules that push down from above
- Determined by 3 things
o Temp
o Density
o Water vapor
Fronts – boundaries where cold and warm air masses meet
- storms & precipitation are typical
- low pressure systems evident
- air masses move from high to low
o warm to cold
- air doesn’t mix when they meet
o colder air moves under warmer air
o warm air rises
o winds form (from high to low pressure)
- 4 types of fronts:
o Warm
 Warm ari rises over cold air
 Precipitation occurs over a wide area
o Cold
 Cold air pushes under warm air up steeply
 Narrow band of strong storms
 Move at 2x the speed of warm fronts
o Occluded
 Faster moving cold front over takes a slower moving warm
front
 Warm air is forced up
 Strong winds
 Heavy precipitation
o Stationary
 Pressure differences cause the warm and cold front to stop
moving
 Light wind & precipitation
 Can cause flooding
Severe weather
Thunderstorms – warm, moist air masses and fronts
- cumulonimbus clouds form
- heavy droplets fall creating downdrafts
- strong winds associated with these types of storms
Lightning – rapid uplift of air builds electric charges in clouds
- positive and negative charges attract forming lightning
- can leap from cloud to cloud or from Earth to cloud
- thunder results from rapid heating of air due to lightning
o 30,000 C
o 5x + the surface of the sun
o Air expands rapidly then cools quickly
o Condenses and contracts creating a sound wave
Tornados – violent, whirling wind storms
- SW to NE travels
- Most form along a front
- Wind shear
o Difference in wind direction & speed
o Occurs @ different heights
o Strong updraft tilts wind shear & cause rotation in clouds
o Funnel cloud may apprear
o Not all reach/touch Earth’s surface
o Center has an updraft
o Don’t usually exceed 200m width
o Lasts only a few minutes (typically)
o Most occur in US (~700 per year)
 Texas, Oklahoma, Kansas
- NEXRAD and Doppler
o Next Generation Weather Radar
 Nationwide system of radar stations
 Use doppler
o Doppler
 Sends out radio waves continuously
 Tracks storm movement by way it bounces radio waves
 Higher frequency – storm is moving towards radar
 Lower frequency – storm is moving away from radar
 Use colors to indicate frequency fluctuations
 Green – closer
 Red – away
 The two close together indicates rotation
Hurricanes – large, swirling, low-pressure system that forms over tropical
oceans
- most powerful storms
- winds must reach up to 120km/hr to be categorized as a hurricane
- can travel on land
- rotates counter-clockwise in N. hemisphere