Transcript Understanding weather - Rock Springs Middle School

Water in the Air
 Weather is the condition of the atmosphere
at a certain time and place.
 The condition is affected by the amount of
water in the air.
The Water Cycle
 Water is
constantly
being
recycled
through
the water
cycle.
Humidity
 As water evaporates
from lakes oceans and
plants it becomes
water vapor.
 The amount of water
vapor in the air is
called humidity.
As the temperature increases, the amount of
water vapor the air can hold increases.
Relative Humidity
 Relative Humidity is the amount of water
vapor in the air compared with the maximum
amount of water vapor that the air can hold
at a certain temperature
Factors that affect Relative
Humidity
Amount of Water Vapor
 The more water vapor in
the air the higher the
relative humidity.
 The water vapor drops
the relative humidity
drops.
Temperature
 The relative humidity
decreases as the
temperature rises and
increases as the
temperature drops
Measuring Relative Humidity
 A psychrometer is an instrument that is used
to measure relative humidity.
– It consists of two thermometers: one wet bulb
thermometer and one dry bulb thermometer.
– The difference in the temperature readings
between the two thermometers indicates the
amount of humidity.
Measuring Relative Humidity
Condensation
 Condensation: the
process by which a
gas, such as water
vapor, becomes a
liquid.
 Condensation occurs
when saturated air (air
full of humidity) cools.
Dew Point
 Dew point: the temperature at which a gas
condenses into a liquid.
 Air is saturated at dew point
Clouds
 A cloud is a collection of millions of tiny
water droplets or ice crystals.
 Clouds form as warm air rises and cools
Clouds
Cumulus Clouds
 Puffy, white clouds that tend to have flat
bottoms
 Form when warm air rises
 Indicate fair weather
 When they get larger they produce
thunderstorms – cumulonimbus clouds
 Clouds with nimbus or nimbo will produce
precipitation
Stratus Clouds
 Stratus Clouds form in layers
 Cover large areas and often block out the
sun
 Caused by a gentle lifting of a large body of
air.
 Nimbostratus clouds are dark stratus clouds
that produce continuous rain.
 Fog is a stratus cloud found near the
ground.
Cirrus Clouds
 Cirrus Clouds are thin feathery white clouds
found at high altitudes
 Cirrus clouds form when the wind is strong
 They can indicate a change in weather.
Clouds and Altitude
 Clouds
are
classified
by the
altitude
that they
form
Cloud Classification
 Low clouds – have no prefix
– Ex. Cumulus, Stratus
 Middle clouds – Have the prefix alto
– Ex. Altocumulus, Altostratus
 High clouds – Have the prefix Cirro
– Ex. Cirrostratus, Cirrus
Precipitation
 Precipitation is water in solid or liquid form
that falls from the air to the Earth
 There are four major forms
– Rain
– Snow
– Sleet
– Hail
Rain
 The most common form of precipitation.
 Rain is produced when the water droplets
become a certain size.
Sleet and Snow
 Sleet forms when rain falls through a layer
of freezing air.
 Snow forms when temperatures are so cold
that water vapor changes directly to a solid.
Hail
 Hail are balls or lumps of ice that fall from
clouds.
 Hail forms in cumulonimbus clouds due to
updrafts freezing the water droplets.
Air Masses and Fronts
 Changes in weather are caused by the
movement and interaction of air masses.
 An air mass is a large body of air where
temperature and moisture content are
similar throughout .
Air
Masses
 Air Masses are
characterized by
their moisture
content and
temperature
which is
determined by the
area over which
the air mass
forms (Source
Region)
Types of air masses

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

Maritime (m) Forms over water; wet
Continental ( c ) Forms over land; dry
Polar ( P) Forms over polar regions; cold
Tropical (T) develops over the tropics: warm
Cold Air Masses
 Cold air masses are
responsible for
bringing extremely cold
winters.
Warm Air Masses
 Warm air masses bring
warm air weather
systems into the
United States.
 This brings milder
temperatures
 This can also bring
severe weather during
the summer months.
Fronts
 Front: the boundary between air masses of
different densities and usually different
temperatures
 Four kinds of fronts
– Cold front
– Warm front
– Occluded front
– Stationary front
Cold Fronts
 A cold front forms
where cold air moves
under warm air
 Move quickly and bring
thunderstorms, heavy
rain, or snow
 Cooler weather follows
a cold front
Warm Fronts
 A warm front forms
where warm air moves
over colder denser air
 Warm air replaces cold
air
 Bring drizzly rain
 Followed by clear and
warm weather
Occluded Front
 An occluded front
forms when a warm air
mass is caught
between two colder air
masses
 This produces cold
temperatures and
large amounts of rain
and snow.
Stationary Fronts
 Stationary fronts form
when a cold air mass
meets a warm air
mass
 Brings many days of
cloudy wet weather.
Air Pressure and Weather
 Areas that have lower pressure than the
surrounding areas do are called Cyclones.
 These areas the air masses come together
and rise
Air Pressure and Weather
 Areas that have high pressure are called
anticyclones.
 Anticyclones are areas where the air moves
apart and sinks
Air Pressure and Weather
 By keeping track of the cyclones and
anticyclones, meteorologists can predict the
weather.
 Cyclones cause stormy weather
 Anticyclones bring dry clear weather
Severe Weather




Thunderstorms
Tornadoes
Hurricanes
Severe weather safety
Thunderstorms
 Thunderstorms are small intense weather
systems that produce strong winds, heavy
rain, lightning, and thunder.
 Two atmospheric conditions are needed to
produce a thunderstorm:
– Warm and moist air near the Earth’s surface
– Unstable atmosphere
Thunderstorms
Cumulonimbus Clouds
Lightning
 Lightning is an electric
discharge that occurs
between a positively
charged area and a
negatively charged
area.
 Can occur
– Between two clouds
– Between Earth and a
cloud
– Between two parts of
the same cloud
Lightning
 When lightning strikes, energy is released.
 Energy is transferred to the air.
 Thunder is the sound that results from the
rapid expansion of air along the lightning
strikes.
Severe Thunderstorms
 Can produce:
–
–
–
–
–
High winds
Hail
Flash floods
Tornadoes
Lightning that causes
forest fires and burning
down homes
Tornadoes
 Tornadoes occur in 1% of all thunderstorms.
 A tornado is a small spinning column of air
that has high wind speeds and low central
pressure and that touches the ground.
 It starts as a funnel cloud that pokes through
a cumulonimbus cloud.
Tornadoes
Tornadoes
 75% of the world’s tornadoes occur in the
United States.
 Occur in spring and early summer
 They average 120-180 km/h wind speed
Tornado Pictures
Tornado Pictures
Tornado Pictures
Hurricanes
 Hurricanes are large rotating tropical
weather systems that have wind speed of at
least 120km/h
 They are the most powerful storms on Earth.
 Pacific Ocean hurricanes are called:
typhoons
 Indian Ocean hurricanes are called:
cyclones
Hurricane
 Hurricanes happen
over warm tropical
oceans.
 Hurricanes vary in size
from 160 to 1500 km in
diameter and can
travel for thousands of
kilometers.
How Hurricanes form
 Begins as a group of thunderstorms moving
over tropical ocean waters
 Winds traveling in different directions cause
the storm to spin
 It gets its fuel from the contact with the
warm ocean waters
 The hurricane continues to grow as long as
it is over the moist warm source.
Hurricane
Hurricane Damage
 Very destructive
 Average wind speed is
120-150 km/h
 Can knock down trees
and destroy buildings
 Flooding is the most
destructive part.( storm
surge).
Weather Safety
Thunderstorm Safety
 Lightning is attracted to
tall objects
 Crouch down in open
areas
 Stay away from water
Tornado Safety
 Weather forecast: watch
and warning system
 Watch- tornado may
happen
 Warning- tornado has
been spotted
 Find shelter: basement or
cellar
Weather Safety
Weather Safety
Flood Safety
 Weather forecast:
watches and warning
system
 Find a high place
 Always stay out of flood
waters
Hurricane Safety
 Watch weather updates
 Evacuate the area
 Have a disaster supply kit
available with food and
water
 Board up windows with
plywood
 Stay indoors during the
storm
Weather Safety
Forecasting the Weather
 A weather forecast is a prediction of weather
conditions over the next 3 to 5 days
 A meteorologist is a person who observes
and collects data on atmospheric conditions
to make weather predictions
Weather-Forecasting Technology
 High in the sky
– Weather balloons
 Measure weather conditions as high as 30 km above
Earth
 Measures temperature, air pressure, and relative
humidity
 Meteorologists can also measure wind speed and
direction by tracking the weather balloons.
Weather-Forecasting Technology
Weather-Forecasting Technology
 Measuring Air Temperature and Pressure
– Thermometer: a tool used to measure air
temperature
– Barometer: an instrument used to measure air
pressure
 The mercury inside a barometer rises as the air
pressure increases
Weather-Forecasting Technology
 Measuring Wind Direction
– Can be measured by using a windsock or wind
vane
 Windsock: is a cone shaped cloth bag open at both
ends
 Wind vane: is shaped like an arrow with a large tail
and is attached to a pole
Weather-Forecasting Technology
 Measuring wind speed
– Anemometer: an instrument used to measure
wind speed
 Consists of four cups connected by spokes to a pole
 The cups move
 The motion sends an electric current that measures
the wind speed
Weather-Forecasting Technology
Weather-Forecasting Technology
 Radar and Satellites
– Radar is used to find the location movement
and amount of precipitation
– Doppler Radar systems on the weather station
– Weather Satellites provide the Earth images of
weather systems you see on TV weather
reports
Weather-Forecasting Technology
Weather Maps
 National Weather Service (NWS) and
National Oceanic and Atmospheric
Administration (NOAA) use information from
about 1000 weather stations in the US
 This is information from one weather station.
Reading Weather Maps