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




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.
Locate the 4 types of fronts on this
weather map.
Cold Fronts
Warm Front
Stationary Front
Occluded Front
Be a Weather Forecaster
You are planning
to travel to
Alabama in 2
days. The high
temperature
there for today
is 68º F.
Use the map to help you predict whether the temperature in
Alabama will increase, decrease, or stay the same. Explain why
you think so.
Weather Fronts
37
Be a Weather Forecaster
There is a cold front approaching. The
temperatures will probably be cooler
behind the front.
38
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
 A rising barometer reading means higher pressure
and possibly clear fair weather .
 Decreasing barometer reading means a lower
pressure and possibly bad weather.
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
1. Which statement best describes the driving force behind all the weather on
Earth, from local weather systems to large-scale storms?
A.The Coriolis Effect curves the motion of wind due to the Earth's counterclockwise rotation.
B.The Greenhouse Effect ensures that some of the Sun's heat is always
trapped close to the Earth's surface.
C.The Sun heats the atmosphere unevenly, so temperatures and pressures are
constantly equalizing.
D.The ocean holds the heat of the Sun more efficiently and for longer periods
than land.
2.. Air masses move from areas with high pressure (such as the poles of the
Earth) to areas with low pressure (the equator). The temperature and
moisture of an air mass depend on where it is formed.
 A continental tropical air mass is _______ and _______.
A.warm, moist
B.cold, moist
C.cold, dry
D.warm, dry
3. The amount of water vapor in the air is referred to as
A.precipitation.
B.condensation.
C.humidity.
D.temperature.
1. When warm air moves over cold air, a warm front forms. The warm air
tends to rise along a gentle slope above the cold air and form layers of
clouds.
Given this information, which of the following cloud types and weather
conditions would most likely result from an incoming warm front?
 A.stratus clouds and drizzly rain
 B.cumulonimbus clouds and thunderstorms
 C.cumulus clouds and clear weather
 D.isolated cirrus clouds and clear weather
2. Wispy, thin, white clouds can be seen high in the sky on a pleasant day.
Which type of clouds are these?
 A.stratus
 B.cumulonimbus
 C.cirrus
 D.altostratus
3. Sharon woke up on a sunny morning and ate breakfast. Then she
looked outside and saw tall, quickly-forming clouds. The clouds looked
ready to rain. When she turned on the TV, she saw just what she
thought—a forecast for sudden rains. What most likely caused the
change in weather?
 A.a warm front overtaking a cold air mass
 B.lack of wind
 C.a cold front overtaking a warm air mass
 D.an expanding warm air mass
1. If the collected information showed that the temperature was 10°C,
there was little pressure, and the wind speed was 75 mph, what
type of weather would it probably be outside?
 A.warm and windy
 B.warm and sunny
 C.cold and wet
 D.cold and windy
2. A swirling, high-speed windstorm begins over the ocean. It
contains heavy rains. The wind and rain rotate around a center of
low pressure. When the winds inside this tropical cyclone reach 74
mph, which of the following occurs?
 A.hurricane
 B.tropical depression
 C.tropical storm
 D.thunderstorm
3. A local barometer reading on a local weather forecast was 29.8 mm
and falling. What type of weather conditions are most likely to
occur?
A. clear and cool
B. clear and warm
C. fair skies
D. cloudy skies