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Understanding
Weather
Chapter 16
Pages 422 – 449
Page 424
Water in the Air
 Water
can exist in the air as a solid, liquid, or gas.
• Ice = a solid, found in clouds as snowflakes
• Water droplets = Liquid water that exists in clouds
• Water vapor = water in gaseous form
 Weather = The condition of the atmosphere at a
particular time & place.
The Water Cycle
 Water cycle = the continuous movement of water
from water sources, such as lakes & oceans, into the
air, onto & over land, into the ground, & back to the
water source
Pages 425 – 426
Humidity
 Humidity
= the amount of water vapor or moisture in
the air.
 As water evaporates, the humidity increases. But the
air’s ability to hold water depends on air temperature.
(warmer air can hold more water)
Relative Humidity
 Relative humidity = the amount of moisture the air
contains compared with the maximum amount it can
hold at a particular temperature.
 Calculate by: (Present humidity)/(saturated humidity
at the given temperature)
Page 426
Measuring Relative Humidity
 Psychrometer
= an instrument used to measure
relative humidity.
 It has a wet-blub thermometer (measures the rate
of evaporation) & a dry thermometer (measures
the air temperature).
 The larger the difference between the two readings
indicates that there is less water vapor in the air
and thus lower humidity.
Page 427
The Process of Condensation
 Condensation
= the process by which a gas, such
as water vapor, becomes a liquid. It occurs when
saturated air cools.
 Or it can happen if air cools to a point of
saturation for the water vapor present in it.
 Dew point = the temperature to which air must
cool to be completely saturated.
 It must also have a surface to condense on.
Page 428
Clouds
 Cloud
= a collection of millions of tiny water droplets
or ice crystals.
 They form as warm air rises & cools. In order for
water vapor to change physical state, it needs a
surface on which to change.
• Condensation nuclei – are small particles, such as
dust, smoke, & salt, suspend in the air.
 At higher temperatures, water vapor condenses on
these small particles as water droplets.
 At temperatures below freezing water vapor changes
directly to a solid, forming ice crystals.
Pages 428 – 429
Kinds of Clouds
 There
are three basic types of clouds
 Cumulus clouds = puffy, white clouds that tend to
have flat bottoms; form when warm air rises.
Generally indicate fair weather. Cumulonimbus
cloud – larger clouds that produce thunderstorms
 Stratus clouds = layers of clouds; cover large
areas; caused by a large body of air lifting into the
atmosphere. Nimbostratus clouds – dark clouds that
produce light-heavy rain. Fog = Near the ground
 Cirrus clouds = thin, feathery, white clouds found
at high altitudes; form in strong wind; indicate
approaching bad weather if they thicken & lower
Pages 430 – 431
Precipitation
 Precipitation
= water, in solid or liquid form, that
falls from the air to the Earth.
 Cloud droplets start out about the size of a period.
They get larger by colliding & joining with other
droplets. Eventually the droplets become too heavy to
remain suspended & fall to the Earth.
 Rain – liquid water that falls from the clouds.
Snow, Sleet, Hail
 Snow – solid water that falls to the Earth.
 Sleet (freezing rain) – forms when rain falls through a
lay of freezing air.
 Hail – solid water that falls as balls or lumps of ice.
Pages 430 – 431
Measuring Precipitation
Page 432
Air Masses & Fronts
 Changes
in weather are caused by the movement
& interaction of air masses.
 Air mass = a large body of air that has similar
temperature & moisture throughout.
Air Masses
 An air mass gets tis moisture & temperature
characteristics from the area over which it forms.
These areas are called source regions.
Page 433
Cold Air Masses
 There
•
•
•
are 3 source regions that affect the US.
Canada – brings extremely cold weather in the
winter & cool dry weather in the summer
North Pacific Ocean – brings rain & snow to
the Pacific coast in the winter & cool, foggy
weather in summer.
North Atlantic Ocean – produces cold, cloudy
weather in winter, & cool weather with fog in
summer.
Pages 433 – 434
Warm Air Masses
 There
are 4 warm air masses that influence the US
Ocean & 2Gulf of Mexico – In the
summer these bring bring hot, humid weather,
thunderstorms, & hurricanes. In the winter they
bring mild, often cloudy weather.
3Pacific Ocean – has less moisture content & is
weaker than the maritime polar mass = southern
California has less rain than the rest of California.
4Desert region of Mexico & southwest US – only
influences the weather in the summer brings dry,
clear, & very hot weather.
• 1Atlantic
•
•
Pages 434 – 435
 Air
Fronts
masses with different characteristics, such as
temperatures & humidity, do not usually mix.
 Front = a boundary where two different air
masses meet
 Weather at a front is usually cloudy & stormy.
They occur in mild climates because both warm &
cold air masses can be found there.
Page 436
Severe Weather
 Weather
in the mid-latitudes can change from day to
day. These changes result from the continual shifting
of air masses.
 Severe weather = weather that can cause property
damage & even death.
Thunderstorms
 Thunderstorms = are small, intense weather systems
that produce strong winds, heavy rain, lightning, &
thunder.
 Two atmospheric conditions are required to produce
thunderstorms: the air near the Earth’s surface must be
warm & moist, & the atmosphere must be unstable.
Page 437
Lightning
 Thunderstorms
are very active electronically.
 Lightning = a large electrical discharge that
occurs between two oppositely charged surfaces.
 Thunder = the sound that results from the rapid
expansion of air along the lightning strike.
Severe Thunderstorms
 ≈ 10% of thunderstorms are considered severe.
 Severe thunderstorms produce one or more of the
following conditions – high winds, hail, flash
floods, & tornadoes.
Pages 438 – 439
Tornadoes
 Tornadoes
are produced in only 1% of all
thunderstorms.
 Tornado
= a small, rotating column of air that has
high wind speeds & low central pressure & that
touches the ground.
 The
length of a tornado’s path of destruction can vary,
but it is usually about 8 km long & 10-60 m wide.
 Tornadoes
are capable of picking up heavy objects,
such as houses, cars, & store signs, & hurling them
through the air.
Page 439
 Hurricane
Hurricanes
= a large, rotating tropical weather
system with wind speeds of at least 119 km/h.
generally form in the area between 5o & 20o
north & south latitude over warm, tropical oceans.
 They
 Hurricanes
vary in size from 160 km to 1,500 km
in diameter, & they can travel for thousands of
miles.
Page 440
Formation of a Hurricane
 A hurricane
begins as a group of thunderstorms
moving over tropical ocean waters. Winds
traveling in two different directions collide,
causing the storm to rotate over an area of low
pressure.
 Hurricanes get their energy from the condensation
of water vapor. Once formed, the hurricane is
fueled through contact with the warm ocean water.
 As long as the hurricane is over warm water it
continues to grow
Page 441
Damage Caused by Hurricane
 Hurricanes
can cause a lot of damage when they
move near or onto land.
 Their winds knock down trees & telephone poles
& can damage & destroy buildings & homes.
 Most hurricane damage is done by heavy rains &
the storm surge.
 Depending on the strength of the hurricane, as
storm surge can be 1 m to 8 m high.
Page 442
Forecasting the Weather
 Weather
forecast = a prediction of weather
conditions over the next 3 to 5 days.
Weather Forecasting Technology
 In order for meteorologists to accurately forecast
the weather, they need to measure various
atmospheric conditions, such as air pressure,
humidity, precipitation, temperature, wind speed,
& wind direction.
Measuring Air Temperature
 Thermometer = a tool used to measure air
temperature.
Pages 443 – 444
Measuring Air Pressure
 Barometer
= used to measure air pressure
Measuring Wind Direction & Wind Speed
 Windsock or Wind vane = used to measure wind
direction.
 Anemometer = used to measure wind speed
Measuring Weather in the Upper Atmosphere
 Weather balloons carry radiosondes to measure
weather conditions 30 km above the Earth’s surface.
 Radar is used to find the location, movement, &
intensity of precipitation.
 Weather satellites orbiting the Earth provide weather
information that cannot be obtained from the ground.
Pages 444 – 445
Weather Maps
 Station
model = a small circle, which shows the
location of the weather station, with a set of
symbols & numbers surrounding it, which
represents the weather data.
 isobars = lines that connect points of equal air
pressure.