Chapter 12

Chapter 12

Lesson 1

Water cycle

The movement of water through Earth’s

systems powered by the sun's energy

In the water cycle water vapor

enters the atmosphere by

evaporation from the oceans and
other bodies of water and leaves

condensation.

Evaporation

The process by which molecules of liquid water escape into the air after
becoming water vapor. It also can leave through living things like plants and
animals. In plants, the water enters the roots, rises to the leaves, and is released
into the air. Animals also release water vapor whenever they exhale.

Condensation

The process by which water vapor becomes liquid water.

Humidity

Is a measure of the amount of water vapor in the air.

Relative Humidity

Is the percentage of water vapor that is actually in the air compared to the
maximum amount of water vapor the air can hold at a particular temperature.

Relative humidity can be

measured with an instrument

called a psychrometer.

Psychrometer

Has two thermometers a wet bulb thermometer and a dry bulb thermometer.
The wet bulb thermometer has a moist cloth around it so when you swing it air
blows over it and evaporates the water. If the air pressure is low, there will be a
big difference, but if its high pressure it’ll be low.

Clouds form from when water
vapor in the air condenses to

form liquid water or ice crystals

Lesson 2

Dew point.

The temperature at which condensation begins.

Scientist classify clouds into three
main types based on their shape:

cirrus, cumulus, and stratus.

Clouds are further classified by

their altitude

For clouds to form, there need to
be 2 things. Cooling ad particles.

Cooling: Temperature at which condensation begins is called the dew point. If the dew point is above freezing, the
water vapor will form droplets. If the dew point is below freezing, the water vapor may directly change into ice crystals.

Particles: Tiny particles must be present for clouds to form. They form a surface so that the clouds can condense. Most
of the particles are salt crystals, dust from soil, or smoke. Also condenses on solid surface. Liquid water that condense
from the air onto a cooler surface is called dew. While ice deposited on a surface that is below freezing, is called frost.

Cirrus

. Wispy, feathery clouds

.Cirrus -a curl

. high altitudes above 6 km low temperatures

. made of ice crystals and indicate fair weather

Fog

. form near the ground

. forms when the ground cools at night

after a humid day.

Altocumulus and Altostratus

. clouds form between 2-6 km above Earth’s surface

. alto-high

. medium level clouds that are higher than regular cumulus and stratus clouds
but lower than cirrus clouds.

. indicate precipitation

Cumulus

. look like cotton

. cumulus- heap

. form less than 2 km above the ground may extend upward 18 km

. short indicate fair weather

. towering clouds with flat tops or cumulonimbus clouds produce
thunderstorms

. nimbus- rain

Cirrocumulus

. look like cotton balls

. indicate storm is on its way

Stratus

. from in flat layers

. strato-spread out

. cover all or most of the sky and are dull and gray

. as they thicken they may produce drizzle, rain, or snow called nimbostratus

Lesson 3

Precipitation

Any form of water that falls from clouds and reaches Earth’s surface.

Common types of precipitation
include rain, sleet, freezing rain,

snow, and hail.

Rain Gauge

An open ended tube that collects rain

Freezing Rain

. raindrops can sometimes fall liquid water but sometimes freeze when they
touch a cold surface

. this precipitation is called freezing rain

Rain

-Drops of water at least 0.5 mm in diameter.

-Smaller drops of water, drizzle 0.05 mm

-Even small drops of water, mist 0.005mm

Snow

. snowflakes have endless number of
different shapes and patterns with 6
sides or branches

. snowflakes form when water vapor
in a cloud is converted directly into
ice crystals

. they join together into large clumps
of snow in which the crystals are
hard to see

Hail

. hailstone a round pellet of ice larger
than 5 millimeter in diameter.

. forms inside a cumulonimbus cloud
during thunderstorms

. starts as an ice pellet inside a cold
draft. Strong updrafts carry
hailstone many times. Each time the
hailstone goes through the cold
region builds a new layer of ice.

.eventually they become heavy
enough to fall. They can grow large
and cause damage to crops,
buildings, and vehicles

Measuring Rain and SNow

Snow

-Two ways

-Measuring stick

-Melting collected snow
and measuring the
depth of water it
produces

-10 cm of snow = 1 cm of
rain

-Good snow has less
water than bad snow

Rain

-An open - ended tube
that collects rain is
called rain gauge

-Measured by dipping
ruler inside

-Add funnel on top to
make more accurate,
then divide by 10.

-

Sleet

. Sometimes raindrops fall through layers of air thats below 0 degrees celsius
freezing point of water

. when they fall the raindrops freeze into solid particles of ice

.ice particles smaller than 5 millimeters in diameter are called sleet

Flood

Is an overflowing of water in a normally dry area. People try to control floods
ny adding dams and levee.

Small or large many floods occur
when the volume of water in a
river increases so much that the

river overflows it channel

Drought

A long period of scarce rainfall or dry weather. People try to control it by
growing drought resistant plants and saving on water.

Droughts are usually caused by
dry weather systems that remain
in one place for weeks or months

at a time

Lesson 4

Air Mass

Is a huge body of air in the lower atmosphere that has similar temperature,
humidity, and air pressure at any given height

Four types of air masses

influence the weather in North

America: Maritime Tropical,

Continental Tropical, Maritime Polar,

and Continental Polar

Tropical

A warm air masses form in
the tropics and have low air
pressure

Is or a cold air masses form in the north of 50
degrees north latitude and south of 50 degrees
south latitude

Polar

Maritime

Air masses form over oceans. Water evaporates from the oceans so the air can
become very humid

Air masses form over land. Continental air masses have less exposure to lage
amounts of moisture from the bodies of water.

Continental

Maritime
Tropical

. Warm, humid air masses form
over the Pacific ocean.

. Summer bring hot, humid
weather, summer showers, and
thunderstorms.

. Winter bring heavy rain or snow

Maritime Polar

. Cool, humid air masses form over
icy cold North Atlantic ocean.

. Air mass often pushed out to sea
by westerly winds

Continental

Tropical

. Large air masses form over
Canada and Alaska bring bitterly
cold weather low humidity

. Storms may occur when air
masses move south or collide with
Maritime Tropical air masses
moving north

. Hot, dry air masses form mostly
summer over dry areas Southwest
and Northern Mexico

. Bring hot dry weather to southern
Great Plains

Continental

Polar

Colluding air masses can form

four types of fronts: cold fronts,
warm fronts, stationary fronts,

and occluded fronts

The warm air mass is cut off or occluded from the ground

Occluded

Cold Fronts

. Clouds and precipitation accompany
warm fronts

. Fast moving warm air overtakes
slower colder air

. Warm air moves over cold air

. If humid light rain or snow falls

. Air dry scattered clouds

. Warm fronts arrive slowly weather
rainy or cloudy several of days

. Warm fronts passes weather warmer
and humid

Warm Fronts

. Cold air sinks and warm air rises but less dense

. Cold fast air runs into slower warm air. Denser
cold air slides under lighter warm air. Warm air
pushed upward along leading edge of colder air.

.Warm air rises expands and cools

. Heavy rain or snow may fall

. Cold fronts arrive quickly because of leading
edges move ground

. abrupt weather changes/ thunderstorms

. Colder drier air brings clear sky and lower
temp

Stationary

Fronts

. Cold and warm air masses meet
neither one can move another

. Warm and cold air meet water vapor
in warm air condenses rain, snow, fog,
or clouds

. If stalls bring many days of clouds
and precipitation

. Complex weather situation

. Warm air masses caught between 2 cold air
masses

. Denser cool air moves underneath warm air
upward

. Both cold air masses in the middle mix

. Temp near ground becomes cooler

Occluded

Fronts

Fronts

. Bands of HIGH SPEED
WINDS about 10 kilometers
above Earth’s surface

. Generally blow from west to east

. Air masses are carried along
their tracks

Jet Streams

. The boundary where the air
masses meet OF DIFFERENT
TEMPERATURE AND
HUMIDITY

. Storms and changeable weather
often develop along fronts like in
figure 3

.

Cyclone

A circled L on a weather map stands for low and indicates an area of relatively
low air pressure. A swirling center of low air pressure is a cyclone from a greek
word meaning wheel. Cooler air blows inward from nearby areas of higher air
pressure. Winds spiral inward toward the center.

Cyclones and decreasing air
pressure are associated with
clouds, winds, and precipitation

Anticyclones

As its name suggest an anticyclone is the opposite of a cyclone. Anticyclones are
high pressure centers of dry air shown by an H on a weather map. Winds spiral
outward from the center moving towards areas with lower pressure.

The descending air in an

anticyclone generally causes

dry,clear weather

Lesson 5

Storm

A storm is a violent disturbance in the atmosphere. Storms involve sudden
changes in air pressure, which cause rapid air movements. There are several
types of severe storms: winter storms, thunderstorms, hurricanes, and
tornadoes.
All year round most precipitation
begins in clouds as snow. If the

air is colder than 0 degrees

celsius all the way to the ground
the precipitation falls as snow.

Thunderstorms

Is a small storm often accompanied by heavy precipitation and frequent
thunder and lighting. They form when warm is air is forced upward along a
cold front. The warm humid air rises rapidly. The air cools, forms dense
thunderheads with water condenses into rain droplets. Heavy rain falls,
sometime along with hail. Within the thunderhead are strong upward and
downward winds known as updrafts and downdrafts.

Thunderstorms form in large

cumulonimbus clouds, also known

as thunderheads

Lightning

A sudden spark or electrical discharge, as these changes jump between parts of a
cloud between nearby clouds, or between a cloud and the ground

Hurricane

Is a tropical cyclone with winds of 119 km/h or higher. A typical hurricane is
about 600 kilometers across. Hurricanes form in the Atlantic, Pacific, and
Indian oceans. In the Western Pacific they are called typhoons. In the Indian
ocean they are simply called cyclones.

A hurricane begins over warm
ocean water as low-pressure
area or tropical disturbance

The result of a storm surge a dome of water that sweeps across the coast where
the hurricane lands. Storm surges can cause great damage, washing away
beaches, destroying coastal buildings, and eroding the coastline.

Storm Surge

Tornado

A tornado is a rapidly whirling funnel-shaped cloud that reaches down from a
thunderstorm to touch Earth’s surface. If a tornado occurs over a lake or ocean
the storm is called a waterspout. Tornadoes are usually brief but can be deadly.
They may touch the ground for 15 minutes or less and be only a few hundred
meters across. But an intense tornado wind speed may approach 500 km/h.

Tornadoes most commonly

develop in thick cumulonimbus

clouds the same clouds that bring

thunderstorms

If you are caught in a snowstorm
try to find shelter from the wind

During thunderstorms avoid places

where lightning may strike. Also
avoid objects that can conduct
electricity such as metal objects

and bodies of water

Evacuate

You should be prepared to evacuate or move away temporarily. A hurricane
warning means that hurricane conditions are expected for the next 24 hours

If you and you hear a hurricane
warning and are told to evacuate

leave the area immediately

Lesson 6

The safest place to be during a
tornado is in a storm shelter or

the basement

A meteorologist are scientist who study and try to predict the weather

Meteorologist

Meteorologists use maps, charts,
computers, and other technology
to analyze weather data and to

prepare weather forecast

Isobars are lines joining places on the map that have the same air pressure. The
numbers on the isobars are the pressure readings. These readings may be given
in inches of mercury or in millibars

Isobars

Isotherms are lines joining places that have the same temperature. The isotherms
may be labeled with the temperature in degrees Fahrenheit, degrees Celsius or
both

Standard symbol on weather

maps show fronts, areas of high

and low pressure, types of

precipitation, and temperature

Isotherms

The scientists use stuff like automated

weather stations, weather balloons, weather

satellite, and computer forecasts

Automated Weather Stations - gather data from surface locations for temperature, air pressure, relative humidity,
rainfall, and wind speed and direction. NWS established network more 1,700 surface weather observation sites.

Weather Balloons- carry instruments into the troposphere and lower stratosphere. Instruments measure temperature,
air pressure, and humidity.

Weather Satellites- orbit Earth in the exosphere. Cameras on them make images of Earth’s surface, clouds, storms, and
snow cover. Collect data on temperature, humidity, solar radiation, and wind speed and direction.

Computer Forecasts- process weather data quickly to help forecasters make predictions. THe computer works through
thousands of calculations using equations from weather models to make forecasts.